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let us now inquire into the nature and course of the fluid contained in the veins; but so far only as to enable us to understand the function of absorption; the other considerations, relating to the blood, appertaining to the function of circulation.

Sect. III. VENOUS ABSORPTION.

Anatomy of the Venous System.

This system consists of myriads of vessels, called veins, which commence in the very textures of the body, by what are called capillary vessels; and from thence pass to the great central organ of the circulation—the heart; receiving, in their course, the products of the various absorptions not only effected by themselves, but by the chyliferous and lymphatic vessels.

The origin of the veins, like that of all capillary vessels, is imperceptible. By some, they are regarded as continuous with the capillary arteries; Malpighi and Leeuenhoek state this as the result of their microscopic observations on living animals; and it has been inferred, from the facility with which an injection passes from the arteries into the veins. According to others, cells exist between the arterial and the venous capillaries, in which the former deposit their fluid and whence the latter obtain it. Others, again, substitute a spongy tissue for the cells.

A question has also been asked,—whether the veins terminate by open mouths; or whether there may not be more delicate vessels, communicating with their radicles,—similar to the exhalants, which are presumed to exist at the extremities of the arteries, and are the agents of exhalation.

All this is, however, conjectural. It has already been observed, that the mesenteric veins have been considered to terminate by open mouths in the villi of the intestines; and the same arrangement has been conceived to prevail with regard to other veins. Ribes concludes, from the results of injecting the veins, that some of the venous capillaries are immediately continuous with the minute arteries, whilst others open into the cells of the laminated tissue, and into the substance of the different organs.

When the veins become visible, they appear as an infinite number of tubes, extremely small, and communicating very freely with each other; so as to form a very fine net-work. These vessels gradually become larger and less numerous, but still preserve their reticular arrangement; until, ultimately, all the veins of the body empty themselves into the heart, by three trunks,—the vena cava inferior, the vena cava superior, and the coronary vein. The first of these receives the veins from the lower part of the body, and extends from the fourth lumbar vertebra to the right auricle; the second receives all the veins of the upper part of the body; and

into it the subclavian opens, into which the chyle and lymph are discharged. It extends from the cartilage of the first rib to the right auricle. The coronary vein belongs to the heart exclusively. Between the superior and inferior cava a communication is formed by means of the vena azygos.

Certain organs appear almost wholly composed of venous radicles. The spleen is one of these.

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The accompanying figure represents the ramifications of the splenic artery, a, in the substance of that organ; and if we consider, that the splenic vein has corresponding ramifications, the viscus would seem to be almost wholly formed of blood-vessels. The same may be said of the corpus cavernosum of the penis and clitoris, the nipple, urethra, glans penis, &c. If an injection be

thrown into one of the veins, that issue from these different tissues, they are wholly filled by the injection; which rarely occurs, if the injection be forced into the artery. Magendie affirms, that the communication of the cavernous tissue of the penis with the veins occurs through apertures two or three millimeters.—In.0.117. -in diameter.

In their course towards the heart, particularly in the extremities, the veins are divided into two planes ;—one subcutaneous or superficial; the other deep-seated, and accompanying the deep-seated arteries. Numerous anastomoses occur between these, especially when the veins become small, or are more distant from the heart.

We find that their disposition differs according to the organ. In the brain, they form, in great part, the pia mater; and enter the ventricles, where they contribute to the formation of the plexus choroides and tela choroidea. Leaving the organ, we find them situated between the laminæ of the dura mater; when they take the name of sinuses. In the spermatic cord, they are extremely tortuous, anastomose repeatedly, and form the corpus pampiniforme; around the vagina, they constitute the corpus retiforme; in the uterus, the uterine sinuses, &c. The veins have three coats in superposition. The outer coat is cellular, dense, and very difficult to rupture. The middle coat has been termed the proper membrane of the veins. The generality of anatomists describe it as composed of longitudinal fibres, which are more distinct in the vena cava inferior than in the vena cava superior; in the superficial veins than in the deep-seated; and in the branches than in the trunks. Magendie states, that he has never been able to observe the fibres of the middle coat; but that he has always seen a multitude of filaments interlacing in all directions; and assuming the appearance of longitudinal fibres, when the vein is folded or wrinkled longitudinally, which is frequently the case in the large veins. It exhibits no signs of muscularity; even when the galvanic stimulus is applied; yet Magendie suspects its chemical nature to be fibrinous. If so, it is perhaps different from every other tissue in the body. It was remarked, in an early part of this work, that the bases of the cellular and muscular tissues were, respectively, gelatine, and fibrine; and that the various resisting solids could all be brought to one or other of these tissues. To which, then, ought the middle coat of the veins to be attached. Magendie, however, merely states its fibrinous nature to be a suspicion; and, like numerous suspicions, it may be devoid of foundation. Yet we have reason to believe, that the veins are contractile; and the possession of this property would be in accordance with their fibrinous character. Broussais affirms, that this action is one of the principal causes of the return of the blood to the heart. He conceives, that the alternate movements of contraction and relaxation are altogether similar to those of the heart;

but that they are so slight as not to have been rendered perceptible by any process in the majority of the veins, although very visible in the vena cava of frogs, where it joins the right auricle. In some experiments by Sarlandiere on the circulation, he observed these movements to be independent of those of the heart. After the heart was removed, the contraction and relaxation of the vein continued, for many minutes, in the cut extremity, and even after the blood had ceased to flow.

The inner coat is extremely thin and smooth at its inner surface. It is very extensible, and yet presents considerable resistance; bearing a very tight ligature without being ruptured.

In many of the veins, parabolic folds of the inner coat exist, like those in the lymphatics, and inservient to a similar purpose: the free edge of these valves is directed towards the centre of the circulation; showing that their office is to permit the blood to flow in that direction and to prevent its retrogression. They do not seem, however, in many cases, well adapted for the purpose, inasmuch as their size is insufficient to obliterate the cavity of the vein. By most anatomists, this arrangement is considered to depend upon primary organization, but Bichat conceives it to be wholly owing to the state of contraction, or dilatation of the veins at the moment of death. Magendie, however, affirms, that he has never seen the distention of the veins exert any influence on the size of the valves; but that their shape is somewhat modified by the state of contraction or dilatation, and this he thinks probably misled BI

CHAT.

Their number varies in different veins. As a general principle, they are more numerous, where the blood proceeds against its gravity, or where the veins are very extensible and receive but a feeble support from the circumambient parts, as in the extremities. They are entirely wanting in the veins of the deepseated viscera;—in those of the brain and spinal marrow, of the lungs; in the vena portæ and in the veins of the kidneys, bladder and uterus. They exist, however, in the spermatic veins; and, sometimes, in the internal mammary, and in the branches of the vena azygos.

On the cardiac side of these valves cavities or sinuses exist, which appear externally in the form of varices. These dilatations enable the refluent blood to catch the free edges of the valves, and thus to depress them, so as to close the cavity of the vessel; serving, in this respect, precisely the same functions as the sinuses of the pulmonary artery and aorta in regard to the semilunar valves.

The three coats united form a solid vessel,—according to Bichat devoid of elasticity, but, in the opinion of Magendie, elastic in an eminent degree. The elasticity is certainly much less than that of the arteries.

The veins are nourished by vasa vasorum, or by small arteries, which hare their accompanying veins. Every vessel, indeed, in the body, if we may judge from analogy, appears to draw its nutriment, not from the blood circulating in it, but from small arterial vessels, hence termed vasa vasorum. This applies not only to the veins, but to the arteries. The heart, for example, is not nourished by the fluid constantly passing through it; but by vessels, which arise from the aorta, and are distributed over its surface, and in its intimate texture. The coronary arteries and their corresponding veins are, consequently, the vasa vasorum of the heart. In like manner, the aorta and all its branches, as well as the veins, receive their vasa vasorum. There must, however, be

a term to this; and if our powers of observation were sufficient, we ought to be able to discover a vessel, which must derive its support or nourishment exclusively from its own stores.

The nerves, that have been detected in the veins, are branches of the great sympathetic.

The capacity of the venous, is generally esteemed to be double that of the arterial, system. It is obvious, however, that we can only arrive at an approximation, and that not a very close one. The size and number of the veins is generally so much greater than that of the corresponding arteries, that, when the vessels of a membranous part are injected, the veins are observed to form a plexus, and, in a great measure, to conceal the arteries: in the intestines, the number is more nearly equal. The difficulty of arriving at any exact conclusion, regarding the relative capacities of the two systems, is forcibly indicated by the fact; that whilst Borelli conceived the preponderance in favour of the veins to be as four to one; Sauvages estimated it at nine to four; Haller at sixteen to nine; and Keil at twenty-five to nine.

There is one portion of the venous system, to which allusion has already been made, which is peculiar. We mean the abdominal venous or portal system. All the veins, that return from the digestive organs, situated in the abdomen, unite into a large trunk, called the vena portæ. This, instead of passing into a larger vein, into the vena cava, for example, proceeds to the liver, and ramifies, like an artery, in its substance. From the liver, other veins, called supra-hepatic, arise, which empty themselves into the vena cava ; and which correspond to the branches of the hepatic artery as well as to those of the vena portæ. The portal system is concerned only with the veins of the digestive organs situated in the abdomen; as, the spleen, pancreas, stomach, intestines and omenta. The veins of all the other abdominal organs,—of the kidney, supra-renal capsules, &c. are not connected with it. The first part of the vena ports is called, by some authors, vena portæ abdominalis vel ventralis, to distinguish it from the hepatic portion, which is of great size, and has been called the sinus of the vena porta.

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