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.
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.
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.
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.
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.
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.
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.
The veins of the inferior parts are generally more dilated in old age than those of the superior; this arises from the habitual weight of the column of blood, which constantly acting, produces finally a real effect; for, as we have said, the venous circulation is much influenced by mechanical causes, owing to the want of power in the cause that circulates it; hence why varices are infinitely more frequent in the inferior than the superior parts, in which they are hardly ever found.
In women who have had many children, we see this dilatation of the veins of the inferior parts in a very evident manner; very often there are varices in them. Observe that this disease seems to be the companion of old age more particularly than that of every other age. On the contrary, we rarely see aneurisms in old people. The rupture of the veins has been almost constantly observed at this or the adult age. I hardly know an example of it in infancy.
The pulmonary artery does not dilate in old age in proportion to the veins; because, removed from the action of foreign bodies, and provided at its origin with an agent of impulse formed of a firm and resisting texture, it has not been in the habit of yielding like them.
The veins are accidentally developed in two ways. 1st. In cancerous tumours, in fungi, &c. in which more red blood enters, they acquire a size in proportion to that of the arteries; now, as they are superficial, we see more easily their increase than that of the arteries; this increase, which has been taken for a characteristic of cancers and other analogous tumours, is only a consequence of the increase of nutrition. The motion of the blood is as rapid there as in the other veins; there is no obstruction to it. 2d. There are cases on the other hand, in which the veins dilate, because the blood cannot easily circulate in them, and because the velocity of its course is diminished. For example, the whole venous system of the abdominal parietes is often increased in ascites; it is not because there is more blood circulated; there is less than in the ordinary state; but it is because the venous parietes having in part lost their elasticity, like the neighbouring parts, the circulation becomes slower; now the slower it is, the more the blood accumulates and the more the venous parietes are dilated. It is then a kind of general varix in a division of the veins. There is not more blood brought by the arteries, as in the preceding case; the same thing in part happens in old age.
Though in the exposition of the two systems of black and red blood, I have considered the pulmonary artery as making a part with the veins, and the pulmonary veins as a continuation of the arteries, yet their nature is wholly different. There are in truth but two general membranes, forming the two great tubes in which are contained the two kinds of blood, which are every where of the same nature, from the capillary system to the pulmonary. The textures added to the exterior of these two common membranes are essentially different. Thus the texture of the pulmonary artery, though added to the membrane with black blood, is, in point of thickness nearly of the same nature as that of the aorta and its divisions. So the texture of the pulmonary veins, though united to the membrane with red blood, is the same as that of the other veins.
This uniformity in texture supposes an uniformity in the functions, and this is really the case. The mechanical laws of the circulation of black blood are the same in the pulmonary artery as those of the red blood in the aorta. So the laws of the general venous circulation are the same with those of the pulmonary veins; inspection proves this; and, moreover, it must be so, since the relation of the heart to the two kinds of vessels, the veins and the arteries, is the same.
Each system of blood, then, has its two modes of circulation. Sudden motion, generally communicated, and not the progressive undulation of the fluid; a pulsation by a real locomotion, a general straightening of all the divisions of the same trunk at each impulse of the heart; these are the general mechanical characters of the artery with red blood, as well as that with black. Absence of pulsation, slowness in the course of the blood, want of straightening, &c.; these are the general attributes of the veins of each kind of blood.
There are no doubt general modifications that arise from local causes. Thus, on account of the short course of the pulmonary veins, the weight has scarcely any influence upon the blood; they never become varicose; the motion of the fluid is more rapid in them, since they have less time to lose that which is communicated to the blood in the pulmonary capillary system, &c.; thus the artery of the same name, whose branches are less tortuous, does not seem to me to have pulsations as evident as those of the aorta, &c. But these general phenomena are always the same; they are but different modifications.
This is why the general arrangement is nearly the same in the veins and in the arteries, whether they circulate red blood or black. Thus, for example, each of the two arteries go off from a ventricle by a single orifice, necessary for the unity of the impulse of the blood, for the uniformity of its course in the divisions of its great vessels, and for the simultaneous pulsations in all the divisions. On the contrary, the veins pour into the heart the red and black blood by many separate orifices; this is of no consequence, since, as we have seen, the motion of this fluid in the veins is not uniform, but may be accelerated or retarded in a part, by the influences it receives; thus, it may pass with velocity through the opening of the vena cava superior, and slowly through that of the vena cava inferior, &c.
From the preceding considerations, it seems, that if we have no regard but to the mechanism of circulation, that it is almost of no consequence whether we consider with the ancients the small and great circulation by studying first the course of the blood in the artery and the pulmonary veins, then in the aorta and the general venous system; or of studying, as we have done, the course of the blood, first in the pulmonary veins and the aorta, then in the general veins and the pulmonary artery. But if we consider this great function in the important relations of nutrition, secretions, exhalations, for which they furnish the materials, of the general stimulus it carries to all parts, and which is indispensable to the support of life, of the introduction of foreign fluids in the body of the animal, and of the change of these fluids into its own substance; then I think it must be described as I have done it.
There is in the abdomen a system of black blood wholly independent of the preceding, arranged precisely like it, with the difference, that its course is shorter, and that it has no agent of impulse. This system, usually known by the name of Vena Porta, is found in most animals.
It arises from that part of the general capillary system which belongs to the intestines, the stomach, the omentum, the spleen, the pancreas, &c. and generally to all the abdominal viscera connected with digestion. This origin is remarkable. The viscera in the abdomen, foreign to the phenomena of digestion, are also foreign to the origin of this system. The kidnies and their dependancies, as the glandulæ renales, the ureters, the bladder, the urethra, &c. the genital organs, the diaphragm, &c. the abdominal parietes themselves, &c. &c. pour their black blood into the preceding system. Why are the digestive viscera, in their whole extent, different from the others, in the destination of their black blood? To answer this question, we must know the uses of the system of which we are treating; now, of these we are ignorant.
Thus rising from the whole gastric apparatus, this system forms into two or three trunks, which soon unite into a single one, which occupies the superior and right part of the abdomen below the liver.
This common trunk soon divides again into many branches, which spread in the liver into an infinity of ramifications, and are spent upon the texture of that organ.
This system, then, presents the same general arrangement as the preceding; it is composed of two trees united by their lopped summits, that intermix with each other. Place an agent of impulse at these summits, the arrangement will be the same as in the two preceding. The blood is moved from one capillary system to another. Divided at first into small streams, it is formed into masses constantly increasing to a certain point, then it is divided again, and is carried in streams not larger than the first.
In the abdominal portion, the ramifications, the small branches, the branches and the trunks are arranged very nearly as in the general venous system. The ramifications are found in the organs, the small branches in their interstices, most of the branches are situated in the layers of the peritoneum, there accompanying the arteries, and the trunks wind along the subjacent cellular texture. As to the hepatic portion, contained wholly in the liver, it is divided there nearly like the preceding.
The anastomoses present the following arrangement in the system of which we are treating. 1st. Its hepatic portion appears to want them; all the branches, smaller branches, and ramifications, go separately. As the circulation is not subject in the liver to increase or diminution, the solid texture of this organ protecting the vessels, the blood has no occasion for the means by which it can deviate from one place to another. Thus the great divisions of the pulmonary artery and veins, which go immediately into the lungs where they are wholly distributed, do not communicate with each other. Thus the branches of all the arteries and of all the veins contained in the interior of a viscus, as in the kidney, the spleen, &c. are most commonly without communication. 2d. As to the abdominal tree, its anastomoses are very frequent in the smaller branches. We see all along the small intestines arches exactly like those of the mesenteric arteries; less frequent in the great intestines, they are, however, very evident in them, as upon the stomach; in the branches and the trunks they do not exist.
The anastomoses in the system of black abdominal blood are necessary there from the frequent delays that this fluid may experience. For observe, that the circulation is performed for the abdominal portion according to the same laws as in the other veins, and that consequently that the force that can circulate the blood there, can yield to the least effort. Now in the different motions of the small intestines, often too great a fold, the pressure of these organs filled with aliments upon the veins, when we are lying on the back or the side, and which pressure the veins support only by their resistance, and a thousand other causes, impede the course of the blood in one branch, and force it to flow by anastomoses into others. Observe also, that an obstacle which is of no consequence to the red blood, on account of the very strong impetus that is given to it, is very important to the two circulations of black blood, which receive but a feeble impulse.
The influence of gravity is evident in the blood of this system, as in that of the preceding. Thus you see the hemorrhoidal veins, more exposed than all the others to this influence by their position, become much more frequently varicose; and it is even rare to find dilatations in the superior mesenteric veins, splenic, gastro-epiploic, &c. &c. whilst there is no part in which they exist more often than in the hemorrhoidal. Thus we have seen the preceding system dilated rarely above, but very frequently below.
The system with black abdominal blood communicates but very little with the general system; if there are anastomoses, it is only in the last divisions; do these anastomoses exist? I believe we may consider these two systems as independent of each other.
Many authors, Haller in particular, considering that the system of which we are treating is destitute of an agent of impulse, have admitted in them a force of structure superior to that of the other veins; but after examining it attentively, I am convinced that it is precisely the same. The cellular covering, of a peculiar nature, which surrounds it, and which is analogous to that of the other vessels, is only a little more evident; this makes these veins at first appear thicker; but by raising this covering we see that the internal membrane is of the same nature, only perhaps a little less extensible. We do not discover the venous longitudinal fibres so well as in the preceding system; I doubt even if they exist in the trunks, in which we should be able to see them better.
The two portions of this system, the hepatic and abdominal, appear to be completely uniform in their structure. Only the first is every where accompanied by a kind of membrane, which appears to be cellular, but whose nature is not yet well known, and which is called the capsule of Glisson. This capsule, intimately connected with the substance of the liver, adheres more loosely to the veins; so that when they are empty, there is often a space between them and it; it is this that makes them fold up when we cut the liver in slices. I think that we are entirely ignorant of the object of this anatomical arrangement.
The analogy between the systems with black blood, the abdominal and general, supposes them the same in properties, sympathies, affections, &c. I have often irritated the mesenteric veins, upon which it is extremely easy to act, by drawing through a small wound of the abdomen a portion of the intestinal canal; the results have always been the same as in the preceding system. Only when we inject air, the animal does not struggle, does not appear to suffer, and the experiment is not fatal; this proves still more, that it is not by its contact upon the veins or the heart, that the air is injurious, but by its action on the brain.
The common membrane of the system of which we are treating, is distinguished from that of the preceding, in this, that it is wholly destitute of valves. This appears to be owing to two causes, 1st, to this, that the course of the blood being shorter, it has less need of being supported; and 2d, to this, that the middle part of this system, wanting an agent of impulse, there is no reflux as in the preceding system. In fact, at every contraction, the right auricle sends, as I have said, a portion of its blood into the veins, which resist by the valves. Here, on the contrary, the course of the fluid is always uniform from one capillary system to the other; there is no cause of retrograde motion.
This uniformity in the course of the motion of this black blood, is not merely the result of the absence of the agent of impulse, but also of this, that the liver does not present as many obstacles to it, as the lungs do to the preceding black blood. Observe also that the liver occupies in regard to this system, the same place as the lungs in regard to the other; it is the termination of the circulation of which we are treating. Having no dilatation or contraction, deprived of the fluid which acts incessantly upon the lungs, and which, loaded with different foreign substances, can often alter the vital forces of these organs, so as to interrupt the passage of the blood, &c. The liver, having a solid and granulated substance, in which no extraordinary motion can take place, except those of the general locomotion of the organ, is evidently incapable of frequently interrupting the course of the black blood, which the abdominal system sends there. Add to this the want of the agent of impulse, and you will understand, 1st. why, when the abdomen is open, we never see a pulsation, a reflux in the veins of the abdominal system, as we observe in those of the other system; 2d. why we always find there nearly the same quantity of blood; 3d. why, consequently, we do not discover, either in the common trunk that corresponds to the heart, or in its branches, the numberless varieties of dilatation and contraction, which the right side of the heart, and all the great venous trunks so frequently exhibit, so that scarcely two subjects are alike in this respect, whilst here the arrangement is always nearly the same; 4th. why the liver is not subject to the innumerable varieties in size that the lungs are. This deserves a particular consideration. You will rarely find the lungs twice containing the same quantity of blood; the weight varies enormously in this respect. Now this arises from the greater or less obstacles the blood has met with in passing through these organs in the last moments of life. We can make them more or less heavy in an animal, by making him die of asphyxia or hemorrhage, consequently by filling with blood, or emptying the extremities of the pulmonary artery. Whatever on the contrary is the kind of death, the hepatic extremities of the abdominal system, contain always nearly the same quantity of blood; suppose, that more remains than usual in this system at the moment of death, it is generally distributed, because there is no agent of impulse, which, at the last moments, drives the greatest quantity to the liver, as happens to the lungs. We understand from this, why the liver exhibits a firm, resisting texture, not extensible like that of the lungs. Sometimes the blood enters it in greater or less quantity, it is even, more or less heavy according to the kind of death. But these varieties belong only to the hepatic veins, which enter into the vena cave inferior just below the heart; they arise from the greater or less reflux of blood that takes place there, as in all the great venous trunks; they consequently arise almost always from the lungs; so that when we see that they are loaded with blood, the right auricle consequently distended, we may also be sure that the liver contains more of this fluid than usual. But this phenomenon, of which I shall speak when treating of the liver, is wholly disconnected with the system which I am describing.
The mechanism of the circulation of the abdominal part of this system, is precisely the same as that of the veins. As to that of the hepatic part, it is unlike that of any other part of the economy. It has no analogy to that of the arteries, for in them the heart is almost every thing, and here there is nothing to correspond to that system; for certainly there is no kind of contraction in the common trunk of the two trees, as I have been frequently convinced. It is then the same motion, which is continued from the gastric viscera to the liver. There is still much obscurity to be removed concerning this motion, as well as the preceding. Every judicious mind perceives that there is a great void, in reading what has been written upon the motion of the general venous blood, and upon this.
We cannot deny but that external agents do much in this last circulation as in the first. The uniform elevation and depression of the diaphragm, the corresponding motion of the abdominal parietes, the alternate dilatation and contraction of the hollow viscera of the abdomen, the constant locomotion of the small intestines, &c. all these causes certainly have an influence upon the motion of the black abdominal blood; and I even think, that the absence of most of them contributes to retard this motion in the hemorrhoidal veins, and to occasion varices in them.
This influence is not however such as Boerhaave thought, that the circulation could not go on without it. In fact, when the abdomen of an animal is opened, the blood is transmitted the same to the liver, and spouts in the same manner from an open vessel; but we observe a sensible weakness in a short time, and this before the general circulation is enfeebled.
The use of the liver, in being the termination of the black abdominal blood, as the lungs are that of the black blood of all the rest of the body, gives it an importance unknown to all the other secretory organs. Some authors, in observing the enormous size of this viscus compared to the quantity of fluid secreted by it, have suspected that it had another use, besides the secretion of this fluid. This suspicion appears to me, to amount almost to certainty. Compare the hepatic excretories and reservoirs, with the same parts in the kidnies, the salivary glands, the pancreas even, and you will see that they hardly surpass them, and that they are inferior to those of the first. Then compare the size of the liver with that of the kidnies, of the salivary glands, &c. and you will see the difference. If on the other hand, we examine the bile, voided with the stools which it colours, if we open the intestines at the different periods of digestion, as I have done, to see the quantity of this fluid that is poured out; if we keep an animal without food in order to let it accumulate by itself in the intestines; if we tie the ductus choledochus to retain the bile, &c. it is impossible not to be convinced, that the quantity of this fluid is disproportioned to the size of the liver. This viscus is alone equal in size to all the other glands united; now, place on one side, the bile, and on the other, all the secreted fluids, the urine, the saliva, the pancreatic juice, the mucous juices; &c. you will see how enormous the difference is.
Since then the secretion of bile is not the only object of the liver, it must have some other use in the economy. What this is we are ignorant; it is however undoubtedly connected with the existence of the system with black blood of which it is the termination, and is especially relative to this system. The following considerations appear to prove that this use is among the most important.
1st. The liver exists in all classes of animals. In those even in which most of the other essential viscera are very imperfect, it is well developed. 2d. Most of the passions affect it particularly, many of them have an exclusive effect upon it, whilst a great number of the other glands hardly perceive them at all. 3d. In diseases, it takes as evident a part as the first viscera in the economy. In many nervous affections, in hypochondria, melancholy, &c. it has a great influence compared to other glands. We know how easily its functions are deranged. It has undoubtedly no connexion with some affections called bilious, and which are seated exclusively in the stomach, but it certainly has a part in most of them. Since there is no doubt that the jaundice depends wholly upon a serious affection of this viscus, we ought certainly to conclude that the yellow tinge of the face in many of these affections, arises from a cause existing in this viscus, and which is not sufficiently powerful to produce jaundice. Whether in order to produce this tinge, the bile circulates or not with the blood, is of no consequence; it is incontestible that it is occasioned by affections of the liver; now the numerous cases in which it takes place, prove how much this viscus is often affected; there is certainly no gland in the animal economy so frequently. 4th. Shall I speak of organic affections? compare in the examination of bodies, those of the liver, with those of all the other organs of the same class, and you will see that there is no one equal to it in this respect; the kidney approaches it, in the frequency of the alteration of its texture, but it is far from being equal to it. 5th. Who does not know the influence of the liver upon temperaments? Who does not know that its predominance gives to the external appearance, the functions, the passions, the character even, a peculiar shade which the ancients have noticed, and which modern observations have confirmed? Now see if the other glands have a similar influence in the economy. 6th. The liver is, with the heart and the brain, the organ that is first formed; it precedes all the other organs in its development; it is incomparably superior, in this respect, to all the glands.
From all these considerations, and from many others that I might add, we may conclude, I think, that the unknown part which the liver performs in the animal economy, besides the secretion of bile, is among the most important. The study of this part, is one of the points most worthy of arresting the attention of physiologists.
It has been said latterly, that the liver corresponded to the lungs in their functions of removing from the blood its hydrogen and carbon. I know not how this fact can be proved by experiment; but I am positive that the liver does not turn the black blood of the abdominal system into red. 1st. The blood of the right auricle is of the same colour as that of the vena cava inferior; now if the blood went red from the hepatic veins, it would certainly give a brighter tinge to that of the auricle. 2d. Having opened the abdomen and thorax of a dog, I tied with a curved needle the vena cava at its entrance into the heart and above the kidney, then by detaching the liver from behind, I cut the portion intercepted between the two ligatures, and where the hepatic veins opened; the blood came out as black as that of the rest of the system. 3d. Tear out the liver of a living animal, examine immediately its veins, you will see that they contain a blood analogous to that of the others. 4th. This viscus, cut in slices in a living animal, pours out behind an analogous fluid, except some small red streams furnished by the last small branches of the hepatic artery; this is wholly different in the same experiment made upon the lungs.
If the black abdominal blood receives any modifications of its nature in the liver, they certainly have no influence upon its colour, its consistence, or sensible qualities.
The general opinion is that the black abdominal blood serves for the secretion of bile, and that the hepatic artery is only destined to nourish the liver; this is what Haller has adopted; I have also admitted it; but I am far from considering it as clearly demonstrated as it has generally been thought to be; the following observations prove, that we ought to consider it as an hypothesis somewhat uncertain.
1st. It is said that the hepatic blood, blacker, more oily, impregnated with the vapours of the excrements, of a bitter taste even, approaches nearer the nature of bile than the arterial blood, and that it is consequently more proper to form it. I do not know whether this blood has been analyzed comparatively; but I have certainly not found any difference in its external attributes; I did think that in an experiment I observed fatty drops swimming in it; different experiments have convinced me that it was an error. I doubt whether it could ever be demonstrated that the alkaline particles of aliments and of excrements pass into the vena porta; this passage is a gratuitous supposition. 2d. It is said that the volume of the liver is considerable compared to the hepatic artery; this is true; but it is not with the size of this viscus that we should compare that of this artery, to know if it furnishes the materials of secretion, since we have seen that it is impossible that the whole substance of the liver should be destined to secrete bile; it is with the biliary ducts and their reservoir, that we should make the comparison; now this artery is exactly proportioned to these ducts; there is between them nearly the same relation as between the renal artery and the ureter; on the contrary, the biliary ducts are manifestly disproportioned to the vena porta. 3d. It is said that the slow motion in this vein, is favourable to the secretion of bile. But upon what positive data is this assertion founded? Why is slowness of motion more necessary for this secretion than for others? 4th. It is said that the hepatic artery having been tied, the secretion of bile continued. But when we know the relation of parts, the least reflection is sufficient to convince us, that a ligature of this kind cannot be made without producing a derangement that will prevent us from distinguishing any thing. I attempted it once, but could not finish it; I was almost persuaded of it before. 5th. It is said that the black blood is more proper to furnish the materials of the bile than the red. But what is the reason of it? is it because this blood contains more carbon and hydrogen? But it is then the black blood that furnishes the fat also; now all anatomists are agreed, that it is exhaled from the exhalant extremities of the arteries; the same is true of the marrow, the wax, and in general of all the oily fluids. 6th. A fine injection, made in the hepatic portion of the abdominal system with black blood, passes into the biliary vessels. But a similar passage takes place in an injection from the hepatic artery. 7th. It is said that the black abdominal blood in the spleen has qualities essential to the bile. But the secretion of this fluid can evidently take place without the spleen; many experiments prove this. 8th. It is said that at the instant the vena porta is tied, bile ceases to be secreted; it is undoubtedly less difficult to tie the trunk of this vein below the duodenum than the hepatic artery. How can we examine what is going on in the liver? Do we judge by the fluid flowing from the hepatic duct? But open the duodenum, and you will not very often see the bile running out at the opening of the ductus choledochus, undoubtedly because the air contracts and irritates this duct. This phenomenon, observed after a ligature is applied, is not then conclusive; moreover there does flow towards the time of digestion but too little bile by the ductus choledochus, to be able to estimate it. In fine, what inference can be drawn from an animal whose abdomen is open?
These different reflections prove, I think, that we have not sufficiently direct proofs, to decide whether the secretion of bile is from the abdominal black blood or the red. I do not attribute this function more to one than the other; I say that these things should be subjected to a new examination, and that this example is a proof that the opinions most generally received in physiology, those consecrated by the assent of all celebrated authors, often rest upon very uncertain foundations. We are yet far from the time when this science will be only a series of facts rigorously deduced from each other.
The hepatic artery has been said to resemble the bronchial, and the hepatic vena porta the pulmonary artery; this is true in the general arrangement; but what is the proof of it as it regards the functions? On the contrary, I have proved above that those of the two last vessels are not similar. Let us wait, before deciding, for further and positive researches; let us doubt till then; let us not attribute the secretion of bile to the hepatic artery, nor the vena porta, nor to them unitedly. Certainly it is by one of these three means; but which? what vessel furnishes the secretion of bile? what part does the black abdominal blood perform in the liver, if it is not from it that this fluid is secreted? what, in fine, is the function of the hepatic artery, if it is not connected with this secretion? These are questions to be resolved.
Physicians have also hazarded opinions upon the influence of the black abdominal blood in diseases. Undoubtedly the expression, vena portarum, porta malorum, contains a very true meaning; but certainly in the present state of our knowledge, it is, in a strict sense, only a play upon words. If we would express by it the frequency of affections of the liver, it is without doubt just; but if it is employed to express the influence of the vena porta in diseases, it is vague and does not rest upon any positive fact. The more we open dead bodies, the more we shall be convinced, I think, of the necessity of a precise and accurate language, freed from all these ingenious, hypothetical ideas, which do honour, it is true, to their author, but which retard science, by introducing into it a manner of seeing hypothetically, and contrary to the spirit of observation.
Though this question may be to a certain degree foreign to my object, yet as the black abdominal blood has perhaps a real influence upon the secretion of the bile, as my experiments upon this point determine with precision the course of this fluid, I do not think it useless to relate them here. All that is known further upon the uses, mechanism, &c. of this secretion is to be found in works of physiology, to which I refer.
There has been much discussion to ascertain if there was cystic and hepatic bile, if one was of a different nature from the other, if their quantity increased or varied, &c. Contrary and even opposite opinions have been supported by numerous experiments made upon living animals, as Haller has observed. These experiments, though at first view contradictory, are not so, however, as I was convinced by repeating them at different periods of digestion and during the abstinence of the animal; it had not been done with precision. The following is what I have observed in dogs, which I have used in my experiments.
1st. During abstinence, the stomach and small intestines being empty, we find the bile in the ductus hepaticus and ductus choledochus yellowish and clear; the surface of the duodenum and jejunum tinged by bile which has the same appearance; the gall-bladder much distended by a greenish, bitter bile, much deeper coloured and more abundant if the abstinence has been long. 2d. During digestion in the stomach, which may be prolonged for a length of time, by giving to a dog large pieces of meat, which he swallows without masticating, things are nearly in the same state. 3d. At the beginning of the intestinal digestion, we find the bile of the hepatic duct always yellowish, that of the ductus choledochus deeper coloured, the gall-bladder less full and its bile already becoming clearer. 4th. At the end of digestion and immediately after, the bile of the hepatic duct, of the ductus choledochus, that in the gall-bladder, and that which is found upon the duodenum, are of precisely the same colour as the common hepatic bile, that is, of a clear yellow, and a little bitter. The gall-bladder is about half full; it is flaccid, not contracted.
These observations, repeated a great number of times, evidently prove that this, during abstinence and digestion, is the manner in which the flow of bile takes place; 1st, it appears that at all times the liver secretes a certain quantity, which is increased during digestion; 2d, that which is furnished during abstinence is divided between the intestine that is always coloured with it, and the gall-bladder which retains it, without pouring out any portion of it by the cystic duct, and in which, thus retained, it acquires an acrid character and a deep colour, necessary, no doubt, to digestion which is to follow. 3d. When the aliments, having been digested by the stomach, pass into the duodenum, then all the hepatic bile, which was before divided, flows into the intestine, and even in greater abundance. On the other hand, the gall-bladder pours also that which it contains upon the alimentary mass, which is then completely penetrated with it. 4th. After intestinal digestion, the hepatic bile diminishes, and a part begins to flow into the duodenum, and a part to flow back into the gall-bladder, in which, if then examined, it is found clear and in small quantity, because it has had neither time to be coloured or accumulate.
There is then this difference between the two biles, that the hepatic flows almost in a continued manner into the intestine, and that the cystic flows back, except during digestion, into the gall-bladder, and flows, during this function, towards the duodenum; or rather it is the same fluid, of which a part always preserves the same character that it had at its exit from the liver, and the other assumes a different one in the gall-bladder. The diversity of colour in the cystic bile, according as it has been retained long or not, has much analogy to the colour of the urine, which is found more or less deep coloured, as it has been for a longer or shorter time in the bladder.
As to the course of the bile in relation to the stomach, I believe that this viscus contains a certain quantity of it at all times. When empty, we find there a mixture of gastric juices and mucus more or less abundant, sometimes mixed with small bubbles of hydrogen, which burn when brought in contact with flame, and almost always tinged with a yellowish colour from the bile that has flowed up through the pylorus. Haller says that this reflux of bile into the stomach does not always happen; Morgagni says that it always does in men. I have never opened a dog, in whom it has not been seen when the stomach was empty, especially if it had been so for some time. Human dead bodies are not proper to decide this question, because the kind of disease alters almost inevitably the course, the nature, and even the colour of the bile. I shall say in another volume what conclusion we should draw from this, as it respects bilious vomitings.
In a state of fulness, it has sometimes appeared to me impossible to estimate the reflux of the bile; in other states, between the alimentary mass and the parietes of the stomach, I have seen yellowish, gastric fluids; but this mass itself never has this colour.
The bile that flows into the stomach has always appeared to me to be hepatic bile, from its light colour. I think that I have opened a sufficient number of living animals to convince me, that this bile is hardly ever found very green, and that it acquires this colour from the gall-bladder; and that it is this that is brought up by vomiting in some affections. The reflux of this bile appears to be an effect of the affection itself. This observation agrees with that made above, viz. that the hepatic bile alone flows into the duodenum in abstinence. It alone can then, as we may be convinced, flow into the stomach. During intestinal digestion, in which the cystic bile flows, it is evident, that the aliments going continually out of the pylorus, prevent it from passing there and entering the stomach; that which we find during fulness, was there then, or entered there before the peristaltic motion had begun to evacuate this organ.
When we open the gall-bladder in a dead body, we see that the bile there exhibits, according to its diseases, a variety of shades of colour, from that which is black as ink to a kind of transparent fluid. Ought we then to be astonished, if the vomitings in which the cystic bile is brought up, that has flowed into the stomach against the ordinary course of things, should contain matters of such various colours?
In the fœtus, the system of black abdominal blood is not insulated; it becomes a part of the two others, by means of the ductus venosus. There is then truly but one vascular system in the fœtus, whilst after birth, there are three separate ones, two with black blood and one with red.
In the fœtus, it is especially with the umbilical vein that the abdominal system with black blood is continued. The liver is a centre, in which both arrive from two different sides, and in which they unite, in a common trunk. The two columns of blood that they circulate, do not meet directly; their course forms a very remarkable angle.
When we examine attentively the orifice of the ductus venosus in the trunk, made by the union of these two veins, we see that it presents itself naturally to the blood of the umbilical vein; that that of the vena porta, on the contrary, cannot enter there. In fact, there is a little fold in the form of a valve, less evident, it is true, than many others, but yet existing. This fold is only a kind of projection, placed between the end of the vena porta and the ductus venosus, and which contracts the orifice of this, so that it is evidently narrower than the caliber of its own canal. The blood coming from the vena porta and passing at the side of this fold, presses it against the orifice, and thus forms an obstacle; that coming from the umbilical vein, falling, on the contrary, perpendicularly on this orifice, removes this fold, and enters the canal.
It hence follows that the ductus venosus is evidently destined to carry to the vena cava the residue of the blood of the umbilical vein; I say the residue; in fact, as this vein is very large and the ductus small in proportion to it, it is evident that the greatest part of the blood penetrates the liver, by the different ramifications that enter its substance.
The abdominal vascular system is less developed in the fœtus than afterwards; it consequently carries less blood to the liver; this is the same arrangement as in all the other veins. I would observe, that the small quantity, however, which the liver receives in this way, is more than compensated by that of the umbilical vein. This viscus is, then, habitually entered in the fœtus, by a greater quantity of fluid than at any of the other ages. Hence, 1st, why its nutrition is so developed and its size so great; 2d, why it is, in proportion to its size, heavier than in the after ages; 3d, why when we cut it in slices, there flows out a greater quantity of blood; 4th, why, when we dry slices of the liver of a fœtus, of the same thickness as others taken from the liver of an adult, and especially of an old person, they are reduced to a less size.
The disproportion of the size of the liver of the fœtus is more evident, the sooner it is examined after conception; this is the same as with the brain. As the fœtus advances towards birth, the liver approximates in its proportions to the other organs, that which it will have in the adult. From the observations of Portal, it is especially till the seventh month, that the liver is predominant. This circumstance appears to arise from this, that the umbilical vein transmits as much more blood in proportion to the fœtus, as it is less advanced in age.
At this age, the blood of the umbilical vein and that of the vena porta evidently mix, at least in a great measure, in the common trunk. Is their nature analogous? There is no experimental knowledge upon this point. But Baudelocque has many times observed that that of the umbilical vein is redder, and even approximates the nature of arterial blood. I have not accurately observed this fact in any animals except guinea pigs, in whom the want of transparency in the cord does not allow us to see a great difference in the blood of the arteries and of the umbilical vein; but this difference can be in fact more evident in man; now, in this case, the umbilical blood appears to lose this redness in the liver, for very certainly it is uniform beyond this viscus in the circulation of the fœtus, as I have often ascertained.
At the period of birth, the blood ceasing to come by the umbilical vein, the liver becomes only the termination of the black abdominal blood. Then a kind of revolution takes place in this viscus. The different tubes that carried to it umbilical blood do not close up, but they transmit exclusively that of the vena porta, which increases a little in size, because digestion, which begins in the gastric organs, calls to them more arterial blood, and consequently more is returned by the veins. This slight increase does not compensate for the absence of the umbilical blood; thus the liver diminishes proportionally in size in an evident manner.
As to the ductus venosus, it is obliterated by the effect of the contractility of texture. The blood coming in the vena porta, has not, as I have said, any tendency to pass through it, because this canal is not in its direction; it passes rather into the hepatic vessels, and the circulation of the liver is established then, as it will always continue to be.
This then is the difference that birth brings to the hepatic circulation; 1st, less blood and only one kind entering the liver; 2d, an interruption of all communication between the general and abdominal black blood; 3d, proportional diminution of the size of the liver. Hence there is an inverse phenomenon for this organ and for the lungs. The latter increases, the other diminishes in activity and size.
The great quantity of blood that enters the liver before birth, and the size of this organ, compared to the small quantity of bile that escapes from it, are an evident proof then that it is destined for other uses besides the secretion of this fluid. There cannot be a doubt upon this point; it is a proof moreover, that in the adult the disproportion of the organ to the fluid, though less sensible, supposes also in it another important function of which we are ignorant.
There ought to be a precise relation between the obliteration of the ductus venosus, of the foramen ovale and the ductus arteriosus, between the increased activity of the lungs and the diminished activity of the liver at birth, &c. We judge of this relation, without knowing it, because a veil is still spread over the circulation of the fœtus. I would only observe that the predominance of the liver before birth, does not suppose any in the system of black abdominal blood; it arises exclusively from the umbilical vein; thus the proportional volume of this organ is constantly diminishing afterwards, especially on the left side, where this vein is distributed, as Portal has observed. It is difficult to name the period, at which the equilibrium is generally established.
In youth, the abdominal system of black blood, like the general, is weak. It is towards the thirtieth or fortieth year, that it seems to be in its greatest activity; this is the age of gastric diseases, of hemorrhoids, and of melancholy, which is connected with the state of the liver.
In old age, the dilatation of the system of black abdominal blood is much less sensible than that of the preceding system; its vessels have nearly the same caliber as in the adult age; which supposes a less diminution in the velocity of the course of its blood, from the principles established above. It never becomes the seat of any kind of osseous incrustation, a phenomenon that evidently assimilates its common membrane to that of the veins, and distinguishes it in a peculiar manner from that of the arteries.
END OF VOL. I.