Chapter XIII: Section II (8)

When the artery runs but a short course, these two orders of branches remain distinct from each other as far as the organ, as we see around the splenic, the hepatic, the renal, &c.; but if the course is longer, the external branches gradually get into the plexus, and are entirely lost there. This plexus can be followed upon the great trunks; it divides upon each branch, and can be still seen; but such is its tenuity upon the minute ramifications that it disappears there entirely. The spermatic is one of the arteries upon which it can be traced the longest. The arteries of the extremities appear to be destitute of it. In general it is upon those that go to the central organs of internal life, that this network is the most evident. When we deduct from the sum of the filaments coming from the ganglions, those by which they communicate on one part with each other, and on the other with the nerves of animal life, we see that almost all the rest is finally destined to accompany the arteries. This arrangement is wholly different from that of the cerebral nerves, whose filaments are only in apposition with the vessels. These make almost an integrant part of them, the adhesion is so intimate; this certainly supposes a use of which we are ignorant, relative to the circulation, or to the other organic functions. As these vessels distribute every where the materials of these functions, of the secretions, exhalations, nutrition, &c. the organic nerves have no doubt some influence upon them. Neither experiment or observation have taught us any thing upon this point.

The veins are not accompanied by so many organic nerves. It is the same as it respects the absorbent trunks, which go almost every where separate from this system.

The constant union of the arteries with the organic plexuses, an union that presents an arrangement wholly different from that of the ganglions, has undoubtedly an influence upon the action of these plexuses, or rather upon that of the nerves that go from them, by the motion the blood communicates to them. It should be remarked upon this subject, that as nature has placed a crowd of arteries at the base of the brain to agitate it with an alternate motion, she has put also the most considerable plexus of the whole organic system upon one of the places to which the red blood communicates the strongest impulse, viz. upon the trunk of the cœliac.

III. _Structure, Properties, &c._

From what has been said above, it is evident that the nerves going from the ganglions are of two sorts as it respects organization; 1st. those that are identified with the cerebral system, by their white colour, by the possibility of dividing their trunks into distinct cords, and these into filaments, which appear to have nervous coats and medullary substance like the preceding; 2d. those which present only little separate filaments, greyish or reddish, soft, and which are seen in prodigious numbers in the plexuses. Have these a nervous coat and a medullary substance? It is impossible to determine it.

The properties of texture are ascertained with difficulty in the organic nerves. As to vital properties, it is undoubted that the animal sensibility is not as much raised in these nerves as in those of animal life. I have often laid bare the plexuses in the abdomen; then by letting the animal rest a moment, and by irritating them comparatively with the lumbar nerves, I have uniformly made this remark. We know that very frequently the ligature of the spermatic artery is not painful in sarcocele, though the branches coming from the ganglions form for it a plexus like a network, which can in no way be separated from it. If we draw out a portion of intestines by a small wound in the abdomen, the irritation of the sub-mucous layer at the side of the vessels, is hardly felt, though many nerves of ganglions are found at this place. I have had numerous occasions to act in different ways upon the carotid, to which the superior cervical ganglion furnishes branches from above; now, as long as I did not touch the par vagum, the animal remained tranquil. I am far, however, from believing in the absolute insensibility of the nerves of the ganglions; but certainly under the circumstances that I have related, the cerebral nerves would have caused much more pain to the animal.

I think that in a morbid state this sensibility is susceptible of being greatly raised. We certainly cannot deny but that the solar plexus takes a great part in the different sensations that are experienced at the epigastric region; the very acute pains that often attend the formation of aneurisms, are probably owing in part to the distension of the nervous filaments that surround the artery. I have already said that it is probable that the organic nerves are much concerned in the different sensations that are produced by some peculiar neuroses.

These nerves occasion evident sympathies in certain cases. It is to this that must be referred the different affections that Petit de Namur has produced in the organ of sight, by irritating their branches that are accessible. The development of the nerves of the ganglions follows nearly the same laws as that of those organs from which they emanate.

Let us observe in finishing this system, that there is no one that ought to arrest the attention of physiologists more. All the others present a series of phenomena already well known. Of this, we have hardly any knowledge. It does not present as yet, if we may so say, but some of the negative attributes of the nervous system of animal life. Thus it is without doubt that the organic nerves do not have the same part as the preceding in animal sensibility; that they are always foreign to contractility of the same species; that they have no direct influence upon the sensible organic, since as we shall see, we can cut or irritate them without destroying or hastening the motion of the muscles to which they go. But in knowing the parts they do not perform we are ignorant of those to which they are really destined. I have already observed that the difficulty of making experiments upon the ganglions and the plexuses, will much retard the progress of science. We have scarcely any branches upon the exterior upon which we can act.

Scarpa has collected the opinions of all who have preceded him, together with his own, upon the uses of the ganglions. I refer to what he has said upon this subject. As the general point of view in which he has presented these organs, and that in which I offer them here, differ essentially, the account that I have just given of the nerves of organic life has necessarily a general stamp wholly different from that of his work, a work, however, which, like all this author has published, confers the greatest honour upon the state of anatomy at the period in which we live.

I will terminate this article by an important reflection. If the nerves were only divided to form the ganglions, if these presented in their interior only differences of forms, and a very minute division of their filaments, why should they be so constant in animals? Many organs are wanting, vary, are presented under a thousand various forms in their different classes; on the contrary, the ganglions are constant. In those species even in which the cerebral system is imperfect, that of the ganglions is complete in its organization. Animal life diminishes and is contracted in an evident manner in most insects, in worms, &c. and generally in animals without verterbræ. The brain and the nerves become less evidently marked in proportion as this life is less perfect. The organic is, on the contrary, almost in its perfection in these animals. The ganglions and their nerves are also very evident. This remark has struck me in reading the researches of different authors upon the anatomy of the lower classes of animals; now, if the ganglions were not the centres of certain important functions of which we are ignorant, would they be so invariable in the animal organization?

VASCULAR SYSTEM

WITH RED BLOOD.

ARTICLE FIRST.

GENERAL REMARKS UPON THE CIRCULATION.

All authors have considered the circulation in the same way, since the celebrated discovery of Harvey. They have divided this function into two; one has been called the great circulation, the other, the small or pulmonary. The heart, being between the two, is their common centre. But in presenting in this point of view the course of the blood, it is difficult at first sight to perceive the general object of its course in our organs. The method in which I explain in my lectures this important phenomenon of the living economy, appears to me infinitely better calculated to give a great idea of it.

I. _Division of the circulation._

I divide the circulation also into two; one carries the blood from the lungs to all the parts; the other brings it from all parts to the lungs. The first is the circulation of red blood, the second that of black.

_Circulation of red blood._

The circulation of the red blood commences in the capillary system of the lungs, where the blood acquires, by the mixture of the principles that it draws from the air,[6] the peculiar character that distinguishes it from the black blood. From this system it passes into the first divisions, then into the trunks of the pulmonary veins; these pour it into the left auricle of the heart, which transmits it to the ventricle, and this drives it into the arterial system; this spreads it into the general capillary system, which may be truly considered as the termination of its course. The red blood is then constantly carried from the capillary system of the lungs to the general capillary system. The cavities that contain it are all lined with a continuous membrane; this membrane spread upon the pulmonary veins, upon the left cavities of the heart and upon the whole arterial system, may be considered as a general and continuous canal, the exterior of which is strengthened in the pulmonary veins by a loose membrane, in the heart by a fleshy surface, which is delicate in the auricle, thick in the ventricle, and in the arterial system by a fibrous layer of a peculiar nature. In these varieties of the organs that are thus added to it without, this membrane remains every where nearly the same, as we shall see.

[6] It may be remarked, that this work was published at a time when the theory of the oxygenation of the blood was universally considered as explaining in a satisfactory manner the change that is effected in the colour of the blood by the lungs. The experiments of Allen, Pepys, and others, and the Treatises of Ellis, have proved to the satisfaction of most physiologists of the present day, that this change in the colour is not owing to the absorption of oxygen by the blood, but to the extrication of carbon from it. _Tr._

_Circulation of the black blood._

The circulation of the black blood is performed in a manner the reverse of the preceding. It begins in the general capillary system; it is in this system, that its blood takes the peculiar character that distinguishes it from the preceding; it is here that it is formed, by the subtraction probably of the principles of the air that it acquired by terminating its course in the lungs. From this general capillary system, it enters the veins which transmit it to the right cavities of the heart, which send it by the pulmonary artery, to the capillary system of the lungs. This system is its real termination, as it is the commencement of the circulation of the red blood. A general membrane, every where continued, lines the whole course of the black blood, and forms for it also a general and continuous canal, in which it is constantly carried from all parts to the interior of the lungs. At the exterior of this great canal, nature has placed a loose membrane in the veins, fleshy fibres in the heart, and a peculiar fibrous texture in the pulmonary artery; but, like the preceding canal, it remains always nearly uniform, notwithstanding this difference of organs to which it is united externally. It is by the folds of this membrane in the veins that the valves are formed. It contributes to form all those of the right side of the heart, whose cavities it lines, as the preceding enters into the composition of the valves of the left side, which borrows from it the membrane that lines it.

_Difference of the two circulations._

From the general idea that I have given of the two circulations, it is evident that they are perfectly independent of each other, except at their origin and termination, where the red and black blood are alternately transformed into each other, and communicate for this purpose by the capillary vessels. In their whole course they are entirely separate. Though the two portions of the heart are united in one single organ, they may however be considered as uniformly independent in their action. There are truly two hearts, a right and a left. Both would be able perhaps to perform their functions as well if they were separate, as they now do united. When the foramen ovale remains open after birth, I have proved elsewhere that such is the arrangement of the two folds between which it is found, that the black blood cannot communicate with the red, and that the two hearts should equally be considered independent, at least as it respects the course of the blood. This entire separation of the two circulations is one of their most striking characters; it alone proves how much preferable the point of view in which I have presented the circulation, is to that which divides it into great and small, which are evidently confounded and identified.

From what has been said, it appears that the origin and termination of each circulation take place in the two capillary systems, which are, if we may so say, the limits between which the two kinds of blood move. The lungs alone correspond in this respect, with all the parts. Their capillary system is in opposition to that of all the other organs, if we except the parts from which the blood of the vena porta goes. Each capillary system then is at the same time an origin and termination. The pulmonary is the origin of the circulation of the red blood, and the termination of that of the black. The general gives to the red blood its termination, and to the black blood its origin. Observe that this is a great character that distinguishes the two circulations. In fact, the blood not only takes an opposite course at the place where they finish and at that where they begin; but its nature changes also entirely, and in this respect the two capillary systems, pulmonary and general, present to us the most important phenomena of the living economy, viz. the first, the transformation of black blood into red, the second, that of red blood into black.

There are evidently three things to be examined with regard to each of the circulations, 1st. the origin; 2d. the course; 3d. the termination of each kind of blood. In the origin and termination, there is on one hand the mechanical phenomena of circulation, on the other the phenomena of the transformation of the blood. In the course of this fluid there is only the mechanical phenomena of the circulation to be observed.

_General mechanical phenomena of the two circulations._

By examining these phenomena in a general manner, we see, 1st. that the red blood going from the lungs, is formed into larger and less numerous columns, as it approaches the cavities of the heart; that it is in the greatest masses in these cavities, and that from them to the capillary system, it is continually dividing into smaller columns; 2d. that the black blood going from the general capillary system, is formed into columns larger and less frequent, as it approaches the right cavities of the heart; that these cavities contain the greatest quantity of it, and that from them to the lungs, it is successively divided into smaller columns.

The two kinds of blood circulate then on the two sides in branches that diminish as they go from the heart, and increase as they approach it. Represent to yourself for each of the two circulations two trees united at their trunks, sending their branches, one of them to the lungs, the other to all the parts. Each of the two parts of the heart is between these trunks, which it serves to unite, so as to form a general canal of which we have spoken.

Authors have commonly considered the arteries and veins as forming, each by their assemblage, a cone, the base of which is at all the parts, and the apex at the heart. This manner of describing them arises from this, that the sum of the branches is greater in diameter, than that of the trunks from which they arise; now in adopting this idea, it is evident that each half of the heart is at the summit of two cones, which would be united without it. The pulmonary veins represent one, and the aorta the other for the red blood; for the black blood, there is on one part the venæ cavæ and coronary veins, on the other, the pulmonary artery, which form the two cones. In each circulation, one of these cones is remarkable for its small size; it is that of the lungs; the other for its great extent; it is that of all the parts.

Placed between these two cones, each part of the heart should be considered as an agent of impulse which hastens the course of the blood, on one hand towards all the parts, on the other towards the lungs. In fact, if in each circulation these two cones communicate by their apex, it is evident that the parietes of the vessels that compose them would be insufficient to maintain the motion, from the base of one to the base of the other; that is to say from the general capillary system to that of the lungs, and reciprocally from that of the lungs to the general one. The course is manifestly too long, and the vital forces of the vascular parietes not sufficient to admit of this; hence the necessity of the heart.

This consequence leads to another, which is this. As the red blood has a greater extent to go over from the heart to the general capillary system, than the black blood has from the heart to the pulmonary capillary system, it is necessary that the portion of this organ belonging to the first kind of blood, should be endowed with a greater force than that destined to support the motion of the latter. Nature has effected this object by composing the ventricle with red blood of fibres much stronger than those of that of black blood. As to the auricles, as they only receive the blood and transmit it to the ventricles, their thickness is nearly uniform.

From this we see, 1st. that the part the heart performs in the two circulations, is absolutely relative to the mechanical phenomena of the course of the blood, and that, if it has any influence upon the composition, it can only be by the internal motion it communicates to it; 2d. that if the course of the two circulations, of black blood and red, was of less extent, they might do without this intermediate agent of impulse. This is precisely what happens in the system of abdominal black blood, the two trees of which distributing their branches, one to the gastric viscera, the other to the liver, unite by their trunk in what is called the sinus of the vena porta, which occupies exactly the place of the heart in the great system of black blood and in that of red.

It is then possible to conceive, 1st. how the heart may fail; of this we have many examples, in which the two great circulating systems resemble in a certain degree, the abdominal; 2d. how the blood can oscillate from one capillary system to the other, during a considerable time, though the heart, weak, enfeebled, and even in part disorganized, can hardly any longer accelerate the course of this fluid; 3d. how, this organ having entirely suspended its pulsation in syncope, asphyxia, &c. there is still an oscillation, a real progression of the blood from one capillary system to the other, so that if an artery or vein is then opened, it flows a little at the opening. Certainly this oscillation is very weak; it cannot last a long time; but we cannot deny that it exists without the influence of the heart, since the black blood is carried without the agent of impulse, from the intestines to the liver; hence it follows that the cessation of the pulsation of the heart is not a proof of the want of motion in the blood, as some authors have thought. 4th. We know that in many animals of the lower classes, there is no heart, though there are distinct vessels and circulating fluids.

The importance of the part that the heart enjoys in the animal economy is only in relation to the general impulse that it communicates to all the organs and the constant excitement in which it keeps them by this impulse. It does not send to them the materials of secretion, of exhalation and of nutrition; it only in this respect transmits what it receives from the lungs.

II. _Reflections upon the general uses of the circulation._

This leads us to some reflections upon the general differences of the uses of the two circulations; differences which prove the necessity of presenting the single function that results from them in the view in which I have placed it, and not in that in use in the treatises on physiology. The following are the differences.

_General uses of the circulation of the red blood._

It is the circulation of the red blood that alone furnishes the matter of secretions, except that of the bile, a fluid which however deserves a further examination. It is from this circulation that the serous, cellular, cutaneous, medullary exhalants, &c. draw the fluids that they transmit upon their respective surfaces. All the vessels that carry the matter of nutrition of the organs are also continuous with the arteries and consequently their fluids come from the red blood. In the organs even in which the black blood goes, as in the lungs and the liver, there are vessels with red blood evidently for the purposes of nutrition. It is the red blood that communicates to the organs of the whole body that general agitation which is necessary to their functions, an agitation so evident in the brain. The circulation of red blood is then the most important; it is that, whence are derived the great phenomena of the economy.

_General uses of the circulation of black blood._

The circulation of the black blood, on the contrary, having no connexion with any of the functions, seems only destined to repair, if we may so say, the losses the blood has sustained in the preceding one. Observe in fact that a considerable part of the red blood is expended in the exhalations, secretions and nutrition. The principles it borrowed in the lungs and which gave it a vermilion colour, have been left in the general capillary system. It is necessary that the black blood should receive what the other has lost; now a variety of substances enters the great canal that contains it. These substances are internal or external. 1st. The great trunks of the absorbents constantly pour in the lymph of the cellular texture and of the serous surfaces, the residue of the nutrition of all the organs, the super-abundant fat, synovia and marrow. All that which is to be thrown from within out, is first poured into the black blood. 2d. All that which enters in from without, is also received by it. The chyle, the product of digestion, is at first uniformly carried into the general canal, in which it circulates. In the second place, it is with it that are mixed the substances of the air, which pass through the lungs in the act of respiration. In fine, when cutaneous or mucous absorptions take place, the black blood is always the first that receives the product of them.

It follows from this, that the circulation of black blood is, if we may so say, a general reservoir in which is poured in the first place all that is to go out of the body, and all that is to enter it.

In this last respect, it performs an essential part in diseases; in fact it is undeniable, 1st. that deleterious substances maybe introduced with the chyle into the economy, and produce ravages there more or less evident in circulating with our fluids. For this, it is sufficient that the organic sensibility of the chylous vessels should be changed; then they admit what before they rejected, as the glands by changes in their organic sensibility, often secrete fluids that are usually foreign to them. 2d. We shall prove in the article upon the cutaneous system, that it is oftentimes the seat of the absorption of deleterious substances. 3d. We cannot doubt that besides the principles that colour the blood, there often passes through the lungs deleterious miasmata which produce diseases, as my experiments upon asphyxia have proved. The intestines, the lungs and the skin are then a triple gate open, in many cases, to different morbific causes; now these causes that enter thus into the economy are all received in the first place in the black blood; it is not until afterwards that they pass into the red blood.

An evident proof of this assertion is this, that we produce phenomena exactly analogous to those which result from them, by pouring artificially into the black blood those substances that are introduced in a natural way. Thus when a purgative or emetic infusion is introduced into the veins, alvine evacuations or vomiting ensue, precisely as when the substances of these infusions are introduced by friction upon the skin. The experiments of many physiologists leave no doubt upon this point. I am convinced that it is possible to give to animals artificial diseases, by making different substances infused into their veins circulate with the blood. I shall speak of these attempts in the article upon the glandular system. It is sufficient for me to mention them here, in order to prove that the black blood is a general reservoir in which many substances can enter, either naturally, or accidentally, and afterwards disturb the functions by passing into the whole circulating mass. The humoral pathology has undoubtedly been exaggerated, but it has still real foundations, and in many cases we cannot deny, but that every thing arises from the disorders of the humours.

Let us conclude from all that has hitherto been said, 1st. that the essential part which the circulation of the black blood performs in the economy, is to introduce into this blood different new substances; 2d. that that of the system of red blood is to expend on the contrary, the principles that constitute it. One is constantly increasing, the other diminishing; to give is the attribute of one, to receive is that of the other. This sketch, which is perfectly true, and founded upon the most simple observation, appears to me very proper to establish an evident demarcation between the two divisions that I have adopted for the general circulation.

Health supposes a perfect equilibrium between the losses the red blood experiences, and the acquisitions the black blood makes. Whenever this equilibrium is destroyed, there is disease. If the black blood receives more than the red blood expends, plethora follows. That which is called the poverty of the humours, is manifest when more substances go from the red blood than enter the black.

These are I think sufficient characteristic attributes of the two great divisions of the general circulation, to justify the point of view different from other authors, in which I present this important function of the animal economy.

ARTICLE SECOND.

SITUATION, FORMS, AND GENERAL ARRANGEMENT OF THE VASCULAR SYSTEM OF RED BLOOD.

From the general idea that we have given of the two vascular systems, we should form the following of the position in the economy of that with red blood.

1st. The capillary system of the lungs gives rise to many minute ramifications, which soon unite into small branches, then into larger ones, and finally into four great trunks, two for each lung. These trunks open into the left auricle towards its superior part. 2d. This, distinguished from the right by having fewer fleshy columns, by its smaller size, by the greater elongation of its appendix, which is narrower than that of the other, &c. communicates by an oval opening furnished with valves, with the left ventricle, the thickness of whose parietes, the arrangement of the fleshy columns, &c. distinguish it from the right. 3d. From this ventricle goes the aorta, the common trunk whence arise all those that carry red blood to all the parts, where they terminate in the general capillary system.

The first tree of the system of red blood, the trunk of the second and the heart that serves to unite them, are found then concentrated in the cavity of the thorax, whilst the branches of the second trunk are distributed among all the organs of the economy, and even to all its extremities.

It is nearly between the superior third of the body and the inferior, that is found the agent of impulse of the red blood, or the heart. This position is important; it places under a more immediate influence of this viscus, the superior parts, the head especially, all of whose organs, and particularly the brain, require inevitably a very active and habitual excitement from the blood, in order to keep their functions in permanent activity. Thus observe, that in the gangrene of old people, and the affections that arise from the blood not being driven with sufficient force to all the parts, it is the extremity of the foot that is first affected, and that of the head and the hands become much later the seat of mortification. In general, there are many differences between the phenomena that take place in the superior parts, and those that happen in the inferior. We shall see in the dermoid system, that the portion of the general capillary system which belongs to the first, is penetrated with blood with infinitely more ease, than the portion belonging to the inferior parts, as asphyxia, apoplexy, submersion, different cutaneous eruptions, injections even prove, which in young subjects blacken rather the face than the inferior parts; now this difference arises evidently from the relation of position of the superior and inferior parts with the heart.

We have no general remarks to offer here upon the first tree and upon the agent of impulse of the circulation of red blood. In fact, the remarks belonging to the lungs and the heart will be given in the Descriptive Anatomy. It is then especially the second or arterial tree, whose distribution is now to occupy us. It is necessary in this article to examine the origin, course and termination of it.

I. _Origin of the Arteries._

This article comprehends the origin of the aorta at the left ventricle, that of the trunks which arise from it, then that of the branches, the smaller ones and the minute ramifications that go from them.

_Origin of the Aorta._

Most authors have described inaccurately the manner of the union of this great arterial trunk with the heart. This is the manner; the internal membrane of the heart with red blood, after having lined its ventricle, approaches the opening of the aorta, is attached there, forms by its folds the three semi-lunar valves, and stretching afterwards into the artery, covers it in its whole extent. It is this internal membrane that forms the union of the artery with the heart. The peculiar or fibrous membrane is not identified with the fibres of the heart. Its extremity is cut into three semi-circular festoons, which correspond with the semi-lunar valves that they support. These festoons do not extend to the fleshy fibres; there is between them and these fibres a space of two or three lines that the internal membrane alone covers. Between them and consequently between the valves, we perceive three little empty triangular spaces, covered by the membrane also. To distinguish this structure clearly, the origin of the aorta must be dissected well from without, and stripped entirely of the fatty texture that surrounds it. Then by cutting this artery and the ventricle, and by examining when held up to the light the union of one with the other, after having first removed the valves, we distinguish very well by the transparency of the internal membrane and the opacity of the three festoons that commence the aorta, the arrangement that I have just pointed out. It follows from this, that if the artery is accurately dissected on its exterior, and we detach from below upwards the internal membrane that forms the great canal of the circulation of the red blood, the artery is entirely separated from the heart. This entire separation of the fibres of the aorta from those of the heart, would alone be a strong presumption that their nature is not the same if many other considerations did not prove it in the most evident manner.

_Origin of the trunks, branches, smaller branches, &c._

Arising thus from the left ventricle, the aorta divides almost immediately into two branches, one ascending goes to the neck, head and superior extremities; the other descending to the chest, abdomen and lower extremities. The first being very soon subdivided into four principal trunks, differs in this respect from the second, which forms for a long time one trunk only. The latter having to go over a much greater distance than the other, preserves with more certainty by this arrangement the whole of the motion that is given to the blood by the heart; this does not prevent, however, owing to the less distance, the impulse from being more sensibly felt by the superior than the inferior organs, as I have said above. At the superior part of the pelvis, the aorta divides into two secondary trunks. Soon after, subdivisions begin under the name of branches, which are afterwards multiplied under that of ramifications, &c.

Mathematical anatomists have exaggerated the number of the arterial sub-divisions. Many have thought there were a hundred to one artery; Haller reduced the number to twenty, and even less. To ascertain what is really the case, it is necessary to take the arteries at their origin, and follow their course under a serous membrane, the peritoneum for example, where they are every where very apparent; it will be seen in this way, that the sub-divisions are not more numerous than is stated by Haller; I have frequently satisfied myself of this. Besides, the inspection of a living animal whose abdomen is opened, is almost the only means that can be employed without danger of mistake. Injections when too coarse do not fill all the branches; when too fine, they pass into the exhalant vessels, and communicate to the whole serous surface a colour that is not natural to it. It is almost impossible to ascertain by injections, the precise point of the natural circulation. To be convinced of this, inject a dog and open the abdomen of another of the same size; you will see uniformly in one more or less vessels injected, than are seen in the other filled with blood. I frequently performed this experiment, at the time I was engaged in demonstrating the insufficiency of injections, either fine or coarse, to show the quantity of blood in any part.

When they divide, the arteries form among themselves very different angles. Sometimes right angles, as at the middle intercostals; sometimes obtuse, which is more rare, as at the superior intercostals; most commonly they are acute, particularly in the extremities. The origin of the spermatic artery is an instance of the extreme of this last kind of origin.

We observe in general, that wherever there are two divisions, one is larger than the other. The largest follows the original direction of the principal trunk, from which the other is more or less separated. In the interior of the artery, an eminence formed by the fold of the internal membrane, corresponds with the angle entering from without, and breaking the column of the blood, favours the change of its course. This eminence presents an arrangement, that is very variable, which is owing to the angle of origin. 1st. If the angle is a right one, the eminence has a circular arrangement and is equally evident in the whole circumference. 2d. If the angle is acute, as at the mesenteric, then this eminence is very evident between the branch that arises and the continuation of the trunk; it forms even a kind of semi-circular spur or projection, but between the trunk itself and the branch that arises from it, the union of which forms an obtuse angle, this eminence is less conspicuous. The more obtuse the angle is, and consequently the more opposed it is to acute, the less sensible is this second eminence; it has like the other a semi-circular form, and makes by its union with it a whole circle which is oblique; so that the portion of the circle formed by this second eminence is nearer the heart than that made by the other. 3d. If the angle of origin is obtuse, and consequently that formed by the branch with the continuation of the trunk is obtuse, things are then arranged in an inverse manner. There is at the mouth of the artery an oblique circle, the prominent half of which is nearest the heart.

The origin of the arterial trunks is generally pretty uniform; but that of the branches is so variable, that hardly any two subjects have the same arrangement, in this respect. Take for example the hypogastric; it would be impossible to form the least idea of its branches, if, neglecting the manner they separate from each other, you paid attention only to their course and distribution. These numberless varieties in the forms, are a remarkable character in organic life to which the arteries belong. This character must be placed at the side of the constant irregularity of the arteries. There is no symmetry in their general distribution, as in the distribution of the nerves of animal life. Those even of the extremities, that correspond, differ frequently in their mode of origin and the course of their branches.

The branches, smaller branches, &c. arise at distances very near each other. There is hardly any, except the carotid, internal iliac, &c. that runs a considerable course without furnishing some. Thus experiments in which it is necessary to introduce tubes into arteries, to open them, &c. can scarcely be made except upon the first of these, they are prevented in others by divisions that arise from them and hinder us from raising the artery to any considerable extent.

The origin of the arterial trunks, branches, smaller branches and ramifications, does not take place in a gradual and necessarily successive manner. Thus the smaller branches, and the ramifications even, arise equally from trunks and branches; for example, the bronchial, thymic arteries, &c. go from the aorta, and yet they are not so large as most of the divisions of the tibial, which is itself a third division of the aorta.

II. _Course of the arteries._

In their course the arteries present differences according as we examine, the trunks, the branches and smaller branches.

_Course of the trunks and branches._

The trunks are the first divisions continuous with the two great portions of the aorta; such are above, the internal and external carotids, the subclavians, &c.; below, the iliacs, the hypogastrics, &c. Generally they are situated in broad interstices, that contain much cellular texture, as in the groin, the axilla, the neck, the sides of the pelvis, &c. By dividing they form branches that are received into smaller and narrower interstices, and are consequently more exposed to the influence of the neighbouring organs. Both of them are covered almost every where by a thickness of parts that protects them from external injury. Besides this protection that the neighbouring parts and particularly the muscles afford them, they accelerate also the circulation of the blood, and reciprocally the motion of the arterial trunks gives to the neighbouring organs and even to the whole limb, a sensible motion, an agitation that supports its vital energy. This agitation, which it is often difficult to perceive, sometimes becomes very evident upon mere inspection. When the elbow is rested upon the table, and a body of considerable length held in the hand, its extremity is seen to vaccillate, to rise and fall a little at each pulsation. If the legs are crost, being first bent upon the thighs, a spontaneous rising is noticed in that which is supported. To this we must refer also the cerebral motion, that which is communicated to tumours that are situated over a great artery, &c. &c.

The trunks and the branches are accompanied by veins, and surrounded in general with a large quantity of fat, a circumstance that has been considered favourable to the opinion of those who think that this fluid is exhaled by the pores of the arteries. We have already said what should be thought of this opinion.

The direction varies in the trunks and the branches. Usually straight in the trunks, as in the carotids, the internal and abdominal iliacs, it renders the circulation less evident. When these trunks are exposed in a living animal, we do not see any kind of locomotion there, as we do when the curves are great. There is however some exception to this rule as it respects the direction of the trunks; the arch of the aorta is an example of it, so is the internal carotid, which has numerous curves, which are thought incorrectly, to be necessary to prevent the impetus of the blood from producing derangement in the delicate substance of the brain. More tortuous in the branches, this direction occasions the arterial locomotion that constitutes almost exclusively the pulse, according to many physicians.

_Course of the smaller branches, ramifications, &c._

Whilst the trunks occupy the large interstices that are left between several organs, and the branches the smaller ones that separate particular organs, the smaller branches are found in the interior of these same organs, without, however, entering into their intimate structure. Thus in the muscles, they are interposed between their fibres; in the brain, in the circumvolutions; in the glands, between the lobes that form them, &c. By them, an internal motion communicated to the whole organ, facilitates its functions by supporting its partial activity, as the motion of which I spoke above, supports the general activity of the part. Besides, the sudden cessation of life, when the blood ceases to agitate the brain, proves the immediate connexion between this internal motion and its energy. Thus we observe that life is much more active wherever the arteries are numerous, as in the muscles, the skin, the mucous surfaces, &c.; whilst on the contrary its phenomena are weaker and more obscure in the less vascular organs, as in the tendons, the cartilages, the bones and the other white parts.

In the smaller branches, the windings are much more evident than in the branches. Injections make them very conspicuous, especially in the brain; but as they depend principally upon the cellular texture they disappear in part, if we separate from it the vessels of all the parts. Do these windings diminish the rapidity of the circulation, and does the straightness of the arteries increase this rapidity as much as physiologists suppose? I think they have exaggerated the effects of the direction of the arteries; the following are proofs of it. 1st. If in living animals we expose the hollow organs, as the stomach, intestines, &c. alternately in a state of fulness and in that of vacuity, I have observed that the circulation is almost equally rapid in both cases, though fulness renders almost straight the vessels of these organs, and that emptiness, by forcing them to wrinkle, increases their curves. 2d. I have opened the carotid artery of a dog, and having observed the force with which the blood is thrown out, I have also opened both sides of the thorax; immediately the lungs are collapsed and consequently the windings of their vessels increased; notwithstanding this no diminution in the force with which the blood escapes from the artery, after having gone through the lungs is immediately sensible. It is only gradually that the force is destroyed by the influence of causes, that it is not my object to examine. 3d. If in another animal, an artery being open, we open also the wind pipe, and by a syringe affixed to the opening suddenly exhaust all the air the lungs contain, this organ is immediately reduced to a very small size; the vessels become much bent, and yet I have observed in this case that the blood goes from the open artery with as much force as before, for a considerable time. 4th. Finally, after having opened the abdomen of a living animal, I have alternately contracted and stretched the mesentery, whose numerous arteries had been first opened; no difference is discoverable in the force with which the blood is thrown out, in either case.

Let us conclude from these experiments, that the influence of the direction of the arteries upon the course of the blood, is much less than is commonly thought, and that all the calculations of mathematical physicians upon the delay of the blood from this cause, rest upon unsubstantial foundations. There is no doubt that when the fore arm is strongly bent, the pulse is weakened, stops even, and it is essential when we feel the pulse that the arm should be extended; but this phenomenon does not depend upon the angle the artery forms; it arises from this, that the muscles that press it, contract its caliber and even obliterate it. This is so true, that the different curves of the internal carotid are much more evident than the single curve that the brachial then forms, and yet the circulation is performed there very well. Besides, open an intercostal artery which has but few curves, the force with which the blood will be thrown out is not stronger than it would be from the radial, &c. If the whole arterial system was empty and the blood going from the heart filled it successively, as this fluid would strike against the arterial curvatures, it would undoubtedly experience some delay. It is on this account that in our injections a tortuous artery is slower in filling; as the spermatic for example often remains empty. But in a number of tubes full of fluid, it is wholly different; the impulse received at the beginning of them is suddenly propagated into all the cavities that form them, and not by a successive progression, as I shall say hereafter.

The arterial curvatures are adapted to the different states in which the organs may be found. We see them very evident in those which are subject to an alternate dilatation and contraction, for example in the intestines, the lips, and the whole face. In the fœtus, when the testicle is in the abdomen, the artery is very tortuous. When this gland descends, the artery untwists and takes the straight course it has in the adult. In the motions of the womb, the bladder, the pharynx, the tongue, &c. these curvatures perform an important part in the preservation of these organs. In the fractures of the lower jaw, they prevent the rupture of the artery that traverses this bone, a rupture which the displacing of the bone would produce without them. By them the arterial system is preserved unhurt in the violent and oftentimes forced motions that the limbs perform.

The flexibility of the arteries would be insufficient for these motions; in fact, when an artery is extended longitudinally, its diameter is diminished. By accommodating themselves to the motions of our parts, the vessels would impede then the circulation, because there would be less space for the blood to move in. Hence why the arteries of all the parts subject to alternate dilatations and contractions, being uniformly tortuous, can without the aid of their extensibility, have very different degrees of extent. I would remark upon this subject that the locomotion of the arteries, observed by Veitbrecht, is far more evident at the time of the contraction of the hollow organs, or of the flexion of the limbs, than during the dilatation of the one or the extension of the others. I have invariably made this remark upon living animals. We can by emptying or distending the intestines, the stomach, the bladder, &c. make their arteries beat more or less strong, &c. &c.

_Anastomoses of the arteries in their course._

Anastomosis is the union of many branches, which mingle the columns of blood that each brings. There are two kinds of anastomoses; sometimes two equal trunks unite, sometimes a small trunk is joined to a large one.

The first has three varieties. 1st. Two equal trunks sometimes unite at an acute angle, and form but one; it is thus that the ductus arteriosus and aorta are blended together in the fœtus; that the two vertebrals produce the basilary trunks, &c. &c. 2d. Two trunks communicate at certain places by a transverse branch; such are the two anterior cerebral, before they go between the hemispheres. 3d. Two trunks unite and form an arch; this is the case with the mesenteric; then branches arise from the convexity of this arch. We see then that there are three kinds of anastomoses between equal branches; one of these is that in which two columns of blood are united together and take a direction between the two first; another in which two columns follow their first direction, and only communicate with each other; finally in the last, two columns meet each other by their extremities, in an opposite direction, and the blood escapes afterwards by secondary vessels.

The second kind of anastomoses is that of considerable branches with smaller ones; it is extremely frequent, especially in the extremities; it has no varieties.

It is almost always in regions remote from the heart that anastomoses are met with. We find scarcely any in the trunks that arise from the aorta. They begin to be more frequent in the branches, as in the mesenteric, the cerebral, &c. The more the smaller branches are subdivided, the more numerous are the anastomoses. In the last ramifications they are so numerous that they form an inextricable network. This arrangement is calculated to facilitate the circulation, which the anastomoses favour in places, where the motion of the blood is liable to experience obstacles. It is on this account that in the cavities in which the influence of the neighbouring parts upon the motion is less sensible, the anastomoses become more frequent, as in the brain, the abdomen, &c.; whilst they are more rare in the muscular interstices of the extremities, &c. It is not then a tree with distinct branches that forms the arterial system, but a tree all the parts of which communicate together, more frequently as they are the further removed from the origin.

The principal object of the anastomoses, that of obviating the obstacles the blood experiences in its course, is fulfilled in many cases. Thus, after the ligature of a wounded artery or one with an aneurism, after the spontaneous obliteration of one of these vessels, we see the anastomoses between the fine branches, above and below this obliteration or this ligature, continue the circulation in the part. These collateral vessels then increase often in size; but more frequently still, the course of the blood is supported almost entirely by the capillary vessels.

Anastomoses suppose then the vitality of the arteries. It is because these vessels are not inert, but act themselves upon the fluid they contain, that the circulating phenomena are subject to so many variations; that oftentimes, and especially by the influence of the passions, the spasm of their extremities, principally of the capillaries, obliges the blood to flow back, a reflux which is favoured by the anastomoses. This reflux is necessary also in inflammations, in the different engorgements of our organs, &c. How would the circulation be able to go on, if all the small branches went to their respective destinations, without communicating among themselves? Would not the least embarrassment occasion a troublesome stagnation there?