Part 34

The vein is then sensibly dilated; then it contracts. But if you apply the finger above, you do not experience a sensation analogous to that of the pulse; you will perceive only a wave of blood which flows back. The reason of it is plain; 1st, there is no locomotion; 2d, as the venous parietes are loose, they could not strike the finger sufficiently strong, if there was a similar change of place. Observe, that it is less the blood than the artery itself which by its firm texture gives the sensation of the pulse; if it could straighten itself when empty, as it does when it is full, it would produce nearly the same sensation; this is a remark that should be added to what I have said upon the pulse in the preceding system.

The contraction of the veins in the motion of the reflux, of which we are treating, is only the contractility of texture. When the heart ceases to propel the blood in its cavity, it contracts, after having been dilated; it is nearly the same in the dead body, in which we fix a syringe in the veins; when they are very full of water, if we draw back the piston a little, immediately the fluid returning, the vein contracts; it is as when it contracts from a puncture that evacuates the blood; this does not imply any irritability.

I believe that sometimes this reflux may depend upon an irregular motion of the heart, which contracts in an opposite direction to the ordinary one, though there is no obstacle in the lungs. What induces me to think so, is, that frequently in experiments, at the moment the animal begins to suffer much, the reflux takes place before the lungs have had time to be disturbed. A very remarkable thing in experiments is the quickness with which pain disturbs the motion of the heart, accelerates it, renders it irregular, &c. We can always at will hasten respiration, by making the animal suffer; now the acceleration of the pulse is always prior to that of respiration, which appears to be determined by it. I am persuaded that if the diseases of the heart were as frequent on the right side as the left, they would often produce this reflux and this pulsation of the veins.

The limits of the reflux of the venous blood vary. Haller has observed it as far as the iliacs. In general, it rarely goes beyond the great trunks, on account of the valves. I have demonstrated in my Researches upon Death, that the colour of those who die of asphyxia, of those who are drowned, &c. does not depend on this, because it cannot evidently extend to the capillary system, which receives the black blood that colours it, from the arteries that then circulate that kind of blood.

The reflux of the black blood in the veins, produced in the preceding cases, either by an obstruction in the lungs, or by a sudden derangement in the action of the heart, takes place in a natural state, though in an infinitely less degree. In fact, when the right auricle contracts, all the blood does not pass into the corresponding ventricle; the veins being open, a portion flows back into them. It is difficult to determine the extent of this natural reflux, of which all authors have spoken. When the thorax is opened, we observe it distinctly; we might then ascertain its extent; but in this case, respiration not being performed as usual, it is evident that we cannot judge by it of what ordinarily takes place.

_Insensible Contractility._

This property, which, like the preceding, is inseparable from the organic sensibility, exists in the veins as in the other parts; it presides only over nutrition; it appears more evident than in the arteries; at least the diseases which particularly increase it are more frequent in the veins. The texture of these vessels is often inflamed. 1st. Bell relates cases of it, the effect of external violence. 2d. Every one is acquainted with the inflammation of the hemorrhoids. 3d. The cicatrization of venous wounds after bleeding is a product of inflammation. Without doubt this cicatrization is promoted by the want of impulse, to which the arteries are subjected; but certainly these last would not in like circumstances heal so fast, if they did at all. When an artery has been tied, it is necessary that its parietes, inflamed by the action of the thread, most often cut by it, and brought into contact, should form adhesions, that the cure may be complete, and the ligature come off without danger. Now, nothing is more difficult and slower than their adhesion, from the difficulty with which the arterial texture inflames. Hence the frequency of hemorrhage after the operation for aneurism and other great operations. The blood often bursts out at the end of twenty, thirty, or forty days; the surgeon should always be upon his guard when he has tied these great trunks, from the want of disposition in the arterial texture to inflame. Frequently when the artery is obliterated, it is not by inflammation. Whilst the ligature stops the blood, the portion of artery comprised between it and the first collateral branch, closes gradually by the contractility of texture, and forms a kind of ligament, which arrests the blood after the thread has fallen off. I do not know but that these cases are more numerous than those of inflammation. Now the veins always adhere soon when they are tied; their wounds cicatrize immediately. In great wounds it is almost always useless to tie them at the first moment, on account of the valves, as I have said above, and afterwards, because the cut ends contract, and soon inflame and adhere. If there are venous hemorrhages, it is at the time of the injury, and not as long after as in the arteries.

Every thing proves, then, that the vital activity is much greater in the venous than in the arterial system, in respect to the tonic powers. The absence of the cellular texture in the second and its presence in the first, may have an influence upon this phenomenon.

_Observations on the motion of the Black Blood in the Veins._

From what has just been said, it appears, that the blood is beyond the influence of the heart when it arrives in the veins. It is evident, then, that the veins can have no pulse. 1st. This phenomenon depends upon a single impulse, suddenly received by the contraction of the left ventricle; now, the blood is poured from all parts by the capillary system into the veins, this agent of impulse is wanting; the cause of the pulse does not exist in the veins. 2d. The necessary conditions for its production in the texture of the vessels in which it takes place, are elasticity and resistance, which are also wanting in the veins. They are only susceptible, then, either of a pulsation which occasions the reflux of the blood in the derangement of the lungs, or in the irregular motions of the heart, or of an undulation of which they are the seat, when arterial blood accidentally circulates in them; now, in either, the heart is the principle of motion, and it could not exist without it.

This is what takes place in the venous motion. The capillary system, by its insensible contractility, pours continually into the venous system a certain quantity of blood. This fluid, added to what is already there, communicates a general motion to it. Now, as the whole venous system is constantly full, it is necessary that while the fluid enters at one side it should go out at the other; if not, the venous parietes would dilate; but, as they have a resistance by which they can act to a certain point upon the blood, this fluid not being able to dilate the veins, flows towards the heart.

The impulse given by the insensible contraction of the capillary system, is too weak, however, to extend instantaneously from one extremity of the veins to the other, especially where the blood rises against its weight. As this fluid enters these vessels, the weight of that which is before it not being overcome, it would produce a general dilatation, and the blood would not reach the heart; but the valves counteract this, by supporting at short distances the column of blood. Weakness of the venous parietes and the existence of valves are necessarily connected. If the veins were as strong as the arteries, unable to dilate when the blood enters them, they would necessarily transmit the surplus to the heart, if they were destitute of valves; but on the other hand, their circulation would be every instant embarrassed.

It appears that it is not only the insensible contraction of the capillary system which propels the blood in the veins; but that the ramifications of these vessels have a kind of absorbent power, by which they draw blood into this system. Now the insensible motion produced by this power tends evidently from the ramifications towards the trunks, as in the lymphatics; then, when, on the one hand, the blood is propelled in the veins, and, on the other, as it were, attracted by them, it is evident that the primitive source of motion that it obeys, is in the capillary system.

This impulse communicated to the blood, exceeds but very little the resistance which this fluid experiences in its motion; so that the least resistance deranges this motion. Hence, as we have seen, the necessity of anastomoses. Hence also the necessity of other assistance to aid this motion, such as, 1st, the muscular action, the influence of which we cannot doubt, when we see the flow of blood in venesection accelerated by the motion of the muscles of the fore-arm, the palpitations of the heart, produced by the blood that flows there after a rapid circulation; when we observe that varices are as rare in the veins situated among the muscles, as they are common in the sub-cutaneous ones, &c.; 2d, the pulsation of those arteries which are in many places joined to the veins, and which communicate to them a kind of motion; 3d, the motion of certain parts, like that of the brain, which continually rising and falling, accelerates the circulation of the blood of the sinuses in an evident manner; so also the constant locomotion of the gastric viscera, propels it in the veins of the abdomen, and that of the pectoral viscera, in those of the thorax. It is so true that the veins derive assistance to their circulation from external motions, that if a limb is a long time immoveably fixed when fractured, these vessels often dilate. 4th. External frictions, if they are not so violent as to embarrass the venous circulation, evidently facilitate it; this is one of the advantages of dry frictions. 5th. A slight compress, not sufficient to check the venous blood, often promotes its circulation, when the external organs are weakened. We know, since the time of Theden and Desault, the advantage of tight bandages, for varicose ulcers, even for varices, &c.

Since the principle of the motion of the venous blood is generally spread throughout the whole general capillary system, instead of being concentrated, like that of the arteries, in a single organ, it is evident, that this motion cannot be uniform, that it must vary according to the state of the capillary system in the different parts; that it can be more rapid in some veins, and slower in others. This is in fact what we see, especially externally where the veins are more or less swelled, according as the blood circulates there more or less rapidly. In the arteries on the contrary, the motion is every where the same; it is a general and sudden shock, an impulse, which, every where felt at the same time, is necessarily every where uniform; so you never see some arteries more full, others more empty, as it happens in the veins.

There are numerous researches to be made on the motion of the blood in the veins. Notwithstanding all that authors have written upon this question, there is an obscurity in it in which we perceive but few rays of light. These difficulties arise from this, that we do not know precisely what is the kind and form of motion communicated to the blood in the capillary system, what is the influence of the vascular parietes upon this fluid, &c. &c. Our knowledge upon this point is reduced to certain views which I have just presented, and which are particularly relative to the parallel between the motion of the blood in the veins and the arteries. I believe that this parallel carried further at some future day, will throw much light upon the venous circulation; in fact, as the first motion is much more easily understood than the second, we must proceed from what is known to what is unknown, and place in opposition what we are acquainted with in one, with that which we seek to know in the other. This is the summary of this parallel, though imperfect; 1st, General pulsation in the arteries, absence of this general pulsation in the veins. 2d. Rapidity of the course of the blood in the arteries; slowness of the same course in the veins. 3d. Greater capacity and thinner parietes in the veins; less capacity and greater thickness in the parietes of the arteries. 4th. Necessity for accessory assistance for the venous circulation; the inutility of this assistance for the arterial circulation. 5th. The blood flowing per saltem, from the second, the uniform flow from the first. 6th. The susceptibility of the blood in the veins, to be influenced by its gravity and other accessory causes; there is some of this influence in the arterial motion. The following are the phenomena, which, from what we have just said, evidently depend upon the existence of an agent of impulse at the origin of the arteries, and of the absence of this agent at that of the veins.

1st. Constant uniformity of the motion in the arteries; variety of motion in every part of the venous system; 2d, dilatation and contraction generally the same in all the arteries of dead bodies; extreme variety in this respect in the veins of the different parts; these are the other phenomena which arise from the unity of impulse in the first, and from the varieties of the principle of the motion of the blood in the second, &c.

Some authors have insisted much, in explaining the causes of the difference of the arterial and venous motion, upon this, that in the arteries the blood is propelled in decreasing vessels, to the capillary system that resists; in the veins on the contrary it flows in vessels always increasing till it arrives at the right auricle, which offers no resistance. But the black abdominal blood is also carried without the agent of impulse, in a series of decreasing tubes to the capillary system of the liver, and yet the motion is analogous to that of the veins.

_Sympathies of the Veins._

The sympathies of the veins are very obscure, like those of the arteries. As the textures of these two kinds of vessels are rarely affected, as inflammation and the different kinds of tumours do not frequently exist in them, and as they are hardly ever the seat of pain, we know but little of the influence they exert upon the other textures. However when we transfuse substances into the vessels, we have often seen acrid and irritating ones upon being introduced into the veins, produce sudden convulsions in different muscles.

As to the influence that the other organs when affected, exert upon the veins, we know also but very little. As they are every where disseminated, like the arteries and the nerves, it is difficult often to know if it is the vein itself or the organ that it forms, which is the seat of the sympathetic phenomenon.

ARTICLE FOURTH.

DEVELOPMENT OF THE VASCULAR SYSTEM WITH BLACK BLOOD.

I. _State of this System in the Fœtus._

The veins have in the fœtus an arrangement inverse of that of the arteries; they are in proportion much less developed. It is not in the great trunks, as in the venæ cavæ, subclavians, iliacs, &c. that we should compare these vessels, because the reflux of the blood at the moment of death often dilates these trunks, so as to make us believe that they are much larger than they really are in a natural state. It is in the branches and the ramifications that we should make the comparison; now it is easy to see there, that the veins nearly equal the arteries, but are not superior to them, as is uniformly the case in the adult.

However, the side of the heart with black blood, and the pulmonary artery which make a part of the system with the veins, are proportionably larger than these. This arises not only from their receiving and transmitting the blood of these vessels, but also that of the umbilical vein. It is to this last circumstance that must be attributed also an anatomical fact always existing in the fœtus, viz. that the very short trunk of the vena cava, which is extended from the liver to the heart, is found in proportion much greater than the trunk of the superior vena cava, which is not the case in after life.

The less development of the venous system, compared with that of the arteries, appears to arise in the fœtus from this, that much substance being employed for nutrition which is very rapid in the early periods, less returns by the veins. This phenomenon however is not peculiar to the black blood. We shall see that the excretories transmit less fluids by the glands, and that the exhalants pour out less upon their respective surfaces. Much blood enters the general capillary system of the fœtus; hence why the arteries are very large. There remains in the organs, much of the substances that it contains, to nourish them; but little goes out of the general capillary system for secretions, and exhalations; little returns by the veins.

The more the fœtus advances in age, the more of this blood is carried in the veins. In the early periods, almost all remains in the organs to form them. Towards the period of birth, these things approximate to what they will be in the adult.

In this general phenomenon of the venous system in the fœtus, the proportions are always preserved between the veins of the different parts, according to the increase of them. It is thus that most of the superior parts, the brain in particular, being in the fœtus the seat of a more active nutrition than the inferior, the veins there are also more developed.

We can hardly distinguish fibres at this age in the venous parietes, though they no doubt exist. I have only remarked, that they then contain much fewer small vessels in proportion than the arteries, whose trunks are covered with them, as it is easy to see upon the aorta.

Though less dilated than afterwards, the veins appear to be as strongly organized; their parietes are very resisting; they dilate less easily; this continues during the whole of youth. It is to this that I attribute the absence of varices at that age. As on the one hand less blood circulates in the veins, and on the other they appear to be in proportion more resisting, it is evident that they must yield less.

II. _State of this System during growth and afterwards._

A remarkable revolution takes place at birth, as we have seen, in the system of black blood. The right auricle and ventricle receive the whole of the blood, of which a part until then went immediately to the right side by the foramen ovale. This difference has not much influence upon the size of the right auricle and ventricle; differences only in their form take place, which I shall point out in the Descriptive Anatomy.

During the first years of life, the veins have a real inferiority as it respects the arteries. This inferiority continues during the whole time of growth; of this you may be satisfied by examining the external veins; they are never as evident, or as much developed in children as in an adult. Compare the arm of a man with that of a child, and the difference will be perceptible.

The proportion of the cerebral veins over the others, is gradually lost as we advance in age, because the brain does not continue to predominate so much in its nutrition.

At the period of puberty, and towards the end of growth in height, the veins partake of this general plethora, which seems to manifest itself, and which is, as we have seen, the source of many diseases.

When the growth in length and thickness is completed, the veins begin to have a larger diameter; they become more prominent externally; it appears that more blood constantly passes through them. Make the muscles of an adult man contract strongly, and you will see all the veins considerably swelled. The same experiment will not produce a proportional effect upon a young man; ligatures applied show the same difference.

III. _State of this System in Old Age._

In the last years of life the veins become much developed compared to what they are in youth; we can say that in this respect, the two extreme ages exhibit an inverse arrangement. In considering the external appearance in the two ages, we may be convinced by the examination of the superficial veins, of the truth of this assertion.

Let us not think, however, that this greater development supposes an addition of substance in the venous parietes, as for example, the increased size of the bones depends upon the super-abundance of the phosphate of lime. It is a simple dilatation of these parietes, which are weakened, and become more slender, instead of increasing. This dilatation is owing to the loss of their elasticity and to the greater quantity of blood they carry. In fact the motion of decomposition evidently predominates in old age over that of composition. More substance is taken from the organs than is added to them, at this period. I know not but that the bones receive a greater quantity of the substance that nourishes them. In all the other organs, an opposite phenomenon is evident; hence their horny hardening, their withering, if I may use the term. Now, as the system with black blood is that in which is poured all the residue of the decomposition of the organs, it is not astonishing that it should be dilated in old age; so the system with red blood, which carries the materials of their composition, predominates in the first year of life.

The superabundance of black blood in old age however, is to a certain degree deceptive; it depends in part upon the slowness of the circulation in the veins, in which the blood, moved with difficulty on account of the weakness of the capillary system, tends to stagnate, and dilate them, as I have said before; so that though there would be less black blood returning from the organs, there would be more in the veins, than in the adult; the velocity of the circulation then would be much less. There takes place in the whole system, what exists in a varix, for example, in which the blood accumulates because its velocity is diminished. It is not necessary then to believe, that the superabundance of the black blood in old age, supposes a plethora like that of the red blood in infancy, in which, on the one hand, the arteries contain more fluid, and on the other they propel it with greater velocity. We know from this that the dilatation of the veins in old age is a further proof of the principles established above; viz. that the capacity of the veins is always in an inverse ratio to the velocity of the fluids that go through them. It admits of but an inaccurate comparison, though it may give an idea of what passes in the venous system; a river which is very broad above a bridge, flows slowly; but its bed being much contracted under the arches, its velocity is much increased; so that the equilibrium may be established. So in the veins, there is little velocity and much capacity in old age, and much velocity and but little capacity in infancy.

Anatomists know very well the difference of the arteries and the veins at the two extreme ages of life; they choose old subjects to study the veins; on the contrary, these subjects are wholly improper for arterial injections, which succeed so well, and sometimes too well, in infants, in whom every thing appears to become vascular, and in whom the examination of the veins would be very difficult, and even impossible.