Part 31

The general vascular system with black blood arises as we shall see, from the whole of the great capillary system, is collected towards the heart in great trunks, and terminates in the pulmonary capillaries. As the portion of the heart that belongs to it will be examined hereafter, and the pulmonary artery, by its peculiar membrane, has great analogy with the peculiar membrane of the other arteries, we shall now particularly examine the veins; but we shall describe in a general manner, the common membrane that is spread upon the whole system with black blood.

ARTICLE FIRST.

SITUATION, FORMS, DIVISION AND GENERAL ARRANGEMENT OF THE VASCULAR SYSTEM WITH BLACK BLOOD.

We shall now examine the veins as we have the arteries, in their origin, course and termination. Only we shall do it inversely, to accommodate our ideas to the course in which the blood flows in their channels.

I. _Origin of the Veins._

This origin is in the general capillary system. I shall point out in this system, how the veins are continued with the arteries. I would only remark here that these vessels never arise from any organ that the arteries do not enter, as the tendons, the cartilages, the hair, &c.; which evidently proves that the blood is not formed in the general capillary system; it leaves there the principles that made it red, it perhaps acquires there new ones; it is modified in fact, but never created.

It is not as easy to distinguish accurately the veins at their exit from this system, as it is the last arteries at their entrance into it, because the valves prevent injections from penetrating so far. It is in subjects that have died of asphyxia, apoplexy, &c. that the venous ramifications can be best observed. We see then that they are soon divided into two orders; one accompanies the last arteries, the other is distinct from them.

In the greatest number of organs, venous branches go out at the same place that the arteries enter. There are however some exceptions to this rule. In the brain, for example, the arteries enter below, the veins go out above. In the liver, the first enter below, the others go off behind, &c. This circumstance is, in general, indifferent to the circulation, which goes on the same whatever may be the relation of the arteries with the veins. In those places where the small veins go out at the same time that the small arteries enter, sometimes more or less of cellular texture serves to unite the small vessels that are in apposition, sometimes there is a space between them, as in the muscles, the nerves, &c.

Besides the venous origins corresponding with the arterial terminations, there is an order of veins which is separated from the arteries at the exit from the capillary system. This order is particularly remarkable at the exterior of the body. We see that all the organs that are found there furnish, 1st, veins that go to the interior to accompany the arteries; 2d, others that go to the exterior to become sub-cutaneous, and to form trunks of which we shall soon speak. In many internal organs, the same venous division is observed.

It follows from this general arrangement, that many more veins go from the capillary system than there are arteries that enter it. This is the principle of the disproportion of the capacity existing between the system with red blood, and that with black, a disproportion of which we shall soon speak.

The veins at their origin frequently communicate among themselves. We see many little spaces that arise from their interlacing, in the places where they can be seen, as under the serous surfaces, &c.

II. _Course of the Veins._

At their exit from the general capillary system, as we have just said, the veins are differently arranged. 1st. In the extremities and external organs of the trunk, they form two sets, the one interior, which accompanies the arteries; the other exterior, which is sub-cutaneous. 2d. In the internal organs we frequently make a similar observation; thus there are superficial veins of the kidney, and deep ones, that accompany the arteries; but oftentimes all the veins unite themselves to those that follow the artery.

The cutaneous portion of the veins is very remarkable in the extremities, where there are considerable branches, viz. the saphena in the lower, the cephalic, basilic, and their numerous divisions in the superior. In the trunk and the head, we do not see any great sub-cutaneous branches, except the external jugular in the neck; but there is a number of smaller branches proportioned to the minuter ones that are distributed there.

The external parts, then, are remarkable by the predominance of the trunks with black blood over those with red. Oftentimes these trunks can be traced through the integuments, upon which they show themselves much more than those that are whiter and more delicate; they have, besides, no connexion with the tinge that colours them, which arises from the blood contained in the general capillary system.

In the interior of the body, the veins almost every where accompany the arteries; they follow the same distribution; so that they are not commonly described, because the course of the arteries is sufficient to represent theirs. Usually a common cellular space receives both the trunks of the two sorts of vessels and those of the nerves. Sometimes, however, the veins are separate, as the azygos, for example, which has no corresponding arterial trunk, and which on this account requires in descriptive anatomy, like the superficial ones of the trunk and the extremities, a particular examination and an accurate dissection to obtain an idea of it.

The deep-seated veins have a caliber much more considerable than that of the arteries; most usually they are more numerous, as in the extremities, where each artery is almost always accompanied by two veins.

III. _Proportion of the capacity of the two systems with Black and Red Blood._

After the observation I have just made upon the origin and course of the veins, it is evident that their sum total has a capacity much greater than that of the arteries. This assertion it is easy to prove in detail, wherever there is an artery and vein united, as in the kidneys, the spleen, the extremities, &c.; where the arteries are separate from the veins, as in the brain, the liver, &c. it is not less sensible. Finally, there is, as I have just said, a division of sub-cutaneous veins, which is evidently one more than the arteries have.

Many physiologists have endeavoured to calculate the relation of capacity between the two systems with black and red blood; but this relation varies too much ever to be the subject of any calculation. Is it upon the dead body that the attempt is made? But the veins will be more or less dilated according to the kind of death; in apoplexy, asphyxia, drowning, &c. they have a diameter almost double that which they exhibit when the subject has died of hemorrhage, because the first kind of death accumulates much blood in the veins, and the second deprives them of it. We can give a greater or less capacity to the veins of an animal, according to the manner in which we kill him, as we can enlarge or contract the right cavities of the heart by similar means. You can never find the veins exactly equal in any two subjects, though there may be a great resemblance as to size, age, &c. Is it upon a living animal that the attempt is made? But, besides its being very difficult, you will not then have a result uniformly applicable, because the veins vary in diameter as they are more or less full. Examine these vessels in subjects in whom you can see them by the transparency of the integuments; sometimes they are more, sometimes less apparent; their size sometimes appears double, at others, hardly distinguishable. Certainly after drinking copiously, by which the black blood has received a great augmentation of its fluid, the vessels are more dilated than in an opposite state. The veins are remarkably contracted after death from hunger. I have often observed the same phenomenon in dropsies, phthisis, marasmus, &c. Always when the mass of blood is diminished, the veins contract by their contractility of texture. The arteries are infinitely less subject to variations of diameter, on account of their firm and compact texture, though, however, they show much of it.

Let us reject, then, every kind of calculation upon the proportions of capacity of organized canals. We can only calculate what is fixed and invariable; but that which varies at every instant can only be the object of general assertion. Besides, of what importance are the rigorous proportions that some physicians have endeavoured to establish between our parts? They are nothing in the explanation of the phenomena of health and disease. Let us, then, be content with this general assertion, that the venous capacity surpasses the arterial. It may be said, that in a given time there is more blood in one than the other.

The same observation applies to the two sides of the heart, one of which belongs to the same system with the veins, and the other to the one with the arteries. The right has commonly a greater capacity than the left, not precisely under the relation of the fleshy texture, but under that of the fluid that distends it; this is so true, that if in an animal whose thorax is opened, the blood is made to accumulate in the left side by ligatures, and the right is emptied by puncturing it, the last will be less in size than the first. Always when we find it larger than the other in the dead body, except in diseases of the heart, it is because it contained more blood at the moment of death; in fact, as this fluid ordinarily stops first in the lungs, it flows back to this side of the heart, which is almost always the largest.

This is the great difference between inert cavities and those that possess life, viz. that these last can change their capacity every moment, whilst the others remain always the same. In the living animal, the right side of the heart has almost always a greater capacity than the left, because the quantity of blood it contains is greater.

There are, then, two things generally true, viz. 1st, that the great tree that terminates the system with red blood is in general of less capacity than the great tree that commences the system with black blood; 2d, that the same observation is applicable to the two sides of the heart, which correspond with these two trees.

As to the tree that terminates the system with black blood, compared with that which commences the system with red blood, the same thing does not hold true. The pulmonary artery and the veins of the same name exhibit a disproportion of capacity, less it is true, than in the other parts, a disproportion which is real, however, and which, notwithstanding what many authors have said, is in favour of the veins. How does this happen? it would seem that since the one is continuous with the veins and propels the same fluids, it ought to have the same proportion of diameter; and that since the others are continuous with the arteries, that they also should be in proportion to them. This arises from the difference of the velocity of the blood; in fact, this fluid circulates quicker in the pulmonary artery than in the veins of the same name, since it has the impulse of the heart, which these last want; then, in a given time, as great a quantity of blood passes through it, though the diameter of this artery is smaller; what do I say? if it was equal, the circulation could not go on. In the same way, if the aorta equalled in capacity the two venæ cavæ and the coronaries united, and the blood had the same velocity there, the circulation would cease.

The four pulmonary veins united are a little larger than the aorta, which, however, transmits all the blood received from them. Why? Because the impulse that the left ventricle communicates, makes, in a given time, more blood pass by the aorta than by the four pulmonary veins. These two things, 1st, the velocity of the fluid; 2d, the capacity of the cavities in which it circulates, are then in an inverse order in the two opposite trees that form each vascular system. In that with red blood, there is less velocity and greater capacity from the pulmonary capillary system to the agent of impulse; and from this agent to the general capillary system, there is, on the contrary, greater velocity and less capacity. In the vascular system with black blood, there is less velocity and greater capacity from the general capillary system to the agent of impulse; and from this agent to the pulmonary capillary system, there is more velocity and less capacity. Without this double opposite arrangement, it is evident that the circulation could not take place.

There is, however, a remark to be made upon this subject; it is, that the capacity of the four pulmonary veins united, is not so much larger than the aorta as that of the venæ cavæ and the coronary is than the pulmonary artery; and this is the reason of it; as the pulmonary veins run a very short course, the impulse, on the one hand, that the red blood has received from the pulmonary capillary system, is preserved there more; on the other hand, this fluid is there free from numerous causes of delay that the blood experiences in the venæ cavæ and coronaries; then the velocity is greater there, and the capacity should be therefore less. If the lungs were situated in the pelvis, the pulmonary veins would certainly have a greater capacity, because having a greater extent to go over, the velocity of the blood would be more retarded.

We can easily understand now the cause of many arrangements that have engaged the attention of anatomists; viz. 1st, why the sum of the arteries coming from the aorta has less capacity than that of the veins going to the right auricle; 2d, why the four pulmonary veins surpass also in diameter the artery of the same name; 3d, why these four veins are not exactly in proportion with the aorta, which is really a continuation of them; 4th, why the venæ cavæ and coronaries are so disproportioned to the pulmonary artery, which is, as it were, their continuation.

If there was no agent of impulse in the two systems with red and black blood, their capacity would be every where nearly the same, because the velocity of the fluid would be every where nearly the same. This is precisely what happens in the system with black abdominal blood, in which the hepatic portion of the vena porta is nearly as large as the intestinal one, because there is no heart between the two.

The velocity is less in the general veins and in the pulmonary, because they have not at their extremity an agent of impulse; we see there only a capillary system. The opposite reason explains the velocity of the course of the blood in the general arteries and in the pulmonary. We have seen in the preceding system, that the presence of an agent of impulse at the origin of the two great arteries, requires there a considerable resistance of this texture, whilst the absence of this agent requires but little resistance in the veins.

We see, then, clearly, why these three things, 1st, weakness of the parietes; 2d, slowness of the motion; 3d, great capacity, are the attributes of the veins with black and red blood; why these three opposite things, 1st, strength of the parietes; 2d, velocity in the motion; 3d, less capacity, characterize the arteries of both sanguiferous systems.

We see also from this why, though the red and black blood form in their whole course a continued column, though the common membrane over which they pass may be in the whole extent of each system nearly the same, the organs exterior to this membrane are, however, very different.

The inverse ratio of the velocity of the motion with the capacity of the vessels, appears to me so evident, that we might be able to judge nearly by the inspection of the vessel of the velocity of the blood that runs through it, if many causes did not, as I have said, make the vascular parietes vary at the moment of death. We know that all the causes that lessen in the veins the velocity of the blood, increase their capacity; it is thus that we make them prominent by ligatures; that pregnancy enlarges those of the inferior extremities, that long-continued standing produces the same effect, &c.

It is to the same reason that must be referred the explanation of the following phenomenon; viz. that the relation of the arteries and the veins is not every where the same; thus the renal, bronchial, thymic veins, &c. are in general smaller in proportion to their arteries, than the veins of the spermatic cord in proportion to the artery of the same name, than the hypogastric veins in proportion to the corresponding artery. The blood has less difficulty in circulating in the first than in the second, where it rises against its weight; hence why the veins of the inferior parts, especially at a certain age, surpass their arteries more in diameter, than those of the superior parts exceed theirs.

_Ramifications, Small Branches, Branches, Angles of Union, &c._

The veins present in their course, as it respects branches, smaller branches and ramifications, an arrangement analogous to that of the arteries, except that it takes place inversely. The ramifications are nearest the origin; they soon unite into smaller branches, these into branches, and these last into trunks.

The ramifications and most of the small branches are found in the interior of the organs. The first make an integral part of these organs, and are between their fibres, &c.; the second lie in their great interstices; in the glands between the lobes, in the brain, between the circumvolutions, in the muscles between the fasciculi, &c.

In going out of the organs, the small venous branches run into the branches, which take, as we have seen, two positions, one sub-cutaneous, the other deep. The sub-cutaneous branches go in the extremities between the aponeurosis and the skin, in the trunk between this and the cellular layer that covers the muscles. The deep branches lie in the interstices that the organs have between each other, accompanying almost every where the arteries. The cerebral branches have a peculiar arrangement; they are placed in the interstices of the dura mater, and form with them what are called sinuses.

The venous branches differ from the arterial in this, that they are infinitely less tortuous; this is remarkable under the skin and in the interstices of the organs. This is a reason that would prevent locomotion, supposing that there was an agent of impulse at the origin of the veins, and that their parietes were not so loose. Hence a series of arterial tubes is really longer than a corresponding series of venous tubes; this facilitates the motion of the black blood, which has a less extent to go over, and which besides would find causes of delay in the curvatures, greater than the red blood, because this is driven by a strong agent of impulse, and the other is not.

The venous branches unite to form a certain number of trunks that are connected with those that are immediately discharged into the right auricle; these trunks are the internal jugulars, the iliacs, the azygos, the subclavians, &c. They are still less tortuous than the branches; they have, like the arterial trunks, deep positions, far from external agents, from which many organs defend them, as a hemorrhage from them would be followed with serious consequences.

The trunks, branches, smaller branches and ramifications do not always arise necessarily from each other, in the manner we have just pointed out. The branches are often united to the trunks, the ramifications to the branches, &c. &c.; as it is with regard to the arteries.

The angles of union vary; sometimes they are right angles, as in the lumbar, renal veins, &c.; sometimes they are obtuse, as in some of the intercostals; most commonly they are acute.

The arrangement of the smaller branches and the branches is as variable at least in the veins as in the arteries; they partake, in this respect, of the general character of irregularity that the organs of internal life exhibit. It is necessary only to attend to the general position and distribution of the branches, smaller branches, &c. Their union with the trunks and among themselves, is different in almost every subject.

_Forms of the Veins._

The same observation may be made upon the forms of the veins as upon those of the arteries.

1st. A trunk, branch, &c. are cylindrical, when examined in a space where they receive no branch. In the dead body they appear flat, which arises from the collapse of the parietes, and this is owing to the absence of blood. But by distending them with air, water, &c. they take their primitive form. In the living body they appear round.

2d. Examined in a considerable extent, a venous branch appears conical, so that the base of the cone is towards the heart and the apex towards the general capillary system. This form arises from the smaller branches, that are successively united to this branch, and increase its capacity as it approaches the heart.

3d. Considered as a whole, the venous system represents three trunks; one corresponds with the vena cava superior, another with the vena cava inferior, and the third with the coronary vein; these three trunks have their apex at the auricle and their base at the general capillary system. Anatomists thus represent the whole of the veins, because the sum of their divisions, like the arteries, has a greater capacity than the trunks from which these divisions arise.

There is however an observation to be made upon this subject, and that is, that the relation between the trunks and their divisions is not as exact in the veins as in the arteries; thus the sum of certain divisions considerably surpasses their trunks, whilst this relation is infinitely less in other cases. But all this arises still from the extreme variation of the venous parietes, according to the quantity of blood they contain; thus in dead bodies, sometimes the branches are much dilated by this fluid, and the trunks remain the same; sometimes an opposite phenomenon is observed. 1st. This last takes place especially when the lungs are obstructed; then in fact the blood flows back to the right cavities of the heart, then into the great corresponding venous trunks; these are then almost equal in capacity to the divisions they furnish, sometimes they even surpass them. 2d. When in the living subject, a limb has been situated for a long time perpendicularly, when standing has been long continued, for example, then the branches are more dilated than the trunks. Now as these causes of dilatation vary ad infinitum, the dilatations themselves are very variable.

From these varieties in the dilatation of the venous branches and trunks separately, it is evident that the relation existing between them is extremely variable, that it is affected by the manner of the death, by the diseases that have preceded it, by the habits of the subject, &c. Let us disregard then, upon this point, as upon every other, all calculations, even if they have a solid basis, if they do not lead to a useful result.

Injections are also a deceptive means of estimating this relation; in fact, they dilate the trunks much more than the branches, and especially than the smaller branches. The internal jugular injected, for example, becomes of an enormous size when compared with the sinus that empties into it. The two venæ cavæ, the azygos, the subclavians, &c. dilate a little less than the jugular; but their size however is remarkable when they are injected, in comparison with that of their branches injected.

_Anastomoses._