Part 24

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?

I would remark upon this subject that the anastomoses furnish the first proof of a truth which we shall soon demonstrate more in detail, viz. that in the great trunks, the blood is especially influenced by the heart, and that in the capillaries, it is exclusively by the vascular parietes. In fact it is because the vitality of the arteries is every thing for the motion of the last subdivisions, that the least alterations that they experience give rise to many engorgements that inevitably require anastomoses, which are extremely numerous at the end of the arterial tree. On the other hand, the vitality of the trunks having scarcely any influence upon the blood, it experiences but few obstacles in passing through them; there is less need then of anastomoses, which are in fact more rare there.

If the least cause, the least irritation produced spasm of the trunks, as they produce that of their last divisions, it would be necessary that they should communicate as frequently together. A fleshy texture in the great arteries, and vital properties analogous to the involuntary muscles, would have required inevitably these numerous anastomoses, because a variety of causes influencing these kinds of muscles, they can at any moment increase unnaturally their contraction, diminish their caliber and embarrass the progress of the fluids that traverse them.

_Forms of the Arteries in their course._

Many physicians of the present age have described each artery as forming a cone, the base of which is towards the heart, and the apex towards the extremities. If we examine a portion of it between the origin of two branches, whether after having injected it, or by cutting it perpendicularly when it is empty, or by measuring it when it is full of blood, we find it always cylindrical. Undoubtedly considered in its whole extent, it takes a conical form, the effect of its successive diminution by the branches it furnishes; but in this sense it is less a cone, than a series of cylinders successively joined to each other and always decreasing.

Considered in its general arrangement, the arterial system represents on the contrary, as I have said, a cone absolutely inverted, that is to say, having its base at all the parts and its apex at the heart; so that the aorta has a diameter less considerable in proportion, than that of the sum of all the branches. We have a proof of this by comparing a trunk with two branches that succeed it; these surpass it in diameter, and the relation being always the same in all the subdivisions, we conceive that the capacity of the arterial system goes uniformly increasing.

This relation of the trunks and the branches has been exaggerated however by mathematical physiologists, who attributed to the last over the first a predominance much greater than really existed. A cause of error upon this point may arise from measuring the arteries at their exterior after having injected them; in fact, the caliber of the trunks is greater, in proportion to their parietes, than that of the branches separately examined; that is to say, other things being equal, the aorta has parietes thinner in proportion to its cavity, than the cubital artery; hence, without doubt, why aneurisms are rare in the branches, and frequent in the trunks, especially when the diseases arise from a local cause; for if they are the effect of a general disease, oftentimes the little arteries, the radial particularly, are also affected; I have already seen two examples of it. This observation upon the proportions of the arterial parietes proves the impossibility of judging of the relations of diameter between the two, at least by examining them in the interior.

Besides, these relations are necessarily very variable, according as the vital forces which vary themselves so much, increase or contract the caliber of the small arteries; and in this point of view, this examination cannot have the importance that was attached to it by the ancients, whose works are filled with calculations upon this point.

III. _Termination of the Arteries._

After being divided, subdivided, and having the peculiarities we have just examined, the arteries terminate in the general capillary system. To point out where this system begins, and where the arteries end, would be very difficult. We can prove that there the blood ceases to be entirely under the influence of the heart, and circulates by the influence of the insensible organic contractility of the vascular parietes; but how can this line of demarcation be rendered evident to the eye?

Authors in treating of the termination of the arteries, have considered their continuity with the excretories, the exhalants, the veins, &c.; but it is evident that the general capillary system is between the arteries and these vessels. Thus I shall treat of their origin in speaking of this system, which is spread in all the organs, but has essential differences according to the different systems, under the relation of its continuity with the arteries. In fact, 1st. there are systems in which these vessels are distributed in great quantity, and in which consequently the general capillary system contains much blood; such are the glandular, the mucous, the cutaneous, the animal and organic muscular, &c. 2d. Other systems receive but few arteries, as the osseous, the fibrous, the serous, &c. and consequently have but little blood in circulation in that portion of the general capillary system that belongs to them. 3d. Finally the pilous, epidermoid, cartilaginous systems, &c. destitute of arteries, contain only white fluids in the division of the general capillary system that has its seat in them.

ARTICLE THIRD.

ORGANIZATION OF THE VASCULAR SYSTEM WITH RED BLOOD.

I. _Textures peculiar to this organization._

The red blood circulates, as I have said, in a membrane arranged in the shape of a great canal, variable in its form, extended from the pulmonary capillary system to the general one, and having every where the greatest analogy. At the exterior of this membrane, nature has added a fibrous coat for the arteries, fleshy fibres for the heart, and a peculiar membrane for the pulmonary veins. I shall speak here only of the arterial coat. The fibres of the heart and the membrane of the pulmonary veins will be examined, one in the organic muscular system, the other in the system with black blood. As to the internal membrane of the arteries, which is also that of the whole system with red blood, we shall examine it in a general manner.

_Peculiar Membrane of the Arteries._

This membrane is firm, compact, very apparent in the great arteries, less evident in the last divisions where it is insensibly lost. Its colour is usually every where the same. If the branches appear red in living animals, and the trunks yellowish, it arises only from the transparency of the one which allows the blood to be seen, and the opacity of the others. The colour of the arterial fibre is yellowish. However it assumes in certain cases a greyish appearance. I have often observed in arteries exposed to maceration, that it reddens in a very evident manner at the end of some days, or rather that it takes a rosy tinge, very analogous to that of the cartilages of the fœtus and of the fibro-cartilages of the adult, submitted to the same experiment. This result is however less uniform in the arteries than in those two systems, in which it is never absent. Sometimes the internal membrane reddens also, but never the external or cellular; on the contrary, the longer this remains in water the whiter it becomes. When the fibrous coat of the arteries has continued some time with this redness, it gradually loses it, if maceration is continued. This phenomenon is often more evident in the branches than in the trunks. For example, the arteries at the base of the cranium become red very frequently in the dead body, by remaining in the fluids with which this part is moistened. We see, in opening the cranium, this redness which does not belong to the blood left in the arterial cavities, as we may be easily convinced.

The thickness of the peculiar membrane of the arteries is very evident in the great trunks. It constantly diminishes; a circumstance that distinguishes it essentially from the internal membrane, which I have found almost as thick in the tibial artery as in the aorta. It has been thought that in certain arteries, as in the cerebral, the fibrous coat is entirely wanting. There is no doubt that in the vertebral and internal carotid it is thinner in proportion than in equal trunks situated in the muscular interstices; but by examining attentively these arteries, I have clearly distinguished circular fibres in them. Has the thinness of their parietes an influence upon the sanguineous effusions, which are, as we know, so frequent in the brain? I cannot say. These effusions take place only in the capillaries, the trunks are never the seats of them; now it is impossible to examine these capillaries. I have sought in vain to ascertain by injections the vessels torn in apoplexy. Besides, this hemorrhage does not resemble that of the serous membranes; it is not an oozing through the exhalants of the ventricles; for these cavities are very rarely the only seat of it. Almost always these effusions take place even in the cerebral substance, generally nearer the posterior than the anterior lobe. The cerebellum is rarely affected by it. When the tuber annulare becomes so, there are often small partial effusions there, separated by medullary partitions that remain uninjured.

As to the arteries of the other parts of the body, their peculiar membrane presents generally a pretty uniform arrangement. It has appeared to me however, that in the interior of the viscera, of the liver, of the spleen, it is rather thinner than in the intermuscular spaces, and even than in the muscles.

This membrane is composed of very distinct fibres, adhering to each other, easily separated however, arranged in layers, in such a manner that after having raised the cellular covering, we can without difficulty separate these different layers from each other; it is this that has made many authors believe that the great arteries were composed of a great number of coats. The fibres that form these layers are circular or nearly so; the external ones appear to be attached to the compact cellular texture that is contiguous. In fact, by raising this, a number more or less considerable adheres to it always in an intimate manner. As to the internal membrane, it does not appear to furnish any attachment; we raise it easily, without bringing with it any arterial fibres. The manner of the adhesion of these fibres with the compact neighbouring texture, appears to me to have great analogy with the origin of the organic muscular fibres, which are attached in a great number of places, to the sub-mucous texture.

When a branch arises from a trunk, the circular fibres of the last separate and form on each side a half ring, whence arises a complete one, which embraces the small rings formed by the circular fibres of the arising branch. These circular fibres go even to the eminence of the common membrane, which is seen within the arterial cavity and of which I have spoken; so that the whole thickness of the peculiar membrane serves as a support to their origin. But there is but little continuity between the two kinds of fibres. Those of the branch do not arise from those of the trunk; it is the internal membrane that serves to fix them together, as fibres of communication. Dissection shows easily these branches set at their origin in the ring which arises from the separation of the circular fibres. We remark this at the origin of the intercostals and lumbars upon the aorta, &c. When two trunks of an equal size go off, as the iliacs, the last circular fibres of the primitive trunk which they formed, interlace intimately with the origin of each of the two circular layers, that arise at the fork that separates this origin. Thus the last rings of the aorta cannot be separated from the first of each iliac.

There are no longitudinal fibres in the arteries.

What is the nature of the arterial fibre? Almost all anatomists have considered it the same with the muscular. But if we examine them attentively, it is easy to be convinced of their differences. The want of red colour does not establish these differences, since in man even, some parts really muscular, as the intestines, want this colour. But the muscular texture is soft, loose and very extensible; the arterial texture on the contrary is firm and solid, breaks before it yields. We can observe this, by tying an artery tight. The two internal coats are cut; the cellular alone is not, though the ligature is immediately applied to it; we observe, by opening the artery, a section corresponding with the thread, exactly similar to what a cutting instrument would have made.

I have often repeated this experiment, pointed out by Desault, upon the dead body, and upon living animals; the result which is very uniform, explains the frequency of hemorrhages after the operation for aneurism. There is undoubtedly no texture so brittle, if I may use the word, as the arterial, none consequently less proper to be embraced by ligatures. Why is it that this should be the only one in which it is necessary to apply them? This phenomenon alone would distinguish the arterial texture from the muscular. In fact, the preceding experiment, made upon a portion of intestine in which the fibres are arranged like the arterial, would produce a flattening, an approximation of these fibres, but would not cut them.

Moreover compare the properties of texture of the arteries with those of the muscles; compare their vital properties, by examining the articles in which I treat of these properties; compare their development, and especially the different morbid alterations to which the two are subject, and you will see that there is not a single relation in which they have the least analogy. The aneurism of the heart and that of the arteries have nothing in common but the name. In one there is a rupture of the arterial fibres, a dilatation of the cellular coat; in the other an unnatural increase, a real development of muscular fibres which preserve their appearance and their properties.

Notwithstanding the ease with which the arterial fibres are broken in cases of aneurism, they enjoy in a natural state a very considerable degree of resistance and force; another character that distinguishes them from the fleshy texture. The following are the proofs of this resistance, which takes place both transversely and longitudinally. 1st. If we tie the carotid artery above, and drive a fluid afterwards into it, great force must be employed to break its texture. The same thing happens when we force in air instead of a liquid. Frequently the efforts of a man are insufficient to produce a rupture; thus the force of the heart can never cause it suddenly; so that the formation of aneurisms takes place only from the long continued action upon the arterial parietes; I doubt whether these tumours can be formed, without a previous alteration of the arterial texture, by the force of the impulse of the blood alone against the weak parietes of the arteries. 2d. The resistance of these parietes takes place longitudinally also. If we draw in a contrary direction, the two ends of an artery and of a muscle, we effect with more difficulty the rupture of the first, when the dead body is the subject of this comparative experiment. But upon the living the effect is opposite; the vessel yields to a very strong action made upon it; it would be necessary that this action should be incomparably greater to divide the muscle. This difference arises evidently from the vital properties of the latter, which in this case contracts violently, whilst the artery can make no further resistance than from the nature of its texture. Besides, this longitudinal resistance to distension is less than the lateral resistance opposed to the injection; experiments prove it, and it arises without doubt from this, that no fibre, in the first case, is found directly opposed to the effort.