Part 32

The veins communicate in general more frequently than the arteries. 1st. In the ramifications there is a real net work, the anastomoses are so numerous. 2d. In the smaller branches, they are not so frequent. 3d. In the branches, they are still less numerous; but still we find many of them, and it is this that particularly distinguishes these branches from the arterial, which are almost always separate from each other.

The communications between the branches of the veins unite immediately in an evident manner the cutaneous to the deep division; thus there is a communication between the cerebral sinuses and the temporal, occipital veins, &c.; between the external and internal jugular by one and even two considerable branches; between the basilic, cephalic, and their numerous divisions spread upon the fore arm, on the one hand; on the other, the brachial, the radial and cubital, by different branches that penetrate deep into the muscles; between the saphena and crural, tibial, peroneal, by analogous branches.

Though separated, the two great venous divisions can then evidently supply the functions of each other by mixing their blood. Hence why, 1st, by agitating the muscles of the fore arm, we increase the throw of blood in venesection, though the muscles do not furnish many branches to the open vein, which then receives the blood from veins, from which it is forced out by the muscles; 2d, why in external pressure that obstructs or even entirely stops the motion of the superficial venous blood, the circulation continues as usual; why, for example, if a ligature is left for a long time applied to the arm, the superficial veins at first swelled, gradually become empty, by pouring their blood into the deep ones; why, notwithstanding tight bandages in fractures and luxations, the venous blood returns as usual to the heart, though it passes in less quantity by the superficial veins. 3d. If a strong band is applied high up on the leg, and the saphena vein is injected below, it does not fill above the band, but the injection goes into the crural. In the same manner the internal jugular may be filled by the temporal, &c.

The anastomoses between the superficial and deep veins are more necessary in man than in any other animal, on account of his clothing, by which the neck, the ham, the arms, &c. are subjected to compressions that would be dangerous without these anastomoses. We can say that upon them alone is founded the possibility of a variety of modes in clothing. That they show in fact that these modes are less dangerous than some physicians have thought; that the danger of apoplexy from a tight cravat, of varices from tight garters, &c. is much less than they have said.

When a single trunk is compressed, the blood passes easily into the neighbouring ones; but if the compression is made upon all the trunks of a limb, a certain time is requisite for this fluid to dilate the anastomoses. It experiences, before this dilatation takes place completely, a kind of stoppage in the capillary system, a stoppage that explains the momentary redness of the fore-arm in women, when the arm is covered with too tight a sleeve, that of the hand or the foot when the bandages of the fore-arm or the leg are too tight.

The mode of the venous anastomoses is very analogous to that of the arteries. Sometimes the smaller branches anastomose with the trunks, sometimes the trunks communicate among themselves.

In the last mode, 1st, there is merely a branch of communication, and this is the most common case; we see this between the jugulars, between the deep and superficial veins of the thigh, the arm, &c. 2d. Two branches unite by their extremities and form an arch, the mesenterics afford an example of this. 3d. Sometimes instead of a trunk, there is an interlacing of the smaller branches that form a real venous plexus; such as that which surrounds the cord of the spermatic vessels.

In general, where there are the most obstacles to the blood, there the anastomoses are the most numerous. Hence why the veins that surround the spermatic cord communicate so frequently together, why the smaller branches of the hypogastric vein which are spread in the bottom of the pelvis, form there a plexus so extended, that it is a real net work in which the course of no one branch can be traced, so numerous are the communications. Notwithstanding this, these two portions of the venous system are the frequent seat of varices; they are even found more frequently dilated in the dead body on account of the difficulty the blood experiences there in rising against its own weight.

This leads us to a general reflection upon the venous system in relation to the anastomoses, and that is respecting the necessity there is for the communications being more numerous in this than in the arterial system. In fact, if we compare the course of the black blood with that of the red, we shall see that there are many more causes to modify that of the first.

The black blood evidently obeys its weight in certain cases. 1st. If we remain standing a short time the veins swell, especially after diseases in which the forces have been diminished; this swelling soon disappears if the leg is inclined; it increases if it remains perpendicular. 2d. There are many cases, in which the forces being very weak, the circulation cannot go on in perfection, except the legs are in a horizontal or inclined position. The influence of position upon many tumours and ulcers of the legs is undoubted. 3d. We know that the first effect of the attitude with the head reversed, is a giddiness produced by the difficulty the blood experiences in rising against its weight. 4th. The valves are particularly destined to counteract the effect of gravity.

Every violent motion communicated to the black blood, and independent of gravity, can also disturb the course of this fluid; it is thus that when we move violently in a circular direction, the venous cerebral blood receives, if we may so say, a centrifugal motion, which, turning it from its natural direction, and preventing it from going entirely to the heart, produces a stoppage of it, and hence the dizziness that is experienced.

It is not only gravity and every other external cause of motion, which influence at every moment the motion of the blood in the veins, but there are also external and internal pressures, and a variety of other mechanical causes.

On the contrary, that of the arteries is independent of most of the causes, of weight especially, and of the internal motion. Why? because the rapidity of the motion is so great which the heart gives to the red blood, that the influence of gravity and every other analogous cause is necessarily nothing. Let us take a comparison; the greater the force with which a projectile is thrown into the air, the less influence the weight has in making it deviate; in the case of the blood its influence is still less. If the blood was driven in empty vessels, gravity would have some effect in the arteries; but in the sudden shock impressed upon the whole fluid that fills them, a shock, the effect of which is felt at the extremities at the same time as at the origin, it is evident that its effect is nothing. For an opposite reason, we can understand why it is so powerful in the veins, in which there is no agent of impulse, in which the parietes and capillary system alone produce the motions, where the motion is consequently slow, &c.

From these considerations, it is easy to see the reason of the very different arrangement that the arteries and veins exhibit in their branches, as it respects anastomoses, which are as rare on one side as they are frequent on the other.

IV. _Termination of the Veins._

The veins terminate by two principal trunks, the superior and inferior venæ cavæ. There is also another, viz. the coronary vein, which empties separately into the right auricle; but as this trunk only brings back the blood that went to the heart, we shall pay but little attention to it in our general remarks, and but little to those small venous branches that empty separately from it into the same auricle.

Some authors have thought, that the two venæ cavæ were continuous and formed but one vessel; but it is easy to see how different their direction is. It is particularly in the fœtus, that their separation can be well perceived, since one corresponds with the right auricle, and the other with the left. There is behind the right auricle a kind of continuity of the membrane between the one and the other; it is the membrane of the black blood that is common to them, and which passes from the inferior to the superior; but in this respect there is no more continuity between them, than between the right side of the heart and the pulmonary artery, between the left and the aorta, &c.

By considering the whole of the trunks and the branches as a cone, we can say that there are two great venous cones distinct from each other; one for all the parts which are above the diaphragm, the other for all those that are below it.

The superior vena cava does not answer, then, entirely to the union of the arteries that form the aorta of the same name, which is only destined to the neck, head, and superior extremities, whilst the other belongs moreover to the chest by the vena azygos. For a contrary reason, the descending aorta has a destination much more extended than the inferior vena cava.

The limit between the two cones of the ascending and descending venæ cavæ, is placed at the diaphragm. It is especially in this respect that we can say that this muscle divides the body into two parts. Has not this arrangement some influence upon the difference that is observed in certain diseases between the superior and inferior parts? Should not this cause be connected with those pointed out under the article upon the fœtus? As yet there is nothing certain with regard to this, but I think it not improbable.

Though forming each a distinct cone, the two venæ cavæ communicate, however, especially in the neighbourhood of their common limit, that is to say, in the neighbourhood of the diaphragm; the azygos is the great means of communication. We know, in fact, that its trunk opens into the right renal, into the vena cava itself, or into some of the lumbar veins, and that the semi-azygos that arises from it, goes also to the left renal or to the lumbar of the same side. This anastomosis is very important; physicians have not paid sufficient attention to it. It proves that when an obstacle is situated in the trunk of the inferior vena cava, a great part of the blood of this trunk can flow into the superior. Much has been said of the compression of this trunk by the enlargements of the liver, in the production of dropsies. But, 1st, it is ascertained by the numerous examinations of dead bodies in modern times, that the production of these diseases belongs to every kind of organic affection; that the lungs, the heart, the womb, the spleen, &c. can likewise occasion them in the latter periods of the alteration of their texture; and that, in this respect, they are but a symptom in the greatest number of cases, and a symptom wholly disconnected with any sort of compression. 2d. By supposing that the liver could exert upon the vena cava an analogous compression, in the place where this vein crosses its posterior part, it is evident that the anastomoses of which I have just spoken, would prevent the effect of this compression, at least in great measure.

By supposing that an obstacle was encountered in the vena cava superior, the same anastomoses would undoubtedly answer the same end; but as the azygos is inserted very near the auricle, as the course of the trunk of the vena cava superior is consequently very short, it is evident that it is especially to counteract the obstacles the inferior may experience, that these anastomoses have been established.

When the blood of this vein passes thus into the superior, it goes through certain branches in a direction opposite to that which is natural to them. For example, suppose that the anastomosis takes place in the renal, which most often happens; then the blood of the trunk of the vena cava enters by one extremity of this vein; that from the kidney comes by the opposite extremity, and both pass into the azygos. A similar motion evidently supposes the absence of valves in the renal, from the vena cava to the insertion of the azygos. Now the renal veins never in fact have these folds; the capsular, the adipose of the kidney, all the lumbar, are also destitute of them, as Haller has seen, and I have ascertained it to be uniformly so. This absence of valves at the places of the anastomoses of the azygos, is a remarkable phenomenon; it proves very well the use that I attribute to the communication of the two venæ cavæ by means of this.

ARTICLE SECOND.

ORGANIZATION OF THE VASCULAR SYSTEM WITH BLACK BLOOD.

I. _Texture peculiar to this organization._

This organization is nearly the same for the whole system, in the common membrane that forms the great canal in which the black blood is contained; but it differs in the textures that are connected exteriorly with this membrane. In the heart the texture is fleshy; it is analogous to the texture of the divisions of the aorta, in the pulmonary artery; it has a peculiar character in the veins; it is this that will now particularly engage our attention.

_Membrane peculiar to the veins._

In order to see this membrane, it is necessary to remove, 1st, the loose cellular texture that unites the veins to the neighbouring parts; 2d, the cellular layer of a peculiar nature that immediately covers them, and of which we have spoken in the article upon the cellular system. Then we distinguish in the great trunks, longitudinal fibres all parallel to each other, forming a very fine layer, often difficult to be seen at first view, but always having a real existence. When the veins are much dilated, these fibres being more separated, are less evident than in a state of contraction. These longitudinal fibres are seen more clearly in the trunk of the inferior vena cava than in that of the superior. In general it may be said, that they are also more marked in all the divisions of the first than in those of the second; dissection has convinced me of this. Undoubtedly this arises from the greater facility that the blood experiences in circulating in the second than in the first of these veins, in which it mounts against its own weight; this is moreover a proof that man was designed to go erect.

I have uniformly made another remark, it is, that in the superficial veins these fibres are much more evident than in the deep-seated ones; the internal saphena is a remarkable example of this. It suffices to open it in its course, to see very distinctly its fibres through the common membrane, especially if it is a little contracted. By cutting the crural vein to compare it with this, it is easy to perceive the difference, which arises without doubt from this circumstance, that the neighbouring parts assist the circulation in the deep veins, whilst there is less of this assistance given to the superficial ones.

The branches have fibres in proportion greater than the trunks; hence the proportional excess of the thickness of their parietes, their greater resistance to the blood, their less frequent dilatation, &c.

At the place where any branch arises from a trunk, we observe that the fibres change their direction and go upon the branch, a circumstance that distinguishes them from arterial branches, whose fibres are not a continuation of those of the trunk.

Venous fibres oftentimes approach each other, are united, and give a greater thickness to the vein; this is frequently observed at the origin of the saphena. I have also seen this arrangement in the hypogastric vein; Boyer has likewise noticed it.

In general, the venous fibre, except in these places, is remarkable for its delicacy, for the little thickness that it consequently gives to the membrane that it forms. The peculiar membrane of the arteries infinitely exceeds that of the veins in this respect; it is this delicacy that favours to a great degree venous extensibility. Observe that the structure of each kind of vessels is adapted to their peculiar circulation. If the blood circulated in the veins with parietes like those of the arteries, its motion would be continually disturbed. A thousand causes retard the venous blood; when its motion is languid, the capacity of the vessels is increased; now, the arterial textures not allowing of this dilatation, it is evident that the circulation would be interrupted. If then the agent of impulse, placed at the commencement of the arteries, requires a firm and unyielding texture, the slow motion of the blood in the veins, the frequent causes that retard its progress, demand a texture of an opposite character.

What is the nature of the venous fibre? Its appearance, its want of elasticity and brittleness, its great extensibility of texture, its softness, its colour, its direction, distinguish it completely from the arterial fibre. Is it muscular? it does not appear to be irritable, and it does not look like the muscular fibres. I believe that it is of a peculiar nature, essentially different from that of all the other textures, having its peculiar properties, life and organization; I do not think it capable of much motion. We have, however, but few data upon this point.

The venous fibre, though infinitely more extensible than the arterial, is also more resisting; it will support without breaking, very considerable weight. The experiments of Wintringam have proved this. In the superficial and inferior veins especially, this is very remarkable.

There is a great difference in individuals as it respects the venous fibres. In some they are very apparent; in others, they are hardly distinguishable upon the great trunks; but then they are always very evident in the branches, particularly in the superficial ones.

There are places in the venous apparatus where we cannot discover either external fibres, or external cellular texture; this is the case with the cerebral sinuses, which have the following arrangement. The jugular vein at its sinus, loses its peculiar texture, and keeps only the common membrane, which, entering the lateral sinus, lines it, and extends below into the inferior longitudinal sinus, and above into the superior; in a word, into all the sinuses of the dura mater. Hence every sinus supposes, 1st, a separation of the layers of the dura mater; 2d, the common membrane of the black blood lining this separation. It is not then upon the dura mater that the blood circulates; it is upon the same membrane that it flows elsewhere; it is easy to establish this fact in the superior longitudinal sinus. This sinus is triangular when considered in relation to the separation of the layers of the dura mater; but in opening it, we see clearly, that the common membrane, by passing over its angles, makes it round; this is very evident. It is easy, also, in many other sinuses, to separate in certain places this membrane from the dura mater; but in the greatest number the adhesion is close, like the union of the arachnoides with the internal surface of the dura mater. This common membrane of the black blood is spread over the folds of the superior longitudinal sinus; it forms a singular net-work, which I shall describe in the cavernous sinuses.

From this general outline, it is evident that the coats of the dura mater supply in the sinuses the place of the venous fibres and their external dense cellular texture; the common membrane is always the same; but the texture that is added to it externally is different. At the place where each cerebral vein opens into a sinus, the common membrane of that sinus connects itself with it in its passage and lines it to its extremities. I know of no author who has thus considered the cerebral sinuses, as having the common membrane of the black blood extended into all the separations of the dura mater. How little soever we examine the internal surface of a sinus, it is easy to see that this surface differs as much from the texture of the dura mater, as it resembles the internal surface of the veins.

The cerebral veins, of which the sinuses are the terminations, are analogous to the arteries of that part in the extreme tenuity of their parietes, a tenuity that appears to be owing to the absence of the cellular coat, and which is so great, that you might believe that there was only a common membrane.

There are no circular fibres in the veins.

_Common Membrane of the Black Blood._

This membrane generally extended from the general capillary system to the pulmonary, is every where nearly of the same nature. It differs essentially from that of the red blood in a great number of respects.

1st. It admits of much greater distension; consequently it is less brittle. Tie a vein, and it will not break, unless the constriction be excessive; it is almost as pliable as the cellular coat. This pliability renders it much easier of dissection than the common membrane of the arteries. 2d. It appears to be much more delicate; we have a proof of this in the valves, which at first view are hardly visible from their extreme tenuity, when they are lying against the external surface of the vein. 3d. This common membrane is never ossified in old age, like that of the arteries; its organization seems to resist the deposition of phosphate of lime. When it does take place, it is an unnatural state; whereas the ossification of the common membrane of the red blood is almost a natural state in old age, as I have before observed. This difference between the two common membranes of the black and red blood, gives a distinctive character to the diseases of the heart. We never see ossification in the tricuspid valves, or in the semilunar valves of the pulmonary artery, whilst they are so frequent on the left side; this is the uniform result of observations made at the Charity; in the bodies of old people dissection has always shown me the same thing. So the pulmonary artery, though analogous to the aorta by its peculiar membrane, is never the seat of these ossifications, because the common membrane differs essentially from its own. This single phenomenon, so striking in both these membranes, would incontestibly prove their organic differences, while it establishes the necessity of considering them in a general manner, whether, in the black blood, they line the veins, the pulmonary artery, and the right side of the heart, or in the red blood, they are spread upon the arteries, the left side of the heart, and the pulmonary veins.

_Of the Valves of the Veins._

The common membrane of the black blood has numerous folds that are called valves. These folds are wanting in the pulmonary artery, except at its origin, where we find the semilunar valves; in the heart, the tricuspid valves are in part formed by this membrane; but the venous valves are wholly made by it; it is with them that we are particularly concerned.