There are then two things in the pulse; 1st, impulse of the blood, sudden and general motion of its mass by the contraction of the heart; 2d, locomotion of the arteries, an effect produced by this fluid upon the arterial parietes which transmit it. The first is the most essential; as to the second, it would vary, if the arterial texture that produced it ceased to be the same; it depends upon this texture, and is not essential to the circulation.
When an artery is cut at the end of its trunk, the locomotion is much less sensible in this trunk, because less resistance is offered there to the course of the blood.
If an artery is opened laterally, it forms two currents of blood in an opposite direction, which are driven towards the opening, and which unite in one throw. One of these currents is direct, the other arises from anastomoses. It is the same as when an artery is cut, and the blood flows at both ends.
If an artery is wholly divided, more blood flows from it in a given time, than passed through it before in the same time to go to the capillary system, which resisted more. We cannot then judge of the velocity of the blood by the throw from the open arteries.
We have seen that the arteries are rarely the seat of diseases either acute or chronic, on account of the obscurity of their vital properties. They can exert then but a very slight influence upon the other organs; thus, except some sympathetic pains that are experienced in aneurism, this influence of the arterial texture upon the other systems is merely nothing. In two or three cases I have seen convulsive motions produced by the injection of a very irritating fluid in the arteries. It is easy to distinguish these sympathetic motions, from those that pain produces in an animal who is struggling to disengage himself; they are violent tremors or stiffness, like tetanus. It may be imagined that these experiments should not be made in the carotids, because the brain, irritated by the injected fluids, would produce convulsions arising from the stimulant that would be then directly applied to it, and not from a sympathetic relation. Besides, death would be the immediate consequence of the experiment, if it was made upon the carotid.
On the other hand, as the arteries have not sensible organic contractility, hardly any animal sensibility, and but little tone, the other organs can with difficulty develop in them sympathies by their influence; for, in order that a vital property should be brought sympathetically into action in a part, it is necessary that it should exist there, and even be conspicuous. Thus the innumerable variations of the pulse, which are the product of sympathies, have all essentially their seat in the heart; the arteries are not connected with them. Now the sympathies make the heart contract or arrest its motion, as stimulants or sedatives directly applied to it, that is to say, by acting on its sensible organic contractility. When an aneurism is broken in a fit of anger, or in the act of coition, a case of which I have seen with Desault, it is the motion of the blood, which is suddenly increased, that is the cause of it; it is not the arterial texture that has been affected by the passion. Besides, upon what can the sympathies act in the arteries? It could not be either upon the elasticity or the contractility of texture, the only properties, however, capable of contracting these vessels. Observe, also, that the sympathies put in action only the vital properties, because they are themselves a phenomenon purely vital. The physical properties and the properties of texture cannot be exercised under their influence; this is an important observation.
Besides, as the arteries are every where spread in the organs, and as they form, if we may so say, a part with them, it would be difficult to distinguish what belongs to them, especially as it respects sensibility, from what is peculiar to these organs.
The fœtus differs essentially from the infant that has breathed, in this, that its two great vascular systems in reality form but one, since the foramen ovale on the one hand, and the ductus arteriosus on the other, form a direct communication between the two. This communication is much more evident at the period nearest conception; these openings contract towards the period of birth. 1st. The foramen ovale is formed, in the first months, by two productions in the form of a crescent, whose concave surfaces are opposite, and leave between them an oval space, which is constantly contracting, because these two productions constantly approximate and have a tendency to cross each other, which in fact takes place after birth. 2d. The ductus arteriosus contracts as the pulmonary artery dilates.
While these two openings are free, which is constantly the case in the fœtus, the two systems evidently make but one, as I have said; whence it clearly follows that the blood that circulates there must be entirely of the same nature, that there cannot be two kinds in the fœtus, as there always is in the adult. This is, in fact, a remarkable difference between the two ages. 1st. I have many times dissected small Guinea pigs in the womb of the mother; their vessels have uniformly contained the same fluid, which is blackish, like the venous blood of the adult. This experiment is easy. The abdomen of the mother being divided, we successively open each of the separate sacs that the womb has for each fœtus. When one of these sacs is laid open, we cut the membranes, then the abdomen of the small animal, leaving the umbilical vessels untouched. The transparency of the parts easily allows us to see the uniformity of the colour of the blood of the vena cava and the aorta. The same remark applies to the superior parts. The carotid and jugular pour out the same blood when they are opened. 2d. I have three times made the same observation upon the fœtus of a dog. 3d. We know that the blood of the umbilical arteries is always black; all accoucheurs have remarked this. 4th. The change of the black blood to red arises from the contact of air in the lungs; the fœtus not breathing, cannot then have this kind of blood. 5th. I have dissected many fœtuses that have died in the womb of the mother; the blood of the veins and the arteries has appeared to me to be uniformly the same. It is true that this is not a very conclusive proof, since the mere standing of the red blood in the vessels, for a considerable length of time, is sufficient to make it black, as Hunter has observed.
The preceding facts are sufficient to establish incontestibly the uniformity of the blood in the two systems of the fœtus; an uniformity that exists at least in external appearance, if it is not real in its intimate composition. It is for the chemists to elucidate this point.
How is it, that the instant the black blood enters the system of red blood in the adult, alarming consequences follow, soon asphyxia, then death, take place, whilst in the fœtus, the black blood circulates with impunity in the arteries? It is a difficult question to resolve, and yet these two contradictory facts are equally true. The difference of the nature of the blood of the fœtus might perhaps serve to remove this difficulty, if we better understood this difference. In fact, though the colour assimilates this blood with that of the veins of the adult, yet it does not appear to be the same; it has an unctuous feel, unlike the other. It is never found in the dead body coagulated like it, but always fluid, like the blood of those who have died of asphyxia. Fourcroy discovered no fibrin in it; he observed that it did not take the vermilion colour by the contact of the air; that it contained no phosphoric salts, &c. It is then very probable that if the black blood is fatal in the arteries of the adult, whilst it circulates with impunity in those of the fœtus, that it arises from the difference of the nature of the one and the other. Besides, observe that there is a very great difference in the functions of the fœtus and the adult. The first scarcely has animal life; it wants many functions of organic. The relation of the organs with each other, is of a nature wholly different from what it will be after birth. No kind of analogy ever can be established between the fœtus and the infant in this respect. Thus we have observed that the experiments upon life and death give a result wholly different in animals with red and warm blood, and in those with red and cold, which approximate nearly the organization of the fœtus in some respects. We cannot then establish any kind of parallel in respect to the injury of the respiratory phenomena, between the fœtus and the infant, an injury, the causes of which I have sought in my experiments, since the organization relative to these phenomena differ so essentially in the one and the other.
Although, as I have said, the blood of the two vascular systems is confounded in the fœtus, yet there is, especially in the first periods, a kind of separation in the general mass of blood, a separation that was first accurately observed by Sabatier, and which is the result of the arrangement of the foramen ovale and the ductus arteriosus. This separation divides the mass of blood into two. The following is the manner in which the circulation of the blood is performed in this respect.
1st. All the blood that the trunk of the inferior vena cava receives, either from the capillary system of the inferior extremities, or from that of the abdomen, or from the placenta by the umbilical vein, instead of stopping in the right auricle, as in the adult, passes entire into the left through the foramen ovale, the superior edge of which is so arranged, that nothing can mix with the blood of the superior vena cava; so that when we examine attentively, we see that it is really with the left auricle that the inferior vena cava is continued. Hence why this auricle is in proportion as much dilated as the right; for it would be very contracted, if it had only to receive the blood of the pulmonary veins, the quantity of which is merely nothing in the first periods of life. From this auricle the blood passes to the left ventricle, which transmits it to the aorta, where it meets the carotids and subclavians, which, by numerous ramifications, carry it to the capillary system of the head and the superior extremities.
2d. After having remained in this system, the blood returns by the different branches of the superior vena cava to the right auricle, where the superior edge of the foramen ovale prevents it from communicating with the other blood; from this auricle it passes to the ventricle, which transmits it to the pulmonary artery, which sends a small part of it that returns to the left auricle by the veins of the same name, but transmits almost the whole of it by the ductus arteriosus to the descending aorta, below the origin of the carotids and the subclavians, which carry the other blood. This is carried by the branches and ramifications of the aorta to the capillary system of the abdomen and inferior extremities; the remainder is afterwards carried by the umbilical artery and lost in the placenta.
It follows from what we have just said, that notwithstanding the continuity of the two great sanguineous systems in the fœtus, there is in the first months after conception, a kind of separation of the blood they contain; that there is even if we may so say, two systems wholly different from those which will afterwards exist in a separate manner in the adult.
The first of these systems has, 1st, for origin all the capillaries of the abdomen, of the inferior extremities, and even those of the placenta; 2d, for common trunks, below the inferior vena cava, above the quadruple branch called the aorta; 3d, for agent of impulse the left side of the heart; 4th, for termination all the capillaries of the head and the superior parts. The second commences in these last capillaries, and is composed, 1st, for its trunks, of the superior vena cava and the descending aorta; 2d, for its agent of impulse, of the right side of the heart; 3d, for its termination, of the capillaries of the inferior parts.
The blood is then evidently divided in the first months after conception into two circulations, which cross, if it may be so said, in the form of the figure 8, as has been remarked by Sabatier; it is carried in each, from one assemblage of capillaries to another of the same vessels. Only instead of moving between the pulmonary capillary system and the general one, as in the adult, it moves between the superior and inferior part of this last system; we may then say in this point of view that the inferior and superior parts of the body are in opposition in the fœtus, as the lungs in the adult are in opposition to the rest of the body.
This complete opposition on the part of the circulation, between the upper and lower part of the body, in the first months of the fœtus, is probably the origin of the difference that takes place afterwards between these parts.
All physicians have observed this difference in diseases. If the median line frequently separates the affections of the right side from those of the left, the diaphragm seems often to be the boundary of many diseases. Who does not know, that scorbutic affections appear particularly below, that serous infiltrations are most frequent there, and that ulcers are infinitely more common in the inferior extremities, and that on the other hand, most cutaneous eruptions take place in the superior parts, &c.? Bordeu, who has said much of the division of the body into superior and inferior parts, considered one pulse as the precursor of evacuations from above, and another as that of those from below, he has however without doubt exaggerated this opposition of the two parts of the body; still it really exists, and I think that it is very probable, that the manner of the circulation of the fœtus is the primary source of it.
After the first months, things begin to change. The quantity of blood passing by the pulmonary artery was at first scarcely any thing, because the dilatation of the ductus arteriosus was so great, that it turned almost the whole of it into the descending aorta. This canal gradually contracting, the pulmonary arteries dilate, and then more blood goes through the lungs, and is brought by the pulmonary veins to the left auricle, which transmits it to the left ventricle, which sends it to the arch of the aorta; then the mechanism of the circulation described above begins to change, and approximate that of the infant, as we shall see.
Still this first mechanism predominates for a long time over the second; hence it happens that during the greatest part of the time that the infant is in the womb of the mother, it is the left ventricle that sends the blood to the superior parts, whilst the inferior receive theirs by the impulse of the right. Now as the parietes of the first are evidently thicker than those of the second, and the heart is further from the inferior than the superior parts, these last receive a stronger impulse than the others. This perhaps is a new source of the difference of the two halves of the body; hence nutrition is more active in that above, hence the degree of vital energy that it preserves a long time after birth, and which makes it susceptible, the head especially, of many more affections than the lower half.
As the period of birth approaches, the pulmonary artery sends more blood to the lungs, and less passes through the ductus arteriosus. For, as I have said, it is only in a gradual manner that the whole of this fluid, contained in the body, comes finally at birth to go through the lungs. Though before it undergoes no alteration there, it does not circulate the less, which is undoubtedly to habituate it to the passage that it is constantly to take after birth. The quantity of blood then is in a direct ratio to the age in the pulmonary artery, and in an inverse one in the ductus arteriosus.
This arrangement evidently requires a corresponding one in the foramen ovale; in fact, if in proportion as the ductus arteriosus contracted, this was not diminished also, all the blood would finally accumulate in the superior parts. For instead of passing from them to the inferior, the whole of it would return to them by the left auricle and the ventricle of the same side. In proportion as the ductus arteriosus is contracted, the foramen ovale being lessened also, the blood of the inferior vena cava, the whole of which cannot pass through there, begins to mix with that of the superior, enters the right auricle, then the right ventricle, afterwards returns by the lungs to the left auricle and ventricle and the aorta. What is the consequence of this? that this artery begins to receive from the left ventricle a much greater quantity of blood than can pass into the carotids and subclavians; a portion of it then goes into the descending trunk and is distributed to the inferior parts.
From what has been said, it appears, that the two portions of the blood of the fœtus are almost wholly separate in the first months; all that comes from the inferior vena cava goes into the ascending aorta; all from the superior passes into the descending, the lungs receiving scarcely any except by the bronchial arteries for their nutrition. But as the period of birth approaches, these two portions of blood begin to mix, and the circulation then has an arrangement between that of the adult and that of the first months. At birth even, the foramen ovale and the ductus arteriosus are much contracted, the circulation goes on in the mother's womb almost in the same way as it does after birth; the whole difference is that the fluid is of the same nature, because respiration has not taken place. The sudden change of the circulation at birth, arises particularly from the introduction of red blood into the economy. As to the mechanical phenomena, they are gradually produced by the gradual contraction of the two openings of communication. The blood gradually ceases to move from the inferior to the superior capillaries; it then goes from both of these to those of the lungs and reciprocally.
In considering the circulating phenomena, we err in supposing that their change is sudden at birth. It is sufficient to examine the foramen ovale and the ductus arteriosus at different periods of pregnancy, to see that they contract successively, and that consequently these phenomena are successive, so that if the fœtus should remain in the womb a long time beyond its period, and the contraction should continue in the foramen ovale and the ductus arteriosus, the blood would circulate as in the adult, from the pulmonary to the general capillary system exclusively, and reciprocally. The only difference would be in the uniformity of its colour, because it would pass through the first system, without coming in contact with the air.
I do not say that the entrance of the air does not suddenly bring to the lungs the remainder of the blood which passed by the ductus arteriosus; but this kind of sudden turn takes place only in a part of the blood of the pulmonary artery; a part already passed through the lungs before birth, though the air cells were empty.
In general, there is a constant relation between the quantity of blood that the right ventricle sends to the lungs, and that which the left sends to the inferior parts. The more the first increases, the more abundant is the second; this last evidently exceeds that which goes to the superior parts. These three things, 1st, the quantity of the blood of the inferior vena cava which mixes with that of the superior, and passes with it into the right auricle; 2d, that which from the right ventricle goes through the lungs and returns to the left auricle; 3d, that which from the left ventricle goes to the descending aorta, constantly increase as the period of accouchment approaches.
The descending aorta does not undergo by these variations any change in its caliber; in fact, it is the same thing to it, whether it receives the blood of the ductus arteriosus, below the origin of the carotids and subclavians, or whether this fluid comes directly to it from the left ventricle, through its arch; its parietes constantly increase in a uniform manner; all depends upon the successive contraction of the ductus arteriosus and the foramen ovale.
The whole vascular system is generally remarkable in the fœtus for its great development. The arteries are in proportion larger, which corresponds with the size of the heart, which is much developed at this age; it is nearly the same as the nerves in relation to the brain.
The development of the arteries however is not like that of the nerves, nearly uniform every where. These vessels follow in general the same order as the parts to which they are distributed. Thus in the superior parts, the cerebral arteries are much more evident than the facial; among these, the ophthalmic is more so than the nasal, the palatine, &c. In the thorax, the thymic arteries are much larger in proportion than afterwards. In the abdomen, all the gastric viscera being very considerable, their arteries are already very large; the supra-renal are much larger in proportion than in the adult. In the pelvis on the contrary, the arterial system is very contracted, because the viscera are small, as they receive but little nourishment. In the inferior extremities, the arteries are a little more contracted in proportion than in the superior, especially in the earlier periods, for towards birth, the proportion is nearer equal.
The arterial texture is infinitely more pliable in the fœtus than in the adult; it will yield more easily to extension; ligatures applied upon the arteries break it less easily. Aneurisms are extremely rare in infants.
Many little arteries wind upon the parietes of the great ones in the fœtus; they are often livid, to see them distinctly, it is necessary, as I have said, to examine them at this age. Does this abundance of vessels dispose the arteries in the first age to inflammations, which are so rare afterwards? I have never observed this alteration.
In the first periods of the fœtus, the layers and arterial fibres are indistinct; we should say that the coat of the artery is homogeneous. But it has however much more consistence than most of the surrounding textures; this consistence corresponds with that of the heart. Destined to distribute every where the nutritive matter, the arteries ought necessarily to precede the other organs in their nutrition. This early growth, always concomitant with that of the heart, would alone prove that the arteries are made to develop themselves, and that the heart does not hollow them out, as Haller has said, in the interior of our organs by the force of its impulse. Besides this mechanical manner of considering their formation is evidently contrary to the known laws of the animal economy.
At the moment of birth, two great revolutions take place in the system with red blood; 1st, a mechanical one, if it may be so said, in the phenomena of the course of the blood; 2d, a chemical one, in the nature of this fluid. The mechanical revolution depends upon the entire cessation of the passage of the blood through the foramen ovale, the ductus arteriosus, the umbilical arteries and veins. The chemical revolution depends upon the formation of red blood. I will now examine this last.
The fœtus finds at birth that all surrounding objects are the causes of great excitement; the cutaneous surface, all the origins of the mucous, are strongly stimulated. The sensations they experience are even painful, because the difference is very great between the waters of the amnios and the bodies with which the fœtus comes in contact at birth, and every abrupt change in the sensations is painful. Habit soon familiarizes this sensation; but it is not less real at birth, and it may be said that this moment is as painful for the infant as the mother. Now, as every lively sensation is generally accompanied with great motions a general agitation succeeds the impression that the fœtus perceives from without; all the muscles move, the intercostals and the diaphragm like the others. The air which already filled the mouth and wind-pipe, then enters the lungs, and there colours the blood red, then it is alternately expired and inspired until death. The first inspiration then is, in this point of view, a phenomenon analogous to all the motions that the change of external excitement suddenly produces at birth in the voluntary muscles of the fœtus.
The respiratory motion is, however, too important, since it commences a new kind of relation between the organs, to depend exclusively upon this cause. I presume that an unknown principle, a kind of instinct, induces the fœtus at the moment of birth, to contract the intercostals and diaphragm. This instinct, which I do not understand, and of which I cannot give the least idea, is the same that makes the infant the moment it comes from its mother's womb, contract its lips, as if to nurse. We certainly cannot say that this motion is an effect of the very acute external impressions that it feels; these impressions produce agitations, irregular motions, as if to get rid of these impressions, and not a uniform motion evidently directed towards a determinate object. If we examine all animals separately at the instant of their birth, we shall see that every one performs particular motions, directed by its instinct. The small quadrupeds seek the breast of their mother; the gallinaceous animals the grain that is to nourish them; the small carnivorous birds immediately open their bills, as if to receive the prey the mother is afterwards to bring to them in the nest, &c.
In general, it is essential to distinguish accurately the motions, which, at the instant of birth, arise from new excitements that the body of the fœtus receives, from those which are the result of a kind of instinct, of a cause of which we are ignorant. I believe that the respiratory motion belongs at the same time to the two causes, and more especially perhaps to the last.
I pass now to the mechanical revolutions of the course of the blood. At the instant the lungs change to red the black blood that enters them by the pulmonary arteries, they receive all that which before passed through the ductus arteriosus; this ceases to transmit any to the aorta, though, however, it often remains still more or less dilated; for at birth it is hardly ever entirely obliterated; this contraction varies singularly at this period. Why does the blood then cease to flow there? As the aliments do not enter the ductus choledochus, the lacteal or pancreatic ducts, though they pass their orifices; so undoubtedly this takes place, because the kind of sensibility of the ductus arteriosus repels the new venous blood of the fœtus, which comes no longer from the placenta, because that which the lungs have reddened will not mix with it. We cannot certainly give any mechanical reason for its not passing; it really does not, and it evidently depends upon the vital laws. Besides, the motion of which the lungs become the seat, the dilatation, and especially the new excitement that the external air produces there, by rendering considerably more active the capillary circulation, facilitate that of the two pulmonary trunks, and give the blood a tendency to pass there rather than through the ductus arteriosus; it is in this way that the lungs attract, as I have said, the blood from the pulmonary artery. Does not the great irritation, of which certain tumours are the seat, draw there more of this fluid? Is it not on this account that the arteries of these tumours dilate and acquire a double or even treble caliber? What takes place in these tumours in a gradual manner, happens suddenly to the blood that still passes by the ductus arteriosus at birth, and which was very much diminished, as I have said, by the successive contraction of this canal.
For the same reason that all the blood of the pulmonary artery goes through the lungs, the foramen ovale is closed; in fact, this foramen is so arranged at birth, that its valves approximate so as to cross, as it were; so that when they are pressed against each other, the communication of the auricles is really closed. Now the red blood entering the left auricle by the pulmonary veins, pushes the valve of the foramen ovale corresponding to this auricle, against the other, and consequently opposes the blood of the vena cava inferior that endeavours to enter there. This blood flows back to the right auricle. Now when this contracts to drive the blood into the ventricle, far from forcing it through the foramen ovale, it necessarily brings the two valves against each other, and obliterates it. By examining with care the state of the heart of the fœtus, it is evident that when the blood enters the left auricle by the pulmonary veins, the right by the venæ cavæ and the valves are crossed, it is impossible that the blood can pass them either in contraction or dilatation.
Though the foramen ovale may be open at birth, still the black blood ceases to pass through it; I say further, oftentimes this foramen remains open during the whole of life. Many authors have related examples of this. I have seen a great number, though this assertion may appear extravagant at first. It is impossible from the arrangement of its two valves, for the blood to pass through it. When the two auricles contract at the same time, the blood which is forced by them from without within, brings the valves together, and thus itself creates an obstacle to its passage. In the greatest number of cases, the adhesion of the two valves crossed, is extremely weak; they are rather in contact than united; so that by forcing between them the handle of a scalpel, they are easily separated and hardly any traces of rupture are found. If they were arranged so that the blood could insinuate itself between them, it would soon separate them and re-establish the communication. Authors need no longer attempt to explain, how life is supported when the foramen ovale is open; it is the same as if it was closed, no more blood passes through it.
The obliteration of the foramen ovale, and the cessation of the passage of the blood through its opening, are, as we see, phenomena to a certain degree mechanical. The vital laws perform also, without doubt, their part on this occasion. Who knows if the sensibility of the left auricle, stimulated and modified anew by the red blood, does not repel the black which tries to enter it by the foramen ovale? We see every day in the economy, fluids passing at the side of openings, without entering them, though they may be wide, for the sole reason that their sensibility is not in relation with these fluids. Why does the trachea convulsively reject all fluids and solids? why does the air alone enter it? Why does not the blood enter the thoracic duct, which is often furnished with a valve, as I have observed, incapable of opposing its passage, and sometimes even has none? Why does the urethra repel the urine in coition? It is a fault of all authors that they seek only for mechanical causes in all the phenomena of the circulation. Without doubt the course of the blood is a mechanical phenomenon; but the laws that govern this course are vital; it is the same as a bone that is moved by muscular contraction; the effect is the mechanism of the lever; the cause is vital.
The blood no longer passing through the ductus arteriosus, this closes immediately by its contractility of texture; it becomes a kind of ligament, which fixes to a certain degree the aorta and pulmonary artery in their respective position. As to the obliteration of the foramen ovale, it does not arise from this contractility; this obliteration is not made by a contraction, but by a real agglutination of the two valves, between which it is obliquely situated at birth. This agglutination appears to be the effect of a pressure that is made in an opposite direction, upon the partition between the auricles, by the blood that each contains. In fact their fibres are so arranged that they contract from without within; now by contracting thus, they press from each side the blood against the partition, and consequently the two valves against each other. Now this agglutination sometimes does not take place, whilst the contractility is always exerted when the parts in which it exists cease to be distended; the ductus arteriosus is uniformly obliterated.
At the same time that the ductus arteriosus and foramen ovale cease to transmit blood at birth, this fluid is stopt in the umbilical artery and vein. Why does the blood cease to flow in this artery, though its diameter continues very large at birth? The principal cause appears to me to be the nature of the red blood, which is no longer in relation with the sensibility of this artery. A proof of this is, that if some time after the fœtus has breathed, respiration is stopt, and the black blood consequently returns, the umbilical arteries begin to pulsate, and if the ligature is loosened, they pour out considerable blood. Baudelocque has frequently observed this.
In general, when respiration is well established, the blood no longer flows by the umbilical artery, the ligature of the cord is then useless. On the other hand, when this function is badly performed, there is reason to fear hemorrhage of this artery. I confess, however, that there may be other causes for this interruption of the passage of the red blood. These four things, 1st, the cessation of the entrance of the blood into the umbilical vein; 2d, interruption of the passage of that of the inferior vena cava by the foramen ovale; 3d, of that of the pulmonary artery by the ductus arteriosus; 4th, of that of the descending aorta by the umbilical artery; these four things, I say, the three last especially, appear to depend upon a cause that we do not yet understand. The change of the relation of the organic sensibility with the nature of the blood, is perhaps only accessory, since, as I have observed, it is less this property than the action of the heart itself, which is the cause of the circulation in the trunks. This subject deserves the most attentive examination of physiologists.
Respiration being once well established, the lungs are in opposition to the whole body; it sends blood to all the parts, and they all send it to the lungs. The boundary is then rigourously established between the system with black blood and that with red, and things then go on as we have before described.
After birth the vascular system with red blood predominates for some time by its greater development and its more numerous branches, in fact the red blood enters more parts then than it does afterwards. It is sufficient to dissect living animals of different ages, to be convinced of the greater quantity of blood in young animals, that the system contains, of which we are treating; so that, as I have said elsewhere, the two opposite ages of life exhibit an inverse arrangement as it respects the fluids and solids. The first are much more abundant towards the period of conception. The second always predominate more towards the last age.
The predominance of the system with red blood remains evident to the end of the period of growth. We see the necessity of this predominance to distribute to all the parts the materials of their nutrition and growth; in fact, in the adult the arteries contain only what is destined to the first. In the infant, they contain moreover what is destined to the second. Hence the caliber of the arteries is proportionably larger than afterwards, in order to contain more fluid. Injections demonstrate this; and on this account small subjects are not less favourable for the study of the arteries, than of that of the nerves. These vessels are more prominent in them; only the surrounding parts being less developed, we cannot see the connexions so well.
In proportion as the infant advances in age, the equilibrium is gradually established in the system with red blood. In the head, the facial arteries are more evident, and come gradually in their development to the level of the cerebral. In the thorax, the thymus diminishing as the lungs increase, their nutritive arteries follow an inverse order; the bronchials dilate and the thymic contract. In the abdomen less blood goes to the capsular arteries; but most of the others receive as much of it. The pelvis and the inferior extremities have more of it, and their development is proportionably evident.
It is about the period of puberty that the increase in height ceases; the increase in thickness continues always. The genital parts, hitherto without influence, seem to be then a centre of more active vitality than most of the other organs. The portion of the system with red blood that belongs to them, then becomes greater. The first effect that results from it is the secretion of semen, and a general impulse of the individual towards new tastes and desires, towards those relative to the propagation of the species.
Another phenomenon is soon the consequence of this. As the lungs are connected in an intimate, though unknown manner, with the genital parts, they acquire also a predominance with them. Their vital energy is increased, and then begins the period of the affections of this viscus; then, the cause that would in the adult produce a gastric affection, brings on a pulmonary one.
It is truly only at this period, that the predominance of the superior parts, of the head especially, ceases entirely. Thus whilst in infancy the nose is frequently the seat of hemorrhage, in youth it takes place particularly from the lungs. We may consider the increase of the energy of the lungs, which happens shortly after puberty, as the termination of the predominance of the superior parts. The cutaneous eruptions of the cranium, tinea capitis, &c. cease to be as frequent. Convulsions, and all the diseases that arise from the extreme susceptibility of the brain, become also more rare, and seem to give place to a great number of acute pulmonary affections.
It is towards this period, that is, some time after the end of the increase in height, that the diseases that are considered as the product of an arterial plethora, begin especially to manifest themselves; this may be said to be their age, and it arises from the following cause; as the blood contains before puberty, not only the materials of nutrition, but also those of growth, and whilst this continued the whole is expended in the system with red blood. But when the parts cease to increase in length, if this system still continues to receive the materials of growth, a true arterial plethora takes place. About the end of growth generally, some affections appear that indicate a predominance of the blood; as this is however under the influence of temperament, of the mode of life hitherto led, of the season and a thousand other causes, which, making the phenomena of the animal economy vary, rarely permit us to establish exclusive general principles. Thus all that is said upon the disposition to different diseases, in the different ages, &c. is subject to many exceptions.
The predominance of the lungs is gradually lost; the equilibrium is established among all the organs, which, hitherto had each performed a part more or less conspicuous in the phenomena relative to the different ages. As the system with red blood is uniformly, in every part, in proportion to its growth, to which it especially contributes, the equilibrium is in that way established between the different parts at twenty-six or thirty years of age; all the arteries have a proportional size, analogous to what they will always have afterwards. Whilst until then, some predominated, according to the predominance of the growth of the organs to which they are sent.
Towards the fortieth year, the gastric viscera seem to acquire a more decided vital activity; but this activity has no influence upon the size of the arteries that are distributed to these viscera.
Though the growth in height may end about the sixteenth or seventeenth year, that in thickness uniformly continues; so that the internal viscera still grow, and their arteries consequently enlarge, until the last growth ceases. This phenomenon has constantly struck me, in comparing arteries injected in subjects from sixteen to twenty years, with those in subjects beyond thirty-six or forty. In the last they are uniformly larger. It is this difference that first gave me the idea of distinguishing growth, into that in height, and that in thickness. For the development of the arteries is the constant index of the state of the growth of the organs. The period of the cessation of growth in thickness is then remarkable, 1st, by the cessation of the increase of the caliber of the arteries; 2d, by the general equilibrium that is established in their development.
As the arteries grow in the years that succeed the end of the growth of the body, they increase in compactness and thickness. Their fibres become more evident; their elasticity increases; their pliability is lessened; hence why the age of the adult is that of aneurisms. Observe that the density of the arteries follows, in its augmentation, the same proportion as the fleshy fibres of the heart; so that as this is more able to send the blood with force, these are more able to resist it.
In the last years, the system with red blood is remarkable for the following phenomena.
The number of the arterial ramifications is much diminished. As the heart loses its energy, it sends less blood, and with less force. The general vibration that it produces in the whole arterial tree, is less felt at the extremities of this tree. The small vessels that form these extremities gradually contract, are obliterated and become so many little ligaments. Hence why, when the periosteum is separated from the bone, the dura mater from the internal surface of the cranium, only a few drops of blood escape; why the skin, having hardened like horn, no longer exhibits the rosy tint of the preceding ages, especially of youth; why the section of a bone does not furnish hardly any blood, whilst it was so abundant in the fœtus; why the mucous surfaces look pale, the muscles have a dull colour, &c. All anatomists know that injections succeed less in proportion, as the subjects are more advanced in years; that in extreme old age the trunks alone are filled; that the fluids never enter the ramifications; that it is the reverse in young subjects; that even coarse injections oftentimes so fill the ramifications, as to render dissection difficult. I have dissected many old living animals; and the small quantity of blood their vessels contain compared with those of young animals is very remarkable. The general proposition that I have established, viz. that the solids are constantly acquiring the predominance, is perfectly true. This obliteration of the small vessels is remarkable even upon the parietes of the great vessels; we see it in the dead body; I have observed it in the living.
The less quantity of red blood that is found proportionably in old age, is referable especially to the state of nutrition, which is merely nothing when compared with that of infancy. Observe also, that united with the weakness of the motion that animates the blood, it is a cause of the small degree of excitement which the parts have in old age. In fact the use of the circulation is not only to carry to the different parts the materials of the secretions, of exhalations, nutrition, &c. we shall see that it keeps them also in a state of constant excitement by the shock it gives them at its entrance, a shock, the principle of which is evidently in the heart. Now this shock is in a ratio compounded, 1st, of the quantity of the fluid; 2d, of the force with which it is sent. In both respects, the excitement constantly diminishes, as age advances. Observe also that all the functions of the infant, both organic and animal, are characterized by a vivacity and impetuosity that form a remarkable contrast with the slowness and want of energy of those of old people.
The arterial texture always becomes more and more condensed as age advances. The layers that form the fibres of the peculiar membrane become drier, if I may be allowed the expression.
I have said that the internal membrane is very often the seat of a kind of peculiar ossification, which has hardly any influence upon the circulation, except when it is seated at the origin of the aorta.
The caliber of the arteries does not dilate in old age. There is scarcely any except the arch of the aorta, which constantly undergoes an enlargement more or less considerable, which is always without rupture of the fibres, consequently supposes an extensibility of these fibres, and undoubtedly depends upon the habitual and direct impulse that the blood exerts against the concave side of this curvature. I have often examined to see if there was a similar dilatation at those places in the arteries where the curves are very evident, in the internal carotid, for example, at the place where it passes through the carotid foramen; I have not discovered any.
In the last periods, the pulse is remarkable for its extreme slowness; a phenomenon opposite to that of infancy, in which the blood moves with great quickness. These two opposite facts are, after what we have said, foreign to the arteries. They indicate only the state of the forces of the heart, which is the agent of the general impulse of the red blood.
It is the same of the pulse in the last periods of life. It is not a real pulsation of the arteries; it is a kind of undulation, of weak oscillatory motion, and the more obscure, as life is more feeble. Now I am convinced that the heart alone is the agent of this undulation; I am convinced of it by the following very simple experiment. I have laid bare in many dogs, on one hand the carotid, on the other the heart by a section of one side of the thorax, made in such a manner that the other can still perform respiration. By placing the finger upon the artery, I observed that as long as the heart beat by a sudden impulse, that the pulse was kept up as usual, that it was even accelerated, because the contact of the air increased the quickness of the contractions of the heart; but at the end of a little time, this organ began to be weakened in its motions, then it contracted by a kind of general tremor of its fibres. In proportion as the weakness of the motions of the heart increased, the pulse was successively weakened. Then when the tremor extended to all its fibres, the pulsation of the artery changed to a kind of undulation, of feeble oscillation, the precursor of the cessation of all motion.
I shall observe, under the system of the muscles of organic life, that the heart has many kinds of contraction. The principal are, 1st, that which it ordinarily has, in which there is a contraction and a dilatation that succeed suddenly and regularly; 2d, that in which these two motions, retaining their natural character, are irregularly connected; 3d, those in which the fibres only oscillate, and by which the cardiac cavities a little contracted, communicate to the blood a less sudden shock, a general tremor, an undulation, &c. Now with each kind of motion of the heart, there is a peculiar pulse that corresponds. It is easy to be convinced of this upon living animals.
I am astonished that authors who have disputed so much upon the cause of this phenomenon, have not thought of having recourse to experiment to elucidate the question. There are undoubtedly many modifications in the pulse, whose coincidence with the motions of the heart could not be perceived; but that of the slow and frequent pulse, the strong and weak, the intermittent, undulatory, &c. can be immediately discovered, by laying the heart bare and placing the finger at the same time upon the artery. We see then uniformly, during the moments that precede death, that whatever may be the modification of the arterial pulsation, there is always an analogous modification in the motions of the heart; this certainly would not be the case, if the pulse depended especially upon the vital contraction of the arteries. I have had occasion to make these experiments many times, either directly for this object, or with others in view; I have seen the motion of the heart always correspond with that of the arteries. In general the theory of the pulse requires as I have said, new researches; but I have facts enough upon this point to be convinced that the varieties it undergoes in the different ages, as under other circumstances, depend almost exclusively upon the heart, which produces in particular this kind of undulation, of oscillatory motion which is between the pulsation of the natural state and the complete cessation of this pulsation.
I shall speak under the organic muscles, of the accidental development of the left portion of the heart. As to the arteries, new ones are never formed; but oftentimes, those that do exist acquire a remarkable size; this depends on two causes, 1st, on an obstruction to the course of the blood; 2d, on the growth of any tumour.
1st. The dilatation of the arteries by an obstruction to the circulation, is evident in the ligature of aneurismatic arteries, in the spontaneous cure of aneurisms, a phenomenon of which within a few years a great number of examples has been published, &c. Then, sometimes the great collaterals increase in size, sometimes their caliber remains the same, and it is by the ramifications that the communications are made. As the branches dilate, their thickness increases in proportion with their breadth; at least I have twice observed this fact, which is analogous to that which the left ventricle presents when it becomes aneurismatic.
2d. All tumours do not produce a dilatation of the arteries; we see this dilatation in cancers, in those of the breast, of the womb, &c. in osteo-sarcosis or spina ventosa, in the different fungi, &c. In general, most tumours that give great pain to the patients exhibit this phenomenon. We should say even that pain in a part is sufficient to attract there habitually more blood, and dilate the arteries; we know that in the operation for lithotomy, when the patients have previously suffered much, hemorrhage is often more to be feared.
After long and copious secretions or exhalations, I have not observed that the arteries were more dilated in the glands or around the exhalant organs. How large soever the cysts may be, their parietes never contain arteries proportioned to those that are developed in cancerous tumours. The cerebral in hydrocephalus, the mediastinal, intercostals, &c. in hydro-thorax, the mesenteric, the lumbar, the stomachic, the epigastric, &c. in ascites, the spermatic in hydrocele, the renal in diabetes, the branches that go to the parotids after a long salivation, retain their ordinary size, and under some circumstances they become even smaller.
When the arteries dilate in tumours, do their parietes thicken in proportion, as in the preceding case? I have no data, from which I could determine this point.