Part 33

The form of these valves is parabolical; their convex edge is attached, and most remote from the heart; their straight edge is loose, and nearest that organ. There is between them and the vein a space analogous to that of the semilunar valves of the aorta and the pulmonary artery. They have not, like these valves, a granulation upon their loose edge. On a level with the attached edge, the venous texture is firmer; there is a kind of hardening, which makes a prominent line of the same curved form as that edge. This hardening supports the valves, like the one corresponding to it in the semilunar valves. It appears to be of the same nature as the venous texture, the direction of whose fibres are changed to form it. When the common membrane arrives at this prominent line, it is folded to form the valve; so that it seems to be made of two layers, which are separated with difficulty, from its extreme tenuity.

The venous valves exist in the inferior vena cava, as in the superior. In the first, the divisions of the hypogastric, of the crural, tibial, internal and external saphena veins, &c. are full of them. The second presents many of them in the external jugular, in the azygos, in the facial veins, in the veins of the arms, &c. Many veins have no valves, as we see in the trunk of the inferior vena cava, in the emulgents, in the cerebral sinuses, &c.

The size of the valves is always in proportion to that of the trunks in which they are found; very distinct in the azygos, less so in the saphena, and still less so in the plantar veins. If we compare them with the caliber of the trunk they occupy, we shall see that sometimes they can entirely obliterate its cavity, and that at others they are too narrow to produce this effect. All authors have noticed this arrangement; they have thought that it depended upon primitive organization; but I am convinced that it arises wholly from the state of dilatation or contraction of the veins. In the first state, the valves being drawn and not dilating in proportion, become smaller compared to the caliber of the veins, whose cavity they cannot entirely obliterate when they fall down. In the second state, as they do not contract in proportion to the vessel, they become more lax and are capable of closing it completely. All that has been written by authors upon the size of the valves, depends then wholly upon the state of the veins at the moment of death. This is so true that they appear large if the animal dies of hemorrhage, and small if he dies of asphyxia. I have twice proved this fact.

From what has just been said, it is evident that the reflux of black blood takes place much easier and extends much further when the vein is dilated; and that consequently the first pulsation, the effect of this reflux, does not extend as far as the second, nor this as far as the third and so on. It is this that happens in the cases that we have spoken of before. The reflux never extends to the capillary system, especially in parts at a distance from the heart, because there being many valves and each checking in a degree the blood, it soon stops by losing all the motion received from the heart.

The existence of valves is generally constant, but their situation and number are very variable. Sometimes they are very near each other, at others at a greater distance, in this respect there is a great variety. Generally in the small trunks they are nearer, and at a greater distance in the large ones.

They are rarely arranged three by three, most often in pairs, and sometimes they are insulated; this is the case especially in the small vessels, in those of the foot, of the hand, &c. We find in the works of Haller very minute descriptions of the general arrangement, form and position of the vascular folds of which we are treating.

These folds perform, as we shall see, an important part in the venous circulation; by them we are enabled in most operations, to dispense with tying the venous trunks, if they are not very considerable. In fact, without them, the blood poured by the collateral branches into the open vessel, would easily escape by a retrograde motion, and then we might fear the effusion of all that, which passes in the whole course of this vessel, whilst now none can escape except what flows between the opening and the first or second valve.

The valves constitute an essential difference between the veins and the arteries. Let me observe, that the want of them in these last vessels is a proof in addition to what has been already named, of the absence of vital contractility in their texture. In fact if they contracted like the heart to drive the blood, this fluid, tending as much to return towards the heart by the effect of this contraction, as to go to the extremities, there would be at intervals in the arterial tubes, valves to counteract this first motion; now we only see them at the origin of the aorta; why? because it is only necessary in the arteries to resist the effect of the contractility of texture, which, exerting itself without a jerk, by a mere contraction, can return but very little blood to the heart. A single obstacle at the beginning of the arterial system, is sufficient then to prevent the derangement of the circulation, which might be the effect of the reflux caused during the systole by the contractility of texture of the arteries, a reflux which only takes place in certain cases; for ordinarily the return of the arteries upon themselves, is produced as I have said, by their containing less blood, which has been driven through them during the diastole. In order that this reflux might take place, it would be necessary that the effect of the contractility of texture should in the systole exceed what the arteries have lost of blood in the diastole.

_Action of re-agents upon the Venous Texture._

This texture exposed to drying, becomes yellowish, remains pliable and can be bent in any direction; so that the dried venous bands, might in this respect, be applied to uses, to which the arterial could not in the same state.

This texture becomes putrid also more easily than the arterial, but less so than the others, particularly the muscular. To ascertain this, I have exposed at the same time, venous trunks and portions of intestines of fine muscular layers, to the contact of a moist air.

It resists maceration less than the arterial texture, but more than the others; water in which it has been macerated by itself is much less fetid than that in which an equal portion of muscular texture has been placed.

The horny hardening of the venous fibres is very evident when they are plunged into boiling water or the concentrated acids. They contract then more than half, at the same time they become more evident; in this way they can be studied better; I have used it often; their contraction thickens the parietes of the vein. When they are hardened in this way, if they remain in boiling water or the acids, they become soft very soon in the second, more slowly in the first. Boiling acts upon them quicker than upon the arterial fibres; they can also be reduced by long ebullition to a pulpy state, to which we can never bring the arteries.

The caustic alkali seems to have a very remarkable action upon the veins. After remaining a short time in a solution of this alkali, they become diaphanous, diminish in size, do not entirely dissolve, it is true and become liquid, as in the acids, but evidently lose their elementary principles, give a remarkable precipitate, and always render the liquor less strong, by the new combinations which it forms.

II. _Parts common to the organization of the Vascular System with Black Blood._

_Blood Vessels._

The veins have in their texture little arteries and veins, which take very much the same course as in the arteries. They ramify at first in the cellular membrane, send small branches to the neighbouring parts, then penetrating the venous fibres, wind there in a thousand different directions and finally terminate about the common membrane, which when injected has appeared to me to receive more than in the arteries.

_Cellular Texture._

The veins, like the arteries, have around them two kinds of cellular texture; one which is exterior and of the same kind as that which is found in the interstices of all the organs; it contains fat and serum, and serves only to connect the veins with the adjacent organs; the other dense and compact, forms for them a proper coat. No author has yet distinguished the cellular system of this particular texture from that which is generally spread over the organs, though it differs from it so essentially in its filamentary texture, in its dryness, its uniform want of fat and serum, its remarkable power of resistance, &c. When we raise it, by tearing it with the fingers from the veins, it appears as if it was formed of an infinite number of little filaments interwoven with each other.

After having formed this external covering to the veins, this cellular texture of a peculiar nature analogous to the sub-arterial, sub-mucous, &c. passes between the longitudinal venous fibres, separates them, forms for them a kind of sheath, and terminates in the common membrane, which appears to contain it in its texture, and which owes perhaps in part to this circumstance, the great extensibility that it possesses.

I would observe that the presence of the cellular texture in the venous parietes is a distinctive and striking character that distinguishes them from those of the arteries, with which their texture has in other respects no kind of analogy.

_Exhalants and Absorbents._

It appears that there is no exhalation upon the internal surface of the veins. This surface is always moist in the dead body, though the vessels are empty; but I attribute this phenomenon, as in the arteries, to a transudation that has taken place after death. If there was in fact a fluid exhaled, it would prevent the adhesions of the venous parietes, when during life the blood ceases to flow through them. Now every vein that is empty is obliterated into a sort of ligament, like the arteries in similar cases.

There is no more absorption upon the internal surface of the veins, than exhalation. To satisfy myself of this fact, I have tried upon the external and internal jugular veins, the same experiment before noticed, as having been made upon the carotid artery; I obtained the same result and drew from it the same conclusion. I have been induced to make these experiments, from the opinion of many distinguished anatomists, who thought that the absorbents arose immediately from the veins and the arteries. It is possible that this is the case in the smaller branches, in the capillary system especially, as I shall say in the absorbent system; but I do not believe that any thing similar can be demonstrated in the trunks.

It appears then that the exhalants and absorbents of the venous parietes, like those of the arterial, are confined to the nutritive functions, and that they are consequently few. This remark is applicable not only to the veins, but to the whole of the vascular system with black blood.

_Nerves._

1st. The veins differ essentially from the arteries by the few nerves of the ganglions that accompany them. Whilst these nerves form for most of the arteries a kind of covering, they are scarcely spread at all upon the veins. By laying bare the venæ cavæ, jugulars and azygos, it is easy to observe this. 2d. The side of the heart with black blood, receives as many nerves as that with red; this proves that they have no influence upon the contraction, as it is evidently weaker on the right side than the left; whereas if it was produced by the nerves it would be equal, as there is an equal distribution of them. 3d. The pulmonary artery has but very few nerves. I know not as yet the relation that exists between it and the pulmonary veins in this respect.

It appears from this general survey, that the system with red blood has many more nerves than that with black. In fact being nearly equal at the heart, and the difference being very sensible in this particular between the aortic arteries and the veins that go to the right auricle, although the pulmonary artery may have a few more than the corresponding veins, which I think very probable, yet the short course of these vessels would not prevent the disproportion from being very apparent.

ARTICLE THIRD.

PROPERTIES OF THE VASCULAR SYSTEM WITH BLACK BLOOD.

The veins are in general but little elastic, soft, and loose; they partake of the character of many of the animal textures, and are essentially distinguished in this respect from the arteries, which as we have seen are very elastic. We shall now treat of the vital properties and the properties of texture in these vessels.

I. _Properties of Texture. Extensibility._

The veins have in regard to this property, an arrangement entirely opposite to that of the arteries, which are very extensible longitudinally, but very little so transversely.

The veins stretch but little in the first direction. When drawn out of a stump after amputation upon the dead body, they lengthen but little in proportion to what they dilate in varices, though here they experience an actual increase of size. Perhaps however this depends less upon the deficiency of extensibility of texture, than upon the circumstance that the folds are less evident than in the arteries, and of course the development is less. Whatever may be the cause, the fact is certain and uniform.

Few organs, on the other hand, exhibit a greater degree of extensibility transversely, than the veins. In the dead body, they can be enormously dilated, by injections of air, water, fatty substances, &c. In the living, we know the varicose dilatations, which arise in the great trunks, from the obstacles to the course of the blood in the lungs. While the arteries do not appear very often more than to double their diameter without breaking their common and peculiar membrane, the veins treble, quadruple, and even quintuple their diameter without this rupture's taking place.

We have however numerous examples of this accident. Haller has related many in his great work. We see these ruptures take place during pregnancy in the veins of the lower extremities; there are examples of them also in the external veins of the head in violent headaches. We have seen the venæ cavæ, the jugulars, the subclavians suddenly break and produce death. Every one knows of the hemorrhages that arise from the rupture of the hemorrhoidal veins, &c. I think that the extreme tenuity of the parietes of the cerebral veins exposes them to being frequently torn by blows upon the head, wounds upon that part, &c. When there is an effusion in the tunica arachnoides, it can certainly come from no other source than the venous trunks, which being surrounded by a fold of the arachnoides, pass through this cavity to go to the cerebral sinuses. Now we know that this case is very common, and that it even takes place at the same time with that, in which the dura mater being detached from the cranium, is found separated by an effusion. Is not apoplexy a sudden rupture of the venous extremities? I have already observed that we have no data upon this point. All these cases are very different from arterial aneurism; they often take place when the dilatation is infinitely less than in many instances where the veins remain whole. Very commonly this does not happen. The whole of the vein, with the cellular tunic containing it, bursts. The arterial rupture in true aneurisms, is on the contrary uniform; when the dilatation is carried to a certain point it always happens. The two arterial coats break easily, the cellular remains whole. I do not believe that there is a solitary instance of a great aneurism, without rupture. Why? because the arterial extensibility can only yield to a certain point. The ruptures take place then from a want of this property; they are disconnected with this cause in the veins. We do not know yet how they are produced. In a great number of cases certainly, there is an affection of the venous texture; this is undoubtedly the case in hemorrhoids, &c. Let us be content to point out the differences between arterial and venous ruptures, and wait till further observation shall discover to us all their causes.

If we bear in mind, that the arterial fibres are very numerous and all circular, that the venous, on the contrary, are on the one hand longitudinal where they exist, and on the other that they are very thinly scattered on their vessels, we shall then see why the first resist much longer a distension in the direction of their diameter than of their axis, and why the opposite phenomenon is observed in the second, though much less decided.

_Contractility._

This corresponds with the extensibility. Slight in the longitudinal direction, much greater in the transverse. 1st. It produces the contraction upon themselves, of the parietes of the umbilical vein, of any trunk that is tied, &c. 2d. It produces in a trunk that is pricked, the sudden evacuation of the blood contained between the two ligatures by the return of the parietes upon themselves. 3d. It manifests a decided influence on the flow of blood in venesection. 4th. The numberless varieties of caliber that the veins exhibit after death, according to the quantity of blood they contain, are the result of their extensibility and contractility of texture. 5th. During life, the superficial veins appear very various; dilated in summer, contracted in winter, expanded in the warm bath, as we see the saphenas, especially in pediluvium, lessened in the cold bath, prominent by a long continued perpendicular position, flattened by a horizontal one, &c. they present to him who observes them, at different times, numerous varieties. I very much doubt whether those who have calculated so much the capacity of the vessels, the velocity of the blood, &c. would have undertaken their labours, if they had opened many bodies, or made many experiments upon living animals; now all the varieties depend upon the extensibility and contractility of texture.

II. _Vital Properties._

_Properties of Animal Life._

Have the veins sensibility? The following is the result of my experiments upon the subject. 1st. Irritated externally by any mechanical instrument, pain is not produced, as Haller has seen; 2d, a ligature put upon them gives no pain, whether it is done upon living animals, or in certain surgical operations, in great amputations, for example, in which it is recommended to tie the vein as well as the artery. 3d. Irritated internally, they exhibit the same phenomenon. I have many times pushed a stilet very far into one of these vessels, without making the animal cry out. I would observe also, that this is a good method of examining the sensibility of the heart, without producing in the chest a disturbance, that would increase, diminish, or alter this property in any manner, by the general derangement that it would occasion in the economy. I force then a long stilet into the right external jugular vein, opened as it is in the operation for bleeding. This stilet goes to the heart, without any accident, by straightening out the venous angles. The animal oftentimes gives no sign of pain; sometimes, however, he does; the motion of the pulse is always accelerated. We might easily reach in a man, without accident, with a stilet, the right side of the heart, by introducing it into the right external jugular vein. Why, in certain asphyxias, in syncopes which resist all other stimulants, &c. might we not employ this method to re-animate the action of the heart? 4th. When we inject a foreign fluid into the veins, however irritating it may be, the animals rarely show any sign of pain. Urine, bile, wine, the narcotics, &c. are transfused with impunity in this respect. 5th. On the contrary, when a bubble of air enters them, the animal cries out, is agitated, and struggles before dying; is this owing to the contact of the fluid upon the common membrane? I believe not; for usually there is an interval between the cries and the injection of the air. It is possible that the pain happens at the instant when the air strikes the brain, after having passed through the lungs, a passage which is constant, as I have observed elsewhere.

There is evidently no animal contractility in the veins. The same experiments that demonstrate its absence in the arteries, prove it also as it respects the veins. I have made them at the same time upon both kinds of vessels. I refer, then, upon this subject to the preceding system.

_Properties of Organic Life. Sensible Contractility._

This property does not appear to be an attribute of the veins. Haller, by irritating them in different ways, perceived no sensible motion in them. I have usually made the same observation, whether I employed internal or external irritation.

It has appeared to me, however, in two or three cases, that a manifest contraction took place. As the venous fibres are only longitudinal, and as they are very few, it is evident that in admitting that they are muscular, it would be very difficult to observe the effect of irritants applied to them, though it might be real. The question is not, then, fully settled, though I incline much more to the belief that there is no venous irritability. As the venæ cavæ have evident fleshy fibres at their origin, it is evident that they possess at that place the contractility of which we are treating.

A proof of the great obscurity of the sensible organic contractility in the veins, is, that it is never increased in disease. All the organs, in which this property exists, are remarkable for its frequent increase, which constitutes in the heart the quickness and the force of the pulse, in the stomach vomiting, in the intestines diarrhœa, in the bladder incontinence of urine, especially in children, &c. Now the veins never exhibit a derangement, which, corresponding to these, would make us believe in the existence of a power of which this derangement is the excess, if I may so say.

Observe, that this observation is also applicable to the arteries; never in a determinate portion of the arterial system, do we see this local disturbance, this insulated derangement, which certain portions of the intestinal canal sometimes exhibit. The irregularity of the motion of the blood is always general, because it arises from a single cause, viz. the irregular impulse of the heart.

Observe, that this way of discovering the presence or absence of this or that vital force in a part, by the affections which increase that force there, deserves an important consideration in the examination of these forces. Authors have not employed this method of discovering them, of pronouncing consequently upon their presence or absence in the organs.

_Of the Venous Pulse._

The pulsation that the veins have under certain circumstances, must not be taken for an effect of the venous irritability. It is an effect of the reflux of the blood, which not being able to go through the lungs, stagnates in the pulmonary arteries and in the right side of the heart; so that when this contracts, as the blood finds an obstacle in the ordinary course, it flows back whence it came, as when the aliments are unable to pass down, they take the other direction. This reflux takes place, to a certain distance, notwithstanding the valves; it is often very evident in the jugular vein, when animals, submitted to experiments, breathe laboriously; then it is discontinued; it takes place three or four times, then ceases, and returns irregularly; it is observed also in the last moments of life, when the lungs are embarrassed.