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 constitute, in great part, the pia mater; and enter the ventricles, where they contribute to the formation of the plexus choroides and tela choroidea. On leaving the organ we find them situate between the lamina 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. They have three coats in superposition, according to most anatomists: but many modern anatomists are disposed to assign them six. The outer coat is areolar; 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; in the branches than in the trunks. M. Magendie' states, that he has never been able to observe the fibres of the middle coat; but 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 M. Magendie suspects its chemical nature to be fibrinous. It was remarked, in an early part of this work (vol. i. p. 58), that the bases of the areolar and muscular tissue are, respectively, gelatin, and fibrin; and that the various resisting solids may all be brought to one or other of those tissues. The middle coat of the veins doubtless belongs essentially to the former, and is a variety of the tissu jaune of the French anatomists. M. Magendie merely states its fibrinous nature to be a suspicion; and, like numerous suspicions, this may be devoid of foundation. Yet we have reason to believe, that it is contractile; and, of late,2 it has been described as formed of one or two or even more layers between the external and internal coats; these layers consisting of fibres, which agree, in all respects, with the white areolar tissue; and are either quite pure, or mixed in one or other of the layers with a greater or less amount of fibres, resembling those of the middle coat of the arteries in having the anatomical characters of the nonstriated or unstriped muscular fibres. M. Broussais3 affirms, that its contraction 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 in the majority of the veins, although they are very visible in the vena cava of frogs, where it joins the right auricle. In some experiments by M. Sarlandière 1 Op. cit., ii. 242. See, on the researches of recent histologists, Mr. Paget, Brit. and For. Med. Review, July, 1842, ii. p. 242. 2 Quain's Human Anatomy, by Quain and Sharpey, Amer. edit., by Leidy, i. 518, Philad., 3 Op. citat., American translation, p. 391. on the circulation, he observed these movements to be independent of those of the heart. After the organ was removed, and even after blood had ceased to flow,' the contraction and relaxation of the vein continued for many minutes in the cut extremity. Fig. 288. B C The inner coat is extremely thin and smooth at its inner surface, and has an epithelial lining. 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, which are 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 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. M. Magendie affirms, that he has never seen the distension 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 Bichat. Moreover, they are covered by the epithelial coat and consist of tissue like that of fibrous membrane, which, as Mr. Hunter observed, shows, that they are not duplicatures of the lining membrane. Their number varies in different veins. As a general rule, 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 deep-seated viscera; in those of the brain and spinal marrow, and 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 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 serve in regard to the semilunar valves. The valves exist in veins of less than a line in diameter. Diagrams showing Valves of Veins. out, with two pairs of valves. B. Longi A. Part of a vein laid open and spread tudinal section of a vein, showing the apposition of the edges of the valves in their closed state. C. Portion of a distended vein, exhibiting a swelling in the situa tion of a pair of valves. 1 See, on this subject, the remarks on the Circulation in the veins. 2 Précis, &c., ii. 241. 3 Treatise on the Blood, &c., by Palmer, Amer. edit., p. 216, Philad., 1840. VOL. II.-7 The three coats united form a solid vessel,-which, according to Bichat, is devoid of elasticity, but in the opinion of M. Magendie1 elastic in an eminent degree. The elasticity is certainly much less than in the arteries. The veins are nourished by vasa vasorum, or by small arteries, that have their accompanying veins. Every vessel, indeed, in the body, if we may judge from analogy, draws 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, that must derive its support or nourishment exclusively from its own stores. The nerves that have been detected on the veins are branches of the great sympathetic. The capacity of the venous system is generally esteemed to be double that of the arterial. 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 are generally so much greater than those 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 Keill at twenty-five to nine. The ratio between the capacity of individual arteries and veins, is very different in different parts. Between the carotid and internal jugular it is as 196 to 441; the subclavian artery and vein, 3844 to 7396; the aorta and venæ cavæ, 9 to 16; and between the splenic artery and vein, 136 to 676. There is one portion of the venous system, to which allusion has already been made, that is peculiar:-the abdominal 2 Elementa Physiologiæ, lib. ii., sect. 2, § 10, Lausann., 1757. venous or portal system. All the veins, that return from the digestive organs situate in the abdomen unite into a large trunk called vena porta. 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 cave; and correspond to the branches of the hepatic artery as well as to those of the vena porta. The portal system is concerned only with the veins of the digestive organs situate in the abdomen; as the spleen, pancreas, stomach, intestines, and omenta. The veins of all the other abdominal organs,-of the kidney, suprarenal capsules, &c., are not connected with it. The first part of the vena portæ is called, by some authors, vena portæ abdominalis seu ventralis to distinguish it from the hepatic portion, which is of great size, and has been called sinus of the vena porta. 2. BLOOD. Fig. 200. 7 It is not easy to ascertain the total quantity of blood circulating in both arteries and veins. Many attempts have been instituted for this purpose, but the statements are most diversified, partly owing to the erroneous direction followed by experimenters, but, still more, to the variation that must be perpetually occurring in the amount of fluid, according to age, sex, temperament, activity of secretion, &c. Harvey and the earlier experimenters formed their estimates by opening the veins and arteries freely on living animal, collecting the blood that flowed, and comparing this with the weight of the body. The plan is, however, objectionable, as the whole of the blood can never be obtained in this manner, and the proportion discharged varies in different animals and circumstances. By this method, Moulins found the proportion in a sheep to bed; King, in a lamb, th; in a duck, th; and in a rabbit 'th. From these and other observations, Harvey concluded, that the weight of the blood of an animal Portal System. 1. Inferior mesenteric vein: traced by means of dotted lines behind the pancreas (2) to terminate in splenic vein (3). 4. Spleen. 5. Gastric veins, opening into splenic vein. 6. Superior mesenteric vein. 7. Descending portion of duodenum. 8. Its transverse portion which is crossed by superior mesenteric vein and by a part of trunk of superior mesenteric artery. 9. Portal vein. 10. Hepatic artery. 11. Ductus communis choledochus. 12. Divisions of duct and vessels at transverse fissure of liver. 13. Cystic duct leading to gallbladder. (Wilson.) is to that of the whole animal as 1 to 20. Drélincourt, however, found the proportion in a hog to be nearly th; and Moor, ¿1⁄2th.1 Sir George Lefevre2 cites from Wrisberg, that from a plethoric young woman, who was beheaded, 25 pounds [?] of blood were collected; and some recent experiments by Mr. Wanner led to the following results: A bullock, weighing 1659 pounds imperial, yielded 69 pounds of blood, or in the ratio of 1 to 23-81; another weighing 1640 pounds, yielded 65 pounds, or in the ratio of 1 to 23.73; a cow, weighing 1293 pounds, yielded 59 pounds, or as 1 to 21.77; a sheep, weighing 110 pounds, yielded 5 pounds, or in the proportion of 1 to 22-72; another weighing 88 pounds, yielded 4.4 pounds, or as 1 to 20; and in a rabbit, the proportion was as 1 to 25 exactly. An animal, according to Sir Astley Cooper, generally expires, as soon as blood, equal to about th of the weight of the body, is abstracted. Thus, if it weighs sixteen ounces, the loss of an ounce of blood will be sufficient to destroy it; and, on examining the body, blood will still be found in the small vessels especially-even although every facility may have been afforded for draining them. Experiments have, however, shown that no fixed proportion of the circulating fluid can be indicated as necessary for the maintenance of life. In the experiments of Rosa, asphyxia occurred in young calves when from three to six pounds, or from 'd to th of their weight, had been abstracted; but in older ones not until they had lost from twelve to sixteen pounds, or fromth to th of their weight. In a lamb, asphyxia supervened on a loss of twenty-eight ounces, or gth of its weight; and in a wether, on a loss of sixty-one ounces, or d of its weight. Dr. Blundells found that some dogs died after losing nine ounces, or th of their weight; whilst others withstood the abstraction of a pound, or th of their weight; and M. Piorry affirms, that dogs can bear the loss of 'th of their weight, but if a few ounces more be drawn they succumb. From all the experiments and observations, Burdach concludes, that, on the average, death occurs when ths, or ths, of the mass of blood is lost, although he has observed it in many cases, as in hæmoptysis, on the loss of th, and even of th. The following table exhibits the computations of different physiologists regarding the weight of the circulating fluid-arterial and venous. Although the absolute estimate of Hoffmann has been regarded as 1 Haller, op. cit., lib. v. sect. 1, § 2. 2 An Apology for the Nerves, p. 30, London, 1844. 3 Edinburgh Med. and Surg. Journ., July, 1845. 4 Principles and Practice of Surgery, p. 33, Lee's edition, Lond., 1836. • Die Physiologie als Erfahrungswissenschaft, iv. 101 and 334, Leipzig, 1832. |