THE SKELETON, ARTICULATIONS, PROPORTIONS.

CHAPTER III.
OSTEOLOGY AND ARTHROLOGY IN GENERAL—NOMENCLATURE—VERTEBRAL COLUMN.

It is not necessary to emphasise further the importance of a study of the skeleton. By its means we obtain a knowledge of form and proportions; by a study of the several articulations we become acquainted with the complex mechanism by which the whole is knit together, and by which the movements of the various parts of the body occur. Further, the relations of the skeleton to the surface forms of different parts of the body are of fundamental importance. The science of Osteology is the study of bones (ὀστέον, bone; λόγος, description); Arthrology is the study of joints (ἅρθρον, a joint): Myology is the study of muscles (μυς; λογος). The bones are the levers of movement: the articulations represent the fixed points or fulcra of these levers; while the powers which produce motion are represented by the muscles.

Before describing in detail the different parts of the skeleton, it is necessary to consider the method of nomenclature, so that by the employment of proper terms the subsequent descriptions may be more intelligible.

Nomenclature.—In the description of the bones, as of other organs, we have to consider the relation of the portion under consideration to the rest of the body. The figure is always regarded as occupying the erect position, with the face, the palms of the hands, and the toes directed forward. Thus each bone, as well as the other organs or parts, will be found under one or other of two different conditions: either it is median in position, and a vertical plane passing through the longitudinal axis of the body divides it into two similar segments; or else it is lateral in position, and situated outside this median plane. As a type of the first class, we will take the sternum, or breast-bone (see Fig. 11, p. 42). This is a central single bone; it has no fellow, and is composed of two symmetrical portions, one part on the right and one part on the left. As a type of the second class, we will take the humerus (Fig. 18, p. 60), which is a bone situated at the side and one of a pair, inasmuch as there are two, one on the right and one on the left of the median plane. From these two examples it is easy to understand that for the description of each single and symmetrical bone it will be necessary to speak of anterior parts or surfaces directed towards the front of the body, of posterior parts (directed backwards), of lateral portions (right and left), and finally, of parts superior and inferior, looking upwards and downwards (in the case of the sternum a superior and inferior extremity): on the other hand, in the description of a paired and non-symmetrical bone, we shall also have to speak as heretofore of parts superior and inferior, anterior and posterior; but instead of two similar symmetrical portions, one on each side of an imaginary line, it has two dissimilar halves, of which the one looking towards the median plane—towards the axis of the body—is called the internal part, and the other, looking to the outer side (as away from the axis), is called the external part. It is necessary, for brevity and accuracy, to clearly comprehend the meaning of these terms in descriptive anatomy (anterior and posterior, internal and external, superior and inferior) which serve to show the relation of the parts to the skeleton as a whole.

After this first division of bones into single and median, and into double and lateral, if we glance at the skeleton (Figs. 2, 3), it seems at first sight that the various bones present an infinite variety of shape, and defy classification or nomenclature; careful attention, however, will show us that they may be all included in one of the following three classes—viz., long bones, flat or broad bones, and short bones.

The long bones, which usually act as the axes of the limbs (e.g., the humerus, femur, tibia, &c.), are composed of a central portion, cylindrical or prismatic in shape, called the body, shaft, or diaphysis (διαφύω, to be between), and of two extremities or epiphyses (ἐπιφύω, to be at the end), usually marked by protuberances and articular surfaces. The flat bones (e.g., the shoulder-blade and the hip bone) are formed of osseous plates, on which are various surfaces, borders, and angles. Finally, the small bones, which are found in the vertebral column and in the extremities of the limbs, the hand and foot (carpus and tarsus), present a diversity of form in which cylindrical, cubical, and wedge-like shapes can be made out.

Whether the bone be long, flat, or short, it presents prominences and depressions. The projecting portions of bone are called by various names—tuberosities, protuberances, processes, apophyses, crests, spines, tubercles. To some of these names is added an adjective, which shows, more or less exactly, the form of the process or projection. Thus we speak of a spinous process, mastoid process (μαστὸς, a nipple; εἶδος, form), styloid process, &c. The depressions upon the bones are called by various names—fossa, groove, foramen, sinus, canal, notch, cavity, &c. To these also are added names which indicate their shape, as the digital fossa, from its resemblance to the imprint of a finger; the glenoid cavity (γλήνη, cavity), the cotyloid cavity (χοτύλη, a basin); but more frequently still, the added adjective bears allusion to a connection of the cavity with certain organs, as the bicipital groove, that which contains the tendon of the biceps, or the canine fossa, in relation to the root of the canine tooth.

Structure of Bone.—Bone is characterised by its density, toughness and elasticity. If a long bone, such as the femur, is sawn in two lengthwise, its extremities are found to be composed of a delicate network of cancellous, or spongy bone, in the interstices of which marrow and blood are contained during life; the shaft of the bone is composed, for the most part, of a cylindrical tube of dense, ivory-like compact bone, which encloses the hollow medullary canal of the bone, also filled with marrow during life. The dense bone of the shaft is continuous with a thin sheet of hard bone, which covers over the spongy bone of the extremities.

In the case of the flat and short bones, the structure is like that of the extremities of the long bones. The mass of the bone is composed of cancellous tissue, with a surrounding thinner envelope of compact bone.

If a bone is burnt, it loses one-third in weight, becomes brittle, and loses its organic constituents, retaining its inorganic materials—chiefly calcium phosphate and calcium carbonate. If it is subjected to prolonged soaking in an acid such as hydrochloric acid, its inorganic salts are removed, it becomes soft and pliable, it loses two-thirds of its weight and retains only its organic materials—connective tissues. These in boiling produce glue.

In certain situations bone is found in conjunction with a substance which differs from it in its elasticity, its want of rigidity (it is soft enough to be divided by the scalpel), and in its translucent colour. This substance is known by the name of cartilage. Thus the curved bones termed ribs are prolonged at their anterior extremities by a portion called the costal cartilage, which presents the same form as the ribs properly so called. The bones forming the freely movable joints (like the shoulder, hip and knee joints) are capped by thin layers of hyaline articular cartilage, which forms a pliant elastic cushion in relation to the articulation.

Most of the bones, at the commencement of their formation, are constructed solely of cartilage, which is gradually transformed into bone as the animal grows by the deposition in it of lime salts; and this transformation of primitive cartilage into bone may be more or less complete according to the species or age of the animal. With advancing age the bones tend to become more and more calcified. Thus we find that in the skeletons of old people the costal and other cartilages may be more or less ossified.

The Subdivisions of the Skeleton.—The human skeleton is characterised by peculiarities due to the assumption of the erect position, the high development of the brain, and the possession of extraordinary manual dexterity. All these factors leave their impress on the bones of the skeleton, as may be seen by comparing the human skeleton with that of such a quadruped as the dog.

The skeleton is subdivided into axial and appendicular parts. The axial skeleton includes the vertebral column, ribs and sternum, and the bones of the cranium and face. The appendicular skeleton comprises the bones of the limbs. In the following pages, for convenience of description, an account will be given of the vertebral column, sternum, and ribs first; of the limbs second; reserving to the last the account of the skeleton of the cranium and face.

The Vertebral Column.—The vertebral column (Figs. 5, 8) is composed of a number of bones named vertebræ, superimposed on one another, and partially separated from one another by a series of intervertebral discs. The column is subdivided into groups of vertebræ, by reason of its connections with other parts of the axial skeleton, or with the skeleton of the limbs.

The head is poised on the upper end of the column, and causes the peculiarities, to be described later, in the first two vertebræ (atlas and axis). The attachment of the ribs to the sides of the vertebral column causes the separation of three regions: (1) cervical, belonging to the neck, and comprising seven vertebræ; (2) thoracic (or dorsal), belonging to the thorax, or chest, and comprising twelve vertebræ; and (3) lumbar, belonging to the loin, and comprising five vertebræ. The attachment of the hip bones to the sides of the succeeding vertebræ leads to the fusion of the next five vertebræ together, under the name of the os sacrum, which will be described along with the hip bone and pelvis. Finally, below the sacrum are four small, rudimentary vertebræ, known as the coccyx, forming the attenuated remains of a caudal appendage.

There are thus, altogether, normally thirty-three vertebræ: seven cervical, twelve thoracic, five lumbar (constituting together twenty-four movable vertebræ); five sacral, and four coccygeal vertebræ (constituting nine fixed vertebræ, which help to form the pelvic basin).

The vertebral column is intended not only to form an axis for the rest of the skeleton, but also to serve as an attachment, direct or indirect, for all the other bony structures; it also forms a bony canal, within which the spinal marrow is contained. It is for this reason that each of the pieces which compose it, called a vertebra, is a sort of bony ring (Fig. 6). The anterior portion of the ring is very thick, representing the segment of a cylinder, and is called the body of the vertebra (2, Fig. 6); and the vertebral column, considered as the median column of support, is essentially constituted by the superposition of these vertebral bodies upon one another, separated by the intervertebral discs. Behind each vertebral body is an arch, the neural arch, which encloses the neural ring. The spinal or neural canal is formed by the combination and connection together of the neural rings. Each neural arch is comparatively slender, but it gives origin to certain projections or processes, three in number, on each side, of which one directed transversely outwards is called the transverse process (3, Fig. 6). In the thoracic region these give partial attachment to the ribs. The other two—directed more or less vertically, one above, the other below—are called the articular processes, superior and inferior. These serve for uniting together the arches of adjoining vertebræ (5, 5, Fig. 6). Finally, the posterior portion of the neural arch is prolonged backwards as a protuberance, more or less pointed, called the spinous process (1, Fig. 6).

Such are the most important parts which we find in each vertebra, but they present particular characters according to the region to which each vertebra belongs. The description of the sacrum and the coccyx, which are formed of vertebræ welded together, and articulating with the hip bones, will be given with that of the pelvis.

The more important features of the movable vertebræ which contribute to give to the whole column its general form are: (1) the size, particularly of the bodies, of the vertebræ; and (2) the characters of the transverse processes. The bodies of the vertebræ are smallest in the upper thoracic region, and increase in size upwards and downwards from the fourth thoracic vertebra. The bodies are largest and most prominent in the loin; in the neck the vertebræ are broad in the transverse diameter, but their antero-posterior diameters are less. The vertebral column is weakest in the upper thoracic and upper lumbar regions, and most mobile in the neck and thorax. Rotary power in the loin is practically prevented by the shape of the lumbar articular processes, which interlock the vertebral arches in this region.

The spinous processes of the vertebræ, which project more or less obviously in the middle line beneath the skin in different regions, in the cervical region are short and bifid; in the thoracic region they are long, sloped downwards, and “bayonet-shaped”; in the lumbar region they are directed straight backwards, and are “hatchet-shaped.”

Besides these general characters in each region there are certain vertebræ which demand special mention owing to the peculiarities of their shape. These are the first two and the last cervical.

The first cervical vertebra (Fig. 7, A), called the Atlas, because as directly supporting the head, it has been compared to the giant Atlas, supposed by the ancients to support the heavens, is a bony ring with only transverse processes, and on the upper and lower aspects of its lateral portion, two pairs of articular surfaces; the superior articular surfaces are hollow oval surfaces which articulate with the convex condyles of the occipital bone; and by the occipito-atlantoid joints provide for flexion and extension of the head on the spinal column. The inferior articular surfaces are flat and directed downwards to articulate with the axis and form the atlanto-axial joint, which is responsible for the movement from side to side of the head upon the trunk. The axis, or second vertebra (Fig. 7, B C), is so called from the presence on the upper surface of its body of a tooth-like process, the odontoid process (ὀδοὺς, tooth; ἔιδος, form), which projects upwards in an osseofibrous ring formed by a transverse ligament in the anterior part of the ring of the atlas. Ligaments extend from this process to the occipital bone, and it forms a pivot round which the head and the atlas move in the lateral movements of the head upon the spinal column.

In nodding the head the movement occurs primarily at the occipito-atlantoid joint; in shaking the head, the chief movement is between the atlas and axis. These functions, of no moment in the production of surface forms, are of too great an importance in respect of the articulations of the head and trunk to be omitted here.

The seventh cervical vertebra, or vertebra prominens, is so called because of the extraordinary length of its spinous process, which, except in very stout people, forms a projection easily visible beneath the skin; and this projection is also more conspicuous as it corresponds to that part of the neck where the trapezius muscle, represented only by a fibrous layer—not fleshy—forms a flat surface at the back of the neck. In the centre of this surface the projection of the seventh cervical spine appears on the level of a transverse line passing through the superior border of the shoulder (see Fig. 3). It may be observed that when the model bends the head forward the spinous process of the seventh cervical becomes very prominent. It should also be noted that in the majority of cases the spinous processes of the sixth cervical and first thoracic vertebræ also give rise to superficial projections above and below that produced by the vertebra prominens.

We have been disconnecting the vertebræ in order to account for the construction of the vertebral column; we must next see how the different vertebræ are placed one upon the other—how they articulate in such a manner as to form a column, not rigid, but elastic and curved. The vertebræ are placed one on each other so that the inferior articular processes of one fit exactly on to the superior articular processes of the next beneath, and thus throughout the series we see (Fig. 8) that the bodies of the vertebræ are not in contact one with the other, the space which separates them being filled in the living subject by elastic fibrous discs. These intervertebral discs are very thick in the lumbar region, and become thinner in proportion as we ascend to the superior dorsal and cervical regions. They are thicker in the cervical and lumbar regions than in the thorax; and taken together they form one-seventh of the length of the spinal column. Being compressible and elastic, these fibrous discs give to the column, formed by the placing one on another of the bodies of the vertebræ, a certain degree of flexibility, whereas a column formed of bone alone would have been quite rigid.

In addition to the intervertebral discs, a series of ligaments which join together the posterior portions of the neural arches (laminæ) is of great importance. Composed of yellow elastic tissue to a large extent, they are known as the ligamenta subflava. They consist of two short bands placed on each side of the root of the spinous process, uniting the inferior border of the lamina of one vertebra with the superior border of the lamina situated next below it.

The yellow or elastic tissue which composes these ligaments is similar to a piece of india-rubber; it is elastic—that is to say, it is able to stretch, and to return again by its own reaction to its original size when the cause which extended it has ceased to act: so that each movement of flexion of the column in front results in moving the vertebræ on one another, at the same time stretching these elastic ligaments. When the anterior muscles of the trunk which accomplish this flexion cease to contract, it is not necessary, in order to straighten the column, that the posterior muscles of the back should come into play; the elasticity of the ligamenta subflava suffices for this, as they return to their original dimensions and draw together the vertebral laminæ. We may say, then, that there is at the posterior portion of the column within each vertebra a pair of small springs which keeps the column erect, so that the erect attitude of the trunk is maintained simply by the presence of the elastic ligaments; although more is required when a man supports upon his back any extra weight or burden.

The Ligamentum Nuchæ (paxwax) is a large and powerful ligament composed of yellow elastic tissue. It is highly developed in quadrupeds, and is attached between the spinous processes of the cervical vertebræ and the occipital crest, a vertical ridge on the back of the skull. In man it is a rudimentary structure (as the head is poised on top of the vertebral column) and forms a membranous partition separating and giving partial attachment to the muscles of either side at the back of the neck.

Curves of the Vertebral Column.—The vertebral column is subject to a slight lateral curvature, generally towards the right side. Its chief curves, however, are antero-posterior, and are four in number (Fig. 8): two, the thoracic and sacral curves, concave forwards, are primitive embryonic curves; two, cervical and lumbar, convex forwards, are secondary in their origin. The convexity forwards of the cervical region is to be connected with the raising upwards of the head on the trunk; the convex lumbar curve is due to the straightening of the lower limb, which in the course of development is brought into line with the vertebral axis.

These curves (except the pelvic or sacral curve) are to be associated with a difference in the thickness in front and behind of the vertebral bodies, and of the intervertebral discs in the different regions of the spine.

In most animals the vertebral column has but two curves, one the cervical curve, which is convex inferiorly, the other the dorso-lumbar, which is concave inferiorly.

We have now to examine the influence that the vertebral column has in moulding the external form of the body, and to see if the length of the column can be made use of for a system of proportion.

It is evident, in the first place, that the posterior portions of the vertebræ only can affect the outline of the body, the anterior portions, the bodies of the vertebræ, being deeply hidden in the cavity of the thorax and abdomen. Therefore, in the skeleton the posterior surface of the vertebral column (Fig. 9) presents itself under the aspect of a median crest, formed by a series of spinous processes, the spinal crest, on each side of which is a groove bounded laterally by the series of transverse processes (the vertebral furrow). In the living subject these grooves are filled up by powerful and thick muscles, which project in such a manner that in the erect position the back presents a furrow in the median line bounded on each side by these muscles, at the bottom of which furrow the bony structure of the vertebral column is shown only by a series of projections placed one beneath the other, like the beads of a necklace, each one being formed by the summit or free extremity of a spinous process. These projections are well seen in the thoracic region, in which the curvature of the column is convex backwards, and they show themselves still more clearly when the subject bends forward, and thereby increases this curvature. They are not visible in the cervical region, where the ligamentum nuchæ projects to the surface, and a bed of powerful muscles covers them; but we have seen that the seventh cervical, or vertebra prominens—along with the sixth also in many cases—is remarkable for the projection which its spinous process makes. Finally, in the lumbar region these projections are but little marked, the spinous processes here being short and terminated not by points, but by straight borders (Fig. 8).

The measurements of the vertebral column are useful, on the one hand, as absolute measurements of length and height, and, on the other hand, in giving the ratio of its length to the stature of the subject. The height of the vertebral column in the average adult man is from twenty-three to twenty-four inches, being five for the cervical region, eleven for the thoracic, and seven inches for the lumbar region. But as the length of the vertebral column does not serve as a common measure for the total height of the body or for its different parts, it cannot be used as the basis of a system of proportion. A German zoologist, Carus, has advanced the idea that the length of the column forms one third of the height; but this proposition is not exact. On the other hand, it is not easy to measure the column from the atlas as far as the last lumbar vertebra, without taking account of the sacrum and coccyx. It will be more frequently found that the length of the trunk, from the superior limit of the thorax to the inferior limit of the pelvis, gives a measurement more easy to take, and more useful for comparing the general proportions of the body.

It is enough to say here that the proportion of the vertebral column to the height varies according to age and sex, and according as the stature of the individual is very great or very little; the vertebral column is, in fact, in comparison with the height, longer in the infant and in the female than in the adult male; it is also much longer in proportion to the height in subjects of short stature than in tall people. The cause of difference of stature between men and women, infants and adults, long people and short, is principally due to the length of the lower extremities—a question which will be dealt with in a subsequent chapter.