This system, remarkable among all the others by the hardness and resistance that characterize it, has from this double attribute a fitness to serve as a common base for all, upon which they rest, and around which they are suspended and fixed. The whole of the pieces that form it, are connected together for this use, by means of flexible and resisting bands, which with these pieces make a whole that is called a skeleton. The osseous whole, placed in the midst of many organs that it sustains, everywhere continuous in its different parts, has not however, like the primitive systems, continuity of peculiar life from one of its extremities to the other. The bands which connect these different pieces, very different from them in their nature and their properties, produce in them an insulation of vitality, which the different parts of the above systems do not exhibit, because in their continuity their nature is everywhere the same.
Considered in relation to their forms, the bones are of three sorts, long, flat and short. One dimension predominates in the first, viz. length; two are in nearly equal proportions in the second, length and breadth; these two last dimensions, with thickness especially added, characterize the short bones. Let us examine each in a general manner.
The long bones belong in general to the apparatus of locomotion, in which they form a kind of levers that the muscles move in different directions. All are placed in the extremities, in which their whole forms a kind of central column, moveable in different directions. We see them successively diminishing in length and increasing in number, when examined from the superior to the inferior part, from the thigh or the humerus to the phalanges of the toes or the fingers. It follows from this double opposite arrangement, that the top of the limbs is characterized by the extent of its motions, and the bottom by the multiplicity, variety and narrow limits of these motions.
These bones have all an analogous conformation; thick and large at their extremities, they are more slender and usually rounded in the middle or body, as anatomists call it.
The size of the osseous extremities exhibits the double advantage, 1st, of presenting to the articulations large surfaces and consequently more causes of resistance to different displacements; 2d, of contributing to the regularity of the forms of the limb to which they belong. Observe in fact that the muscles and the bones are placed in an inverse direction in the extremities. The middle of the first, which is their largest part, corresponds to the middle of the second, which forms their small portion, whilst the extremities of these compensate by their size for the smallness of the tendons which terminate the others, and which are placed at the side of them. The increase of size of the extremities of the long bones is not sudden; it commences imperceptibly upon the body. We observe upon these different extremities eminences of articulation and of insertion.
The middle or the body has no eminence; prominent lines are seen there, always destined for aponeurotic insertions, and which, when they are very considerable take from the bone its cylindrical form, which it however preserves in the interior; thus the tibia is evidently triangular externally, though within its canal has the form of that of the femur. In general these lines of insertion, always separated by plain surfaces, are three in number upon each long bone, as we see on the humerus, the radius, the ulna, the tibia, fibula, &c. I know not the reason of this law of conformation. Another general observation is, that the body of almost all the long bones is twisted, so that the direction of its superior part is not the same as that of the inferior; by tracing from above downwards one of these lines of which I have just spoken, this may be seen; it is however more evident in the adult than in the fœtus. This change of direction has no uniformity in the course it pursues.
The internal forms of the long bones are very well seen by sawing them longitudinally. The texture of the cells fills them to the extremities; it is, as we shall see, more fine and less abundant in the middle, where the medullary canal exists.
This canal does not exist in the first month of the fœtus, nor as long as the bone is cartilaginous; the osseous state is the period of its formation. All the gelatine of the middle of the bone is then absorbed, exhalation brings no more there, except in the very delicate texture of the cells that this canal contains; this function, which is nothing in the centre, becomes more active on the circumference of the bone. This increase of activity of the external exhalants favours the formation of the compact texture, the development of which takes place precisely at the same time as that of the canal whose parietes it forms; so that at this period of ossification, exhalation and absorption appear to be in an inverse state in the two parts of the bone; one is very active on the exterior in bringing phosphate of lime, with which it encrusts the already existing parenchyma; the other is very active in the interior in removing the gelatine whose absence forms the space from which the medullary canal arises.
There is no well marked medullary cavity except in the humerus, the radius, the ulna, the femur, the tibia, the fibula and clavicle. The ribs and the phalanges, which in their forms resemble them, have much of the ordinary texture of the cells in their centre, and hardly ever any of that more delicate texture of the cells which occupies the centre of the bones above named, and which is only found in the medullary cavity.
This cavity does not extend beyond the body of the bone; where the compact texture grows thinner, it disappears, and is replaced by a great quantity of the texture of the cells, which fills the extremity of the bone. Its form is cylindrical and its direction straight. It does not vary in its form, on account of its asperities or the external prominent lines of the body of the bone, which is only thicker in these places. Its parietes are much smoother in the middle, than at the extremities, where there are already many considerable cellular filaments thrown off. There are in many subjects, delicate, horizontal bony partitions, which interrupt almost entirely its continuity in this place, and appear to divide it into two or three very distinct parts.
The medullary canal serves not only to lodge and defend the medullary organ, but also to give more resistance to the bone; for we know, that of two cylinders formed of an equal quantity of matter, one of which is hollow, and consequently has a greater diameter than the other which is full, the first will resist more than the second, because we can bend and break it with less facility. Full cylinders, equal in diameter to the long bones, would have prevented by their weight, the motions of the limbs; whilst other cylinders of the same weight as the present, but without any cavity, would give too small a surface for the insertion of the muscles. To unite small weight with a sufficient space in the middle of the long bones, is then a great advantage of the medullary canal.
This canal disappears in the first periods of the formation of callus in fractures, because the whole medullary organ is occupied at this place by gelatine, and becomes cartilaginous; then this gelatine gradually re-absorbed, without being replaced, favours the development of a new cavity, and the communication is re-established between the superior and inferior parts of the canal.
I have observed that, in the first age, and while the extremities of the bones are cartilaginous, the medullary canal is shorter in proportion than in the adult; it hardly forms at birth more than the middle third of the bone, the superior and inferior thirds being formed at first by the cartilaginous portion of each extremity, then a texture of cells intermediate between this portion and the canal; so that as we advance in age, its length becomes in proportion greater.
The flat bones have in general, but little relation to locomotion, which they only assist by the insertion of the muscles that go to the long bones. Nature designs them especially to form the cavities, such as those of the cranium and the pelvis. Their conformation renders them very proper for this use. Their number varies according to the cavities with which they are connected; many always unite to form one, and it is this circumstance that contributes in part to their solidity. In fact, external blows losing their force at the place of their junction, fracture them with less ease. If the cranium was only one single piece, its solutions of continuity would be much more frequent than they now are. So that as the sutures ossify in old age, they become more brittle. In children, in whom the ossification is not complete, and the number of whose separate, osseous pieces is consequently more considerable in the head, the pelvis, &c. the difficulty of fractures is very great, because the soft bands which unite the solid parts yield to external bodies, without breaking.
The flat bones are almost all curved, concave and convex on the opposite sides; this arises from their destination in the formation of cavities. Their curve varies according to the place in the cavity they occupy; this curve is the cause of a very powerful resistance, when that mentioned above does not exist. Thus in the first age, the cranium resists by yielding; but as the sutures become more closed, and only one osseous piece is formed, it is by the mechanism of the arch that the brain is protected.
All the flat bones have two surfaces and a circumference. According as the first serve for muscular insertions, or are only covered by aponeuroses, membranes, &c. they are rough or smooth. Towards the middle the bone is thinner; it has more thickness at the circumference, which is either for articulation or insertion. In the first case, this excess of thickness gives more solidity to the joints, which are then made with larger surfaces, as we see in the cranium; in the second, it presents to the fibres more points of origin, as we see on the crista of the ilium and the greater part of its circumference.
The internal forms of the flat bones have but few peculiarities; their two external layers leave between them a space which is filled by the texture of the cells.
The short bones are placed in general in parts where are found united mobility and solidity, as in the vertebral column, the tarsus, and the metatarsus. Always small, they are in great number in the regions which they occupy; their number compensates for their size in the formation of the parts of the skeleton to which they contribute. It is this number also, that gives to these parts the union of the two almost opposite attributes of which we have spoken, viz. solidity, because the external efforts are lost in the numerous bands which unite them, and mobility, because the whole of their individual motions gives a considerable general motion.
There is nothing constant or uniform in the external conformation of these bones; it is modified according to the general plan of the whole, of which they are the parts; thus the different uses of the carpus, metacarpus and vertebral column determine the different forms of their respective bones. These bones have always many cavities and eminences upon their external surfaces, necessary for their numerous articulations, for the insertion of the many ligamentary cords that unite them, and the muscles that move them.
In the interior, these bones have nothing peculiar, except an abundance of the texture of the cells which forms them almost wholly, and exposes them to frequent caries.
Nature is not however regular in the division of bones into long, flat and short. Here as elsewhere, she disregards our methodical descriptions, and shows us the bones sometimes exhibiting the character of long ones and short, and sometimes uniting the attributes of both these last with the flat ones. The basilary apophysis and the superior part of the occiput, the body and the lateral portions of the sphenoid, when placed in contrast, prove this assertion. A bone sometimes by its external form belongs to the long ones, but from its internal organization should be classed with the flat, of this the ribs are an example, &c.
The bony eminences have generally the name of apophyses; they are called epiphyses when the cartilage of ossification which unites them to the bone is not yet encrusted with calcareous substance.
These eminences have four great divisions; viz. those, 1st, of articulation; 2d, of insertion; 3d, of reflection; 4th, of impression.
1st. The eminences of articulation vary according as the articulation is moveable or immoveable; I shall not consider them here, as I should be obliged to repeat it in the chapter upon articulations.
2d. The eminences of insertion are very numerous in the bones; they only give attachment to the fibrous organs, as the ligaments, the tendons, the aponeuroses, the dura-mater; no organ differing from these is implanted into the bony eminences, or generally into the bones, except by means of them; the muscles are a remarkable example of this.
These eminences are usually much less in women than in men, in children than in adults, in weak animals than in carnivorous ones who live by attacking and destroying their prey. The prominence of the eminences of insertion is always an index of the force and vigour of the motions. They are the more developed in proportion as the muscles are. Examine comparatively the skeleton of a strong, sanguineous man, whose muscles are powerfully delineated through the integuments, and that of a feeble, phlegmatic man, whose rounded forms like those of women, do not appear prominent, and you will see the difference.
The form of these eminences of insertion varies greatly; sometimes the muscles are inserted by many separate aponeurotic fibres; then they are small, very numerous and form only little asperities imprinted on a greater or less surface; sometimes it is by a single tendon that the muscle takes its origin, then the apophysis is usually very prominent, and occupies a small space. Sometimes a broad aponeurosis gives rise to the fleshy fibres; it is then a bony line, more or less projecting that gives insertion.
The eminences are in general in proportion to the muscles that are attached to them; for example, in three muscles of nearly equal size, one of which is attached by separate fibres, the other by a tendon, and the other by an aponeurosis, we observe that the sum of the asperities of insertion of the first, the separate apophysis of the second, and the prominent line of the third are nearly equal in the quantity of osseous substance that forms them; so that by supposing that the apophysis was divided into asperities, or extended into a line, or that the asperities were united together, or the line concentrated so as to form an apophysis, this quantity of osseous substance would be found to be about the same.
We understand all the advantage of the eminences for the insertion of muscles, which they render distant from the centre of the bone, lessen the parallelism with its axis and consequently favour their motions in an evident manner.
Are these produced by the pulling of the muscles? This opinion borrowed from the laws of the formation of soft and inorganic bodies, does not accord with the known phenomena of vitality, with the existence of eminences where there is no muscular insertion, and which are often more prominent than these, with the disproportion that exists between the elongation of certain apophyses by muscular insertion, that of the styloid, for example, and the force of the muscles that are attached to it, &c.
The eminences for ligamentary insertion have the advantage, by removing a little the ligament from the articulation, of facilitating its motions; this is especially remarkable in the lateral ligaments of the elbow, the knee, &c.
As to the other eminences of insertion, we can hardly consider in a general manner their respective functions.
3d. The eminences of reflection are those under which a tendon passes, in deviating from its primitive course; such is the hook of the pterygoid apophysis, the malleolar extremity of the fibula, &c. Almost all these eminences have a slope or excavation in one direction, connected in the opposite with a ligament, so as to form a ring for the passage of the tendon.
4th. The eminences of impression are those which arise, when the different organs form on the osseous surfaces excavations that separate these eminences, which in fact only appear because the bone at this place remains at its ordinary level. The cerebral and muscular impressions are given as examples of this arrangement. But are these impressions really the effect of the compression of the organs on the bone, or do they arise from the laws of the osseous development, laws which give to the bones forms accommodated to the surrounding organs? I adopt more readily the second than the first of these opinions, which has been thought very probable from the effect of aneurisms upon bones that are contiguous to them, which are worn and gradually destroyed by them. But let us remark that if the muscles, the brain, and the vessels by their pressure, had upon the bones in a natural state, an action analogous to that of aneurism, the state of the parts ought to be the same as in that case. The compact layer ought to be destroyed where these depressions are, and leave in its place an unequal, ragged surface, but the contrary happens, which makes me think, that what is commonly called the impression of organs, is only a natural effect of ossification.
The osseous cavities are very numerous; those only which are found on the exterior of the bones will be treated of. They are divided, like the eminences, into articular and non-articular. The first will be examined, with the analogous eminences, in the chapter on articulations. Among the second there are cavities, 1st, of insertion; 2d, of reception; 3d, of slipping; 4th, of impression; 5th, of transmission; 6th, of nutrition.
1st. The cavities of insertion give attachment to the aponeuroses of the muscles, to the ligaments, &c. They have the advantage, 1st, of multiplying the insertions of the fibres, without increasing the size of the bone, since a concave surface is evidently more extensive than a plain surface would be which should occupy the space between its edges; 2d, of allowing the muscular fibres more room, and consequently giving them greater length than if they arose from an eminence, which also gives more extent to the motions. The pterygoid, digastric cavities, &c. present examples of this arrangement.
2d. The cavities of reception are those which serve to receive an organ, lodge and defend it; such are the fossæ of the bones of the cranium, those of the ossa ilii, &c. These cavities sometimes belong to the whole of the bone, the form of which is concave, as we see in the frontal bone, sometimes they are hollowed out upon an insulated part, like the maxillary depression of the inferior jaw; they are always destined for an essential part, for a gland, a viscus, &c.
3d. The cavities of slipping are in general found at the extremity of the long bones. They are grooves, more or less deep, in which the tendons glide to go to the place in which they are inserted. All are covered with a cartilage and terminated by a very strong ligamentary ring. Do the tendons by their friction form these cavities? This is the common opinion, but it does not appear to me more probable than the theory of muscular, vascular impressions, &c. These cavities ought to be then so much the deeper, in proportion as the muscles are the more exerted; they ought not to exist in subjects paralytic from their infancy; they ought not to exist in the cartilages of ossification in the fœtus, whose limbs have hardly ever moved; but the contrary of all this is constantly observed. Let us describe then all the different configurations of the bones, as a consequence of the laws of ossification, laws in obedience to which the osseous forms, all primitively determined, are made to develop. The size of the extremities of the long bones favours the existence of these different cavities, which cannot on this account injure the osseous solidity.
4th. The cavities of impression correspond with the eminences of the same name. I have spoken of them above.
5th. The cavities of transmission are especially destined for the vessels and the nerves. We find many of them on the head; they have sometimes the form of a groove, sometimes that of a tube and at others that of a slit, according to the thickness or breadth of the bones which these vessels or nerves traverse in order to go from one place to another. The periosteum lines them; they contain more or less cellular texture. The nerves and vessels they transmit are foreign to the bones.
6th. The cavities of nutrition, on the contrary, give passage to vessels which carry to the bones or the medullary organ the substances that repair them. They are of three sorts.
The first form canals that are seen on the long bones exclusively, and go to the medullary cavity. Each bone has but one of these, situated always on its body, directed obliquely between the fibres of the compact texture, running sometimes from below upwards, sometimes from above down into the cavity of the bone, and thus forming a communication from without to within for the vessel of the medullary organ. This foramen serves particularly for the exhalation and nutrition of this organ, and nourishes the bone only secondarily.
The second kind of cavities of nutrition belong especially to the texture of the cells of the bones. Thus they are seen wherever this texture abounds, in the extremities of the long bones, the circumference of the flat ones, and the whole superficies of the short ones. Their diameter is greater than that of the canal which goes to the medullary cavity; it is less than that of the canals of the compact texture. Their number is very considerable; I have counted a hundred and forty upon the tibial extremity of the femur, twenty upon the body of one of the dorsal vertebræ, fifty upon the os calcis, &c. In general this number is always in proportion to the quantity of the texture of the cells that the bone contains. Hence why there are but few on the flat bones of the cranium, why they are more numerous on the flat bones of the pelvis especially where this texture is abundant, as on the ischium, on the iliac portion of the circumference of the ilium, &c. By pouring mercury into the spongy texture, it runs out from all these foramina, and thus proves their communications. They are irregularly scattered wherever they are. They are not met with on the body of the long bones, because the body contains little or none of the texture of the cells.
The third kind of canals of nutrition is only destined to the compact texture. It consists of an infinite number of little pores which the eye can clearly distinguish, and through which small vessels pass, that go to this texture. An evident proof that they do not go to the texture of the cells, is, that in the preceding experiment, the mercury never finds in them a way to escape externally. It is impossible to determine their number; it is prodigious in childhood. As the bones in old age become filled with calcareous substance, they are obliterated, and the vessels they contain become small ligaments, foreign to osseous nutrition, which continually grows weaker, and is soon annihilated, and allows necrosis to seize upon the bones, if general death does not prevent this partial death of the osseous system.
The peculiar texture of the osseous system forms in it the principal and predominant part, especially as we advance in age. The common organs are in much less proportion.
The texture of the bones, like that of most of the other organs, presents itself under the aspect of fibres whose nature is everywhere the same, but which differently arranged, form two principal modifications; in the one, these fibres being more or less scattered, exhibit many cells; in the other being close to each other, they form a compact substance in which it is difficult to distinguish them. Hence two sub-divisions of the osseous texture, that with cells, and the compact. Authors admit a third one, the reticular; but this is included in the first.
The texture with cells does not exist in the first periods of ossification. The time of its formation is when the phosphate of lime is added to the gelatine of the primitive cartilage, and gives to the organ the bony nature. Then an infinite number of cells is formed in the solid mass of cartilage, because the gelatine, taken up by the absorbents, disappears in the place they occupy. No more is brought by the exhalants, which begin to carry the phosphate of lime to the fibrous cross-pieces, whose interlacing forms these cells; so that the development of the texture of the cells belongs evidently to the disproportion that takes place in the bones at a certain period of their growth, between the functions of the exhalant and absorbent system, until then in equilibrium. We know not the cause of this disproportion, it appears to be a law of ossification. It is by virtue of this law and by an analogous mechanism, that the os ethmoides, at first solid and full when it is cartilage, is hollowed out at the period of its ossification, into a great number of cells. It is thus that the sphenoidal, frontal sinuses, &c. are formed and enlarged.
The formation of the texture of the cells ceases when all the epiphyses have disappeared. At this period it exhibits to us an infinite number of fibres which appear to arise from the internal surface of the compact texture, go in different directions, cross, unite, separate, bifurcate, in a word, pursue such irregular courses, that it is impossible to follow them. Their size is not less variable; sometimes their delicacy is such, that they can hardly be touched without breaking; at others they are quite large. Often instead of fibres there are layers, of more or less considerable size, from which arise other smaller ones, which appear to ramify, and from which result, when they are near each other, species of canals, which are seen very well by sawing transversely the extremity of a long bone, so as to have a segment of half an inch.
The cells which are made by their separation, are of very unequal form and capacity.
All communicate together; the following experiments prove this. 1st. If we make a hole in the extremity of a long bone, or upon the surface of a short or flat one, and pour in mercury, it passes through all the communications, and comes out of the natural foramina on the surface of the bone, which also open into the cells. 2d. Saw a long bone at one of its extremities, cover its whole surface with something that shuts up its pores, then expose it to the sun; the medullary fluid not being able to escape by the external pores, will come out at the sawed place, after passing successively through all the cells. 3d. By varnishing a dry bone, and opening it only in two opposite points, we can force air, water and every kind of fluid through these communications, from one opening to the other.
We can then consider the interior of every bone as forming a general cavity that is filled by many interlaced fibres. I have not observed a sensible difference in the direction of these fibres in the three kinds of bones.
The fibres that form the compact texture are not the same as those of the preceding. These fibres, being in juxta-position, not leaving any space between them, giving by their approximation a remarkable density to the texture they form, have a longitudinal direction in the long bones, are in the form of rays in the flat ones, and cross each other in all directions in the short ones. This triple arrangement of the fibres of the compact texture appears to be wholly owing to the manner of ossification. In fact, when we examine its progress in the primitive cartilages, we see these organs encrusted with the phosphate of lime, in the same direction which these fibres afterwards take. Thus these fibres are very evident in the first age, on the bones of the cranium in particular. When the phosphate of lime, successively deposited on the cartilaginous parenchyma, predominates there, then the whole is confounded in the compact texture in one homogeneous mass. But still there are different circumstances that indicate the primitive direction of the fibres: 1st. When by an acid we remove from bones their calcareous part, then the cartilaginous portion keeps as a kind of mould, the form of the substances that filled it, and exhibits fibres whose direction is the same as that pointed out for the three species of bones. If we wish to separate the cartilaginous layers, it is in this direction that it is most easily done. 2d. The fissures that come in bones long exposed to the air follow in general the natural direction of the fibres. 3d. Calcined bones exhibit nearly the same phenomenon.
The direction of the fibres of the compact texture is changed entirely in the apophyses, in which it does not follow that of the principal bone. In those, which by their form, partake of the character of the long bones, as in the styloid, these fibres are longitudinal; they go in all directions in those, which like the mastoid, the different species of condyles, &c. resemble in their shape the short bones.
The assemblage of the fibres forms, according to anatomists, layers which they have considered as in juxta-position, and held together by little pins according to some, and by the interlacing of fibres according to others. These osseous layers do not appear to me to exist in nature. All the fibres of the compact texture adhere to each other, cross and form a whole that we cannot conceive of in this manner, and which besides does not accord with the irregularity of the distribution of the vessels. Art separates here fibres layer by layer, as it is done in a muscle, in a ligament, &c.; but these layers are wholly factitious; to exhibit the bones as formed by an union of these layers is to give a very inaccurate idea of their structure. It is still more inaccurate to consider these layers as attached to each other by osseous pins, by attraction, or by a glutinous matter which serves as a glue. All these ideas, contrary to anatomical examination, suggested by a false application of the laws of the adhesion of inorganic bodies to the adhesion of organized fibres, now belong only to the history of physiological errors. There is a circumstance, it is said, that very evidently proves the lamellated structure of the bones, it is their exfoliation. It is true that often very distinct layers are separated from the living bone, but these layers are only the product of exfoliation itself. Then in fact the bone dies on the surface; the superficial vessels receive no more blood; this fluid is stopped under the portion deprived of life, the exhalation of the phosphate of lime ceases there, every kind of sanguineous, exhalant and absorbent vessel is destroyed; a slow inflammation, with suppuration, comes on, and fixes the line of demarcation; and as this line is often at the same place, all which is above it becomes an inorganic layer which gradually falls off, and preserves its osseous solidity, because the dead absorbents were not able to remove the phosphate of lime. Besides, nothing is more common than to see exfoliation take place not by layers, and the bone afterwards exhibit an unequal surface, the effect of the inequality of the thickness of the exfoliated portions. Finally, exfoliation often takes place in a direction opposite to that which the layers are thought to have; this is what we see in the separation of the extremity of the long bones, that have been exposed to the air or too much irritated after amputation, in the shedding of the horns of animals, &c. Let us consider the compact texture as an assemblage of condensed fibres, not separated by layers, which we can only consider as imaginary.
The fibres of the compact texture differ in their organic arrangement, from the muscular fibres in this, that frequent elongations unite them to each other, whereas the muscular have only the cellular organ, the vessels and the nerves as the means of union. Such is the intimate juxta-position of the fibres of the compact texture, that they leave between them only pores hardly sensible to the naked eye, but which become so however with a glass, and which the medullary juice and vessels fill. In the rickets this density of texture disappears, and we observe in the middle part of the long bones and under the layer of periosteum more thick than common, an osseous texture, easily bent in all directions, forming an infinity of cells and taking the place of the compact texture that ought to be there. It appears that this change of compact texture into that of cells is made less by the absorption of a part of the phosphate of lime, than by the extension of the osseous fibres which separate from each other, and leave between them spaces that did not before exist; this gives to the bodies of long rickety bones a very considerable thickness. I have many times made this remark.
The osseous textures, considered in the different kinds of bones, are differently arranged. In general the compact forms the exterior, the covering of the bone, and that of the cells occupies the interior. The ossa spongiosa form an exception to this rule, the modifications of which we shall now examine.
1st. In the long bones, the compact texture has a very remarkable thickness in the centre, where it serves the triple purpose, first of protecting the medullary organ, of which it is the covering, then of giving solidity to the bone in this place, which more than the extremities, is exposed to great efforts in locomotion, falls, concussion, &c. and where the bone, traversed only by some very weak fibres of the cells, cannot borrow its resistance but from its external parietes; finally, of thus diminishing without danger the size of the bone in the middle part of the limb, the form of which, becomes by this means much more regular. So that as we go from the centre, we see in a long bone, sawed longitudinally, the compact texture diminish in thickness, and form at last at the extremities only a delicate layer analogous to that which covers the short bones. Thus the power of resistance of the long bones, at their extremity, is less in their compact shell, than in the great quantity of the texture of the cells deposited under it; it is this especially that prevents fractures; hence we see how the proportion of the compact texture and that of the cells being inverse in the two parts of the bone, the manner of their resistance is also inverse.
The texture of the cells differs a little when examined in the medullary canal and in the extremities. In the canal there are extremely delicate filaments, continued from larger fibres which fill above and below the extremities of the bone, and the compact portion which forms the osseous cylinder. Few and scattered at random in the middle of the canal, these filaments approximate each other, and form a kind of net-work, as they go from it; hence the name of reticular substance by which it is designated. But it is not a distinct texture, it is only a modification of that of the cells; a modification, which is especially characterized, 1st, by the delicacy of the fibres; 2d, by the uniform absence of those fine and short layers which frequently belong to this texture in other parts. Besides, the manifest use of this portion of the texture of the cells, too weak to contribute to the resistance of the bone, is evidently to serve as a support to the medullary system, and insertion for its membrane. At the extremities of the long bones, the fibres of the texture of the cells increase a little, approximate each other, are scattered in layers, and give to the bone by their union and number, a remarkable thickness and resistance, without however increasing the weight, which very much favours locomotion, considering that this weight placed at the extremity of the lever would have been very painful to raise.
2d. In the flat bones, the compact texture forms two external layers, the thickness of which is between that of the middle of the long bones, and that of the extremity of the same bones, or that of the short ones. Between these two layers is found the texture of the cells, similar in general to that of the extremity of the long bones, a little more lamellated however, thicker usually at the circumference, often almost wanting in the middle of the bone, where its two compact layers in juxta-position allow a light to be seen through it, when placed behind. In general wherever the broad bones are so thin, from the want of the texture of the cells, there are very strong muscles, which by their thick layers give solidity to the bone. We see examples of this in the iliac, sub-scapular, inferior-occipital fossæ, &c.
3d. In the short bones, the texture of the cells always predominates; the bone is almost wholly formed of it, a delicate layer of compact texture forms only its covering, and in this respect, the organization of these bones is the same as that of the long bones at their extremities; thus the resistance of the bone depends on the whole of its mass, and no part makes a greater resistance than another against fractures. We see, from all that has thus far been said, the successive manner of the solidity of the different bones. In the middle of the long bones, there is hardly any thing but compact texture to which it is owing; in the flat bones it is as much to this texture as to that of the cells; in the extremities of the long bones and in the short ones it is almost to this last only that its solidity is owing.
4th. In the osseous eminences, the compact texture is more abundant than elsewhere, especially in those of insertion, as in the prominent lines of the long bones, which are all formed of it, in the asperities of the osseous surfaces, in their angles. If the eminence is considerable, there enters into it also more or less of the texture of the cells as we see in the spinous and transverse processes of the vertebræ, in the coracoid, mastoid, &c. processes. The eminences of the moveable articulations have in general less of the compact texture, solidity is given to the bone by the articular cartilage. Those of the immoveable articulations, on the contrary, in general smaller, as the sutures of the bones of the cranium, for example, are in proportion more compact than cellular.
5th. In the osseous cavities, all those which serve for moveable articulations, are only furnished with a very delicate compact layer; it is thicker when the articulations are immoveable. In general all the foramina, cavities and canals that transmit from one region to another vessels, nerves or other organs are everywhere lined with a compact layer that defends them from the impression of these parts. The foramina at the base of the cranium, the dental canals, the vidian foramina, &c. are examples of this arrangement.
Whatever may be the modifications under which it is exhibited, the osseous texture has everywhere the same nature; the same elements form it; now these substances are especially a saline calcareous substance and a gelatinous one.
The existence of the saline substance in the bones is proved in different ways. 1st. Combustion, by destroying the gelatinous portion, leaves a friable, brittle body, of a form analogous to that of the bone and which is nothing but this saline substance, which resembles, if we may so say, a moulded body that keeps the form of the mould after it has been taken away. If the combustion is pushed very far, and a red heat produced on the calcined bones, they undergo a semi-fusion, which makes them resemble the state of porcelain; they have then a very compact, fine, semi-vitreous grain, a semi-transparency, and an appearance like that of the vitrified earths. 2d. The long continued exposure of the bones to the air produces an effect very similar to that of the first degree of combustion, though however the gelatine is rarely so thoroughly removed, and the saline portion so perfectly exposed as by the action of fire. Besides, it requires a very long time to produce this effect, especially upon the thick bones; the thin ones are more easily altered; I have often made this observation. After ten years exposure to the air and rain, I have observed that clavicles taken from the cemetery of Clamart, exhibited upon the action of the acids, a cartilaginous parenchyma almost equal to that of a bone that had been some time dried. But this parenchyma finally disappears, and the bone falls to powder, when it is no longer supported by it, and the particles of the remaining calcareous substance have been disunited by time. 3d. In the last stages of all cancerous diseases, the bones have a friability which is only owing to the greater proportion of this last substance, a proportion arising itself from the small quantity of gelatine that is then exhaled in the bones. 4th. When a bone has been for some time exposed to the action of an acid, the nitric for example, a portion of its substance is taken from it by this acid, which is evidently a calcareous salt, as can be seen by mixing it with a solution of an alkali, which uniting immediately to the acid, exposes this salt, by making it precipitate. 5th. Papins digester, by dissolving by the action of water reduced to vapour the gelatinous portion, shows also this saline calcareous part.
Scheele has found that this portion is a neutral salt with an earthy base, the phosphate of lime. Frequently the phosphorus in fresh bones give them a luminous appearance, that can be seen very far in the night. It is sometimes the whole of the bone, sometimes some parts only that become luminous. I have always observed in the illuminated places an oily exudation, either that comes from the medullary juice, or is furnished by the fat of the neighbouring soft parts of the bone.
Different facts as evident as the preceding, prove in a manner not less certain, the existence of a gelatinous substance in the bones. 1st. When in the solution of the bones in the acids, the phosphate of lime has left them, there remains a cartilaginous, flexible, elastic body, yellowish when nitric acid is employed, of the same form as the bone. Now we know that the gelatine especially nourishes the cartilages. 2d. If besides we subject this cartilaginous residue to ebullition, we extract a very great quantity of gelatine which is dissolved in the water and can be afterwards precipitated by tannin. This substance can even be removed from the bones without the previous extraction of the phosphate of lime; it is thus that with bones stripped of every surrounding organ, and reduced to very small fragments or even to powder by the action of a rasp, very nourishing broths and jellies are made. It is not without reason that in the preparation of boiled meat, the bone is left attached to the meat; besides the white organs that surround it, and the medullary oil that it contains, it furnishes to the broth a substance that is peculiar to it. 3d. The combustion of the bones, and especially of their cartilaginous residue, gives an odour exactly similar to that of the combustion of the different animal glues, which, as we know, the gelatine especially forms. 4th. In the different affections in which the bones become soft, the earthy substance is diminished more or less sensibly, and the gelatinous remains more abundant in proportion than common.
These two substances, the gelatinous and saline, which enter essentially into the composition of the bones, imprint upon them very different characters. The phosphate of lime, almost foreign to vitality, is only destined to give to the bones the solidity and resistance that characterize them. The gelatinous substance, on the contrary, has especially the animal character; thus the vital activity is in the inverse ratio of one and the direct ratio of the other, as we shall see. Deprived of gelatine, the bones are not capable of being digested, they offer nothing for the gastric juices to act upon, they cannot extract nutritive matter from them, because they act upon them nearly as water does, which dissolves the gelatinous substance and extracts it from the saline portion. Different animals that swallow fresh bones for nourishment, would die from eating a calcined one; thus the more the bones contain of this substance, the more nourishing they are; those of young animals are on this account more proper to make gelatinous broths, more suitable to be digested raw by the stomachs of certain species, &c. If we expose a bone to the action of an acid, so as to have only its cartilaginous parenchyma left, and afterwards soften this parenchyma in boiling water, it becomes an aliment that can be eaten.
Besides phosphate of lime and gelatine, the bones contain also some saline principles, as the sulphate and carbonate of soda, &c. But this proportion is too small to be noticed. Upon this point, I refer to chemical books, especially to the great work of Fourcroy.
The ancients ranked the bones among the white parts, among the tendons, the cartilages, &c. It is sufficient however to examine the interior of them to see, by the redness that distinguishes them, that much blood enters them. This blood penetrates in three orders of vessels; one belonging to the medullary cavity of the long bones, another to the texture of the cells, and the other to the compact texture. These two last orders distributed in the osseous texture, appear to be especially destined to deposit the phosphate of lime; for in the cartilages of ossification, the white vessels alone carry the gelatine; in other cartilages it is the same; so that I think that this kind of vessels is also destined in the bones which are perfectly formed to nourish their cartilaginous parenchyma, whilst the red vessels belong more to their calcareous portion.
Each medullary cavity has only one vessel, and only one foramen of nutrition. This vessel has a diameter proportioned to that of the bone which it penetrates, and in which it is divided immediately into two branches, without permitting any ramification on the compact texture. These go in an opposite direction to the two extremities of the bone, ramify ad infinitum in the medullary organ, and their last branches are lost in the commencement of the texture of the cells, where they anastomose with the vessels of this texture; that which occupies the medullary cavity under the name of reticular, and the internal surface of the compact texture, receive also some branches. A vein everywhere accompanies the artery, and follows the different distributions of it.
The vessels of the second order belong to the texture of the cells of the long, flat and short bones; they are equal in number to the foramina of this texture, and ramify on its cells; they communicate with those of the marrow and of the compact texture. At death, the small arteries in general remain full of red blood, which indicates their course which their minuteness would conceal, and which injections can rarely demonstrate with accuracy. The accompanying veins of these arteries can hardly be seen.
The blood-vessels of the third order are only the last ramifications of the arteries surrounding the bones, ramifications which enter in great number the compact texture, and stop there. The existence of these vessels may be proved in different ways. 1st. By detaching the dura mater from the internal surface of the cranium, many small sanguineous drops prove their rupture. 2d. By raising on a subject of a middle age the periosteum, we make the same observation. I have remarked that these experiments succeed especially on those that have been drowned, or on animals destroyed by asphyxia, on account of the great quantity of blood their vessels contain. 3d. If we fracture a long bone in the middle, the compact portion, which forms the medullary canal, exhibits small reddish striæ, which are nothing but these small vessels still full of blood, and of which we thus discover a greater or less number, according to the manner in which the blood was arrested in the capillary system at the instant of death. 4th. The saw-dust of the compact texture in living animals is red, though less evidently so than that of the texture of the cells; a proof that these vessels have been divided.
The vessels of the bones are very numerous in childhood; they diminish in the adult, and become scarce in old age. The facility of the formation of callus follows the same proportion in the different ages of life. Often in affections of the osseous parenchyma they have a remarkable development, which much exceeds their natural diameter. Osteo-sarcoma, spina-ventosa, &c. exhibit this, which is much oftener observed in cancerous tumours than any other.
These vessels communicate with each other by numerous anastomoses; this is what we see especially in the long bones, between those of the medullary organ and those of the texture of the cells. By these communications, they mutually assist each other's functions. I have seen the nourishing foramen of the tibia completely obliterated in a body that I injected. A sort of cartilage filled this foramen; the artery formed a real ligament. Yet its bifurcation in the medullary canal was found very well injected, and besides no alteration appeared in the nutrition of the medullary organ, which had probably received as much blood as usual. I found nothing in the neighbourhood of the foramen, which showed the cause of this obliteration, which an exostosis, an affection of the periosteum, or an inflammation can very easily produce.
On the other hand we know that very considerable osseous layers are often taken from the extremity of the long bones by caries, which consequently destroys all the vessels corresponding to these layers, and yet the bone beneath lives, principally by the blood that it receives by the extremities of the artery of the medullary organ. This is nearly what happens to the long bones in the first age, in which the cartilaginous extremities have not vessels of the second order, and in which consequently almost all the blood comes from this same artery of the medullary organ; thus it is much larger in proportion, and the foramen which receives it much more considerable.
Nothing is yet known upon the systems of absorbent and exhalant vessels of the bones, and we can reason upon this point only from analogy. Besides the nutritive process evidently supposes them there.
As to their cellular texture, it appears to be almost nothing; we can even say that in whatever place we break the compact fibres or those of the cells, its filaments are not distinct; but it is their dense and compact texture that conceals them from us. In fact, 1st, when this texture is softened, and the bone has become flesh, as it is called, the cellular texture is very apparent there. 2d. The fleshy granulations, rising on places that have been fractured or laid bare, are only the extension of the cellular texture which has too much calcareous substance to allow it to be seen in the natural state. 3d. After having removed from a fresh bone all this substance by an acid, I have sometimes observed cellular filaments by separating the cartilaginous fibres which form the parenchyma that is left. 4th. When we boil this cartilaginous parenchyma in order to extract the gelatine from it, there remains portions of membranes which are evidently cellular.
We cannot trace the nerves in the bones, the filaments that enter them are so fine; I do not know that anatomy has any positive data upon this point.
The bones have very strongly marked physical properties. Solidity and hardness are their peculiar portion; they derive these properties from the phosphate of lime which penetrates them, thus they are constantly increasing with age, because this substance becomes more and more predominant. Elasticity is another physical property of the bones, which is found combined with the two preceding, but which is in an inverse order; as it is in the gelatinous substance, in the cartilaginous portion of the bone that it resides, it is, like this portion, greater in childhood. In old age, the bones lose entirely their suppleness and elasticity; they break more easily. Elasticity is more evident in the long and small bones, than in those which are larger; the fibula bends and evidently goes back again, this the tibia could not do without difficulty. It is not that the one is more elastic than the other, but it is that its conformation is more favourable to the development of this property.
Although the hardness and solidity of the osseous texture seem to be opposed to every kind of extension and contraction, yet these two phenomena and the properties of texture from which they arise, are often very evident in it.
The extensibility of the osseous fibres is proved by the observation of many diseases, for example, the spina-ventosa, the swelling of the maxillary sinus when it contains a polypus, by the enlargement of the bones of the cranium in hydrocephalus, &c. I would remark on the subject of these different distensions, that often by the influence of analogous causes, the bones which yield and are distended in the above cases, are broken, worn and destroyed in others. A polypus of the nose breaks through the naso-palatine partition, without having first distended it; aneurism of the aorta does not bend the sternum or the vertebræ, but it breaks through and destroys these bones. Whence arises this difference from causes nearly the same? This is not easily determined. The contractility of texture is very evident in the bones, when the cause which distended the fibres is removed. We see the alveoli contract, and become effaced, when a tooth has been drawn from them. The diminution of the thickness of the jaw after cutting the teeth arises only from the contraction of its fibres, which are no longer distended as much, because the root is not so broad as the crown, which had till then been wholly in the bone. The maxillary sinus contracts when a fungus is removed from it, or pus is discharged from the carious bone, &c. If death was not too soon the consequence of the puncture of the head of hydrocephalic patients, I am persuaded that we should see the bones gradually contract, and restore the cavity of the cranium to its natural dimensions. When we remove the dead piece from a long bone in necrosis, the new bone, formed on the exterior by means of the periosteum contracts in an evident manner. In paralysis of the optic nerve, its foramen becomes narrower. The orbit contracts when a cancerous eye has been extirpated. I have dissected the carotid canal in a dog whose carotid I had tied; there was no contraction because the blood coming by anastomoses dilated the artery to the usual size.
This contraction of the bones, by means of the contractility of texture, is not so sudden as that of the muscles, the skin, &c. when they are no longer distended by a tumour, an aqueous collection, &c. This arises from the difference of the organic texture, from the rigidity of osseous fibres owing to the calcareous substance they contain, &c. Thus the organic sensibility is less evident in them.
The bones have hardly any animal properties in a natural state. Their sensibility is nothing; the saw, the mallet, and the chisel act upon their texture almost with impunity; an obscure feeling is the only result of the action of these instruments; fire even can act upon them without making the animal suffer much. But in a morbid state, the sensibility is developed to the greatest extent; we know the horrible pains that attend spina-ventosa, and those not less severe that caries produces in certain cases. If a bone is inflamed, as for example the sawed extremity of a stump after amputation, this bone which in a natural state had borne, without transmitting any painful impression, the action of the saw, becomes as it were a new sensitive organ, to which the least touch is painful. The animal contractility is nothing in the osseous system.
The organic properties give life to this system as to all the others. The sensibility of this kind certainly exists in it; the fluids that penetrate it are felt, and by virtue of this feeling, those are appropriated to it which are proper for its nutrition. But is there in the osseous system a reaction upon these fluids? are there those insensible oscillations which compose insensible organic contractility? Its hardness seems to prevent them. But yet the circulation is carried on there; it performs a constant work, an habitual composition and decomposition, which can hardly be conceived of without reaction on the part of the osseous system. Besides this reaction is more slow, more difficult on account of its structure; and hence without doubt the slowness, of which we shall speak, in the vital phenomena of the osseous system. Sensible organic contractility is foreign to it.
The peculiar life of the bones is composed then of only two vital properties, organic sensibility and insensible organic contractility. From these two properties are derived all the vital phenomena that these organs exhibit, inflammations, formation of tumours, cicatrization of solutions of continuity, &c. This peculiar life is remarkable in general, as I have just observed, when compared with the peculiar lives of the other organs, by its slowness, by the tardy concatenation of its phenomena. All things being equal as to ages, and the different proportions of the earthy and cartilaginous substances, inflammation is more slow there than in the other parts. Callus is remarkable among the other cicatrices by the length of its formation; compare an exostosis in its origin, its progress and its development, with a tumour of the soft parts, a phlegmon for example, and you will see the difference. Who does not know, that whilst suppuration often requires only a few days in the other organs, it is whole months in forming in the middle of the bones? Observe the difference that there is between a gangrene of the soft parts, in which death takes place in a short time, with caries and necrosis of the bones, in which a long period elapses between disease and death of the part. In general we can say, when inflammation exists in a bone, that it is chronic.
This character of the vital properties imprints an analogous one upon the sympathetic relations of the osseous system with the other systems. At first the animal contractility and the sensible organic contractility cannot be put in action in these relations, as they do not exist in the bones. The animal sensibility being developed in them with difficulty and slowly by the diseases that essentially affect them, the sympathies can be brought into action in them only in an obscure manner. These sympathies then should act essentially upon the organic sensibility and upon the insensible organic contractility, and as these two properties are developed slowly, the different sympathies should not be connected with the acute affections of the other organs, and this is what is clearly proved by observation. In fact observe that whilst many other systems respond with great quickness to the acute diseases of an organ, this, as well as the cartilaginous, fibro-cartilaginous systems, &c. remain then almost always in inaction. Let the stomach, the lungs, the brain, &c. be the seat of a severe acute disease, you see immediately many sympathetic phenomena arise in the nervous, vascular, muscular, glandular, cutaneous, mucous systems, &c. &c.; all seem to feel the trouble of the affected organ; each, according to the vital forces that predominate there, exhibit different phenomena, which are only aberrations, irregular developments of these forces; in the animal muscular system, it is the animal contractility which is especially raised; hence spasms and convulsions; in the glandular, the serous, the cutaneous, the mucous, &c. the insensible organic contractility and the organic sensibility principally experience alterations; hence the different sympathetic derangements of the secretions, of the sweat, of the exhalations; in the nervous, it is the animal sensibility which is especially brought sympathetically into action; hence the wandering or fixed pains in different parts; in the organic muscular, it is the organic contractility which is raised; hence the irregular motions of the heart, the stomach and the intestines. In all the acute diseases of an organ there are always two orders of symptoms, the one relative to the affected organ, as are the cough, the pain in the side, the spitting of blood, the difficulty of respiration, &c. in peripneumonia; the others purely sympathetic and arising from the relations which connect the vitality of this organ with that of all the others; now these last are often much more numerous than the others.
Observe the bones in the midst of all that general sympathetic derangement of the systems in which life is very active; they undergo no alteration; their life, more slow than that of the other systems, is not connected with these phenomena which have an acute character; neither is that of the cartilages, the fibro-cartilages, the hair, the aponeuroses, &c. All these systems, remarkable by the same character of vitality, do not respond to the acute affections of the other systems; they are not sympathetically affected, during these affections, at least in an evident manner. Observe all the acute fevers; their numerous phenomena have an effect only upon those systems in which life is very active; those in which it is distinguished by an opposite character, have uniformly no connexion with these phenomena; they are, if we may so say, calm and tranquil in the midst of the tempest which agitates the others. Let us take for example the different eruptions that appear in fevers; it is upon the skin, the mucous surfaces, &c. that they come; they arise during the fever and they disappear with it; now the bones, the cartilages, &c. could not, from their kind of life, admit of this sudden origin and disappearance.
It is then in the slow and chronic affections that we must seek for examples of sympathies of the osseous, cartilaginous systems, &c. In the first stages of the venereal disease, in which it is marked only by acute symptoms, or in which at least its progress is not very slow, as when it appears in buboes, in inflammations of the urethra, &c. it has no influence upon the osseous system; it is only when it is of long standing, when it has, as it were, degenerated, and become chronic, that it makes the bones the seat of pains, of different tumours, &c. Besides, I do not know, that we have yet thoroughly analyzed the osseous sympathies. I have shown only their general character. We shall understand them better, when we have given more attention to the relation that there is in diseases between the affection of each organ, and its kind of vitality.
The bones penetrated by saline substances which tend continually to obey the laws of affinity and attraction, and to make these laws predominate over those of sensibility and organic mobility, seem to hold a middle place in living bodies, between these bodies and inanimate ones. There is truly but one part of their osseous texture which partakes of the vital phenomena, viz. their cartilaginous substance; the other part or calcareous substance is foreign to them; thus the proportion of each of these substances determines in the bones their degree of life. In infancy, in which the first predominates, in the early stages of the formation of callus, in which it is exclusively found, in the softening of the bones in which it remains almost alone, all the vital phenomena become more evident and more powerful. On the contrary, as age accumulates in the bones the saline substance, as in certain animals this accumulation takes place by the natural laws of ossification in some external portions of the system with calcareous base, as in the horns of stags, the shells of crustaceous animals, &c. so life is, if we may so say, successively destroyed in the bones; it becomes nothing, when this calcareous portion predominates considerably; this is what happens in the necrosis which produces the fall of the horns, the casting of the shells of crustaceous animals, &c.
Besides, that which shows the vital energy in an organ, is the rapidity with which inflammation goes through its periods in it, and the frequency of this affection, &c. Now in the bones inflammation is so much the more rapid, in proportion to the greater quantity of cartilaginous texture they contain; observe the periods of the formation of the callus in the different ages, periods which are determined by the duration of the inflammation necessary for its formation, you will see that in infancy they are short, that they are much longer in old age, and that often even consolidation does not take place, whilst it is effected with facility in all the soft parts. The general weakness no doubt of all the vital forces which takes place from the effect of age is one cause of this slowness and this rapidity in the formation of the callus at the two extreme periods of life; but the different proportions of gelatinous and calcareous substances contribute much to it also; for when we compare other cicatrices with this, the cutaneous, for example, we see that age establishes in them an infinitely less sensible difference as it respects the slowness and rapidity of this reunion, than in the osseous system. The bones have not sufficient life to inflame and unite, like the skin, the muscles also exhibit this phenomenon in a very evident manner. I have seen an old man, the neck of whose fractured thigh-bone remained a long time without reunion, and in whom a wound of the face was healed very speedily by the first intention.
Finally, there is a simple experiment that I have often made, and which proves as well as the preceding facts, that the cartilage is truly the animal part of the bone. We know that one of the great attributes of animal substances, is to contract and exhibit the horny hardening when burnt; now when the bone is penetrated with its earthy salt, it has not this kind of combustion; deprived of this salt by an acid, the cartilaginous parenchyma which remains burns in this manner. The flat bones in infancy in which this parenchyma predominates, exhibits also this phenomenon in burning; it forces the calcareous portion, which is in small quantity to obey the impulse that it gives it, and turns it in various directions; but in the adult in whom the calcareous portion is the largest, the bone remains unmoved while the fire penetrates it, and its whole cartilage is taken away, without its fibres being able to obey their tendency to the horny hardening which combustion imprints upon them.
All the bones are united together, and thus form the skeleton. The manner of their union varies, but whatever it may be, it is known under the general name of articulation.
All the articulations can be referred to two general classes. Mobility is the character of the first, immobility that of the second.
One belongs to all the bones which serve for locomotion, to some of those destined to internal functions, as the ribs, the lower jaw, &c. The other is especially met with in the bones, the union of which forms the cavities designed to defend the organs; this we see in the head, the pelvis, &c.
I divide moveable articulations into four kinds, the characters of which are borrowed from the different motions they execute. To understand this division, it is necessary previously to know the motions of the articulations in general. These motions can be included under four species, which are, 1st, opposition; 2d, circumduction; 3d, rotation; 4th, sliding.
1st. The motion of opposition is that which is made in two opposite directions, for example, from flexion to extension, from adduction to abduction, and vice versa. This motion is extensive or limited; extensive when it is made in all directions, first in the four named above, then in all the intermediate ones; limited, when it only takes place from flexion to extension, from adduction to abduction, &c. The thigh in its articulation with the pelvis enjoys an extensive motion of opposition. The tibia in its articulation with the femur has a limited motion of opposition.
2d. Circumduction is the motion in which the bone describes a kind of cone, the summit of which is in its superior articulation, and the base in the inferior; so that it is found successively in flexion, adduction, extension and abduction, or in abduction, extension, adduction and flexion, according to the motion by which it begins, and that moreover it goes through all the intermediate directions. Hence we see that circumduction is a motion composed of all those of opposition, and in which the bone, instead of moving from one direction to an opposite one, as in the preceding case, moves from one direction to another nearest to it, describing thus by its extremity a circle which is the base of a cone of which I have spoken, and which is so much the greater as the bone is so much the longer. We easily understand that among the bones, those only whose motion of opposition is loose, enjoy that of circumduction.
3d. Rotation is wholly different from the preceding motion. In this there was locomotion, a moving of the bone from one place to another; here it remains always in the same place; it only turns upon its axis. The humerus and the femur enjoy this motion which is simple.
4th. Sliding belongs to all the articulations. It is an obscure motion, by which two surfaces go in an opposite direction, by sliding as it were upon each other. In all the other motions, this is met with; but it often exists without them.
It is easy to understand, from these views upon the articular motions, the division into genera of the class of moveable articulations. In fact, there are articulations in which all the motions are united; in others, there is no rotation; in many rotation and circumduction are wanting, and opposition exists only in one direction; some have only rotation. Finally, there are those in which rotation, circumduction and opposition are nothing, and sliding alone remains.
Hence we see that nature moves here as elsewhere by gradation, that from the most moveable articulations to those that are the least so, there are different degrees of decrease, that she descends gradually to the immoveable articulations, that she is finally reduced to the motion of sliding alone, like that which exists at the carpus, the tarsus, &c. There is even an intermediate one between sliding and immobility; it is the articulation of the symphysis pubis; which can be considered with that of the humerus as forming the two extremes of the series of moveable articulations.
All the articulations of which I have spoken are with contiguous surfaces; this is the general character of those which are moveable. There is however an exception to this rule; it is the articulation of the body of the vertebræ, in which there is continuity and mobility. The symphysis pubis is also in part continuous in its surfaces, and yet has sometimes obscure motions. Hence arises a division of the moveable articulations, into those with continuous surfaces, and into those with contiguous ones.