outer mass of areolar tissue, in which such fibres as can be traced run lengthwise, c. This forms the greatest proportion of the venous coat, and the transition to the mid-stratum, b, is closer than that between band the thinnest layer, a, which is lined by the epithelial tissue. The elastic tissue is never so developed as to give the yellow colour which characterises the middle coat of arteries, or to maintain the tubular form in the empty vein. The valves are not confined to the place of union of the venous trunks with the heart, as is the case with arteries, but occur, usually in pairs, fig. 426, in the major part of the venous 426 system after the vessels have gained, in returning from the capillary area, a conspicuous size. They consist,' as Harvey described, of raised or loose portions of the inner membrane of these vessels, of extreme delicacy, and a sigmoid or semilunar shape. They are situate at different distances from one another, and diversely in different individuals; they are connate at the sides of the veins; they are directed upward or toward the trunks of the veins; the twofor there are, for the most part, two together-regard each other, mutually touch, and are so ready to come into contact by their edges, that if anything attempt pass from the trunks into the branches of the veins, or from the greater veins into less, they completely prevent it; they are further so arranged, that the horns of those that succeed are opposite the middle of the convexity of those that precede, and so on alternately.' He further writes, In many places two valves are so placed and fitted, that, when raised, they come exactly together in the middle of the vein, and are there united by the contact of their margins; and so accurate is the adaptation, that neither by the eye, nor by any other means, can the slightest chink along the line of contact be perceived. But if Venous valves, a, femoral vein: b, saphena interna, the probe be now introduced from the extreme towards the more central parts, the valves, like the floodgates of a river, give way, and are most readily pushed aside.'1 The most conspicuous tissue in these valves is of the white fibrous kind, having a general direction parallel with the free border of the valve and remarkable for their minute and equal undulations in this course. The epithelial lining is feebly indicated by scattered nuclei, especially at the extreme margin of the valve. As a general rule the cerebral and myelonal veins, those of the heart, lungs, kidneys, uterus, and liver, both portal and hepatic, have no valves: there are few valves in the external jugular; none in the internal jugular: valves abound in the veins of the pectoral limb, including the axillary, but are not present in the subclavian or precavals. The postcaval and iliac veins have no valves, but they abound in those of the pelvic limb. The spermatic veins have valves, but not the ovarian veins; they are few and incomplete where they have been found in the azygos veins. The varieties in the disposition of the veins exemplify, in mammals, a greater degree of repetition of primary or embryonal steps than do those of the arteries. The cardinal veins, which persist in great proportion in Lizards, fig. 420, unite at with the brachio-jugulars to form a short precaval' trunk2 on each side. That of the left receives the blood from the coronary vein before terminating in the right auricle h. A smaller proportion of blood is returned by the persistent venæ cardinales, z, z, in mammals; but with this exception, the disposition of the great trunks returning the blood from the head, trunk, and pectoral limbs, is essentially such in Lyencephala and most Lissencephala as is exemplified in Saurians. The blood from the left side of the trunk (intercostal or intervertebral spaces) is carried, in the ascending series of Mammals, by progressively increasing anastomosing channels from the left into the right cardinal vein: and, to such an extent in Man, that the right cardinal vein was noted for its want of symmetry as the vena azygos,' while the remnant of the left cardinal was called 'hemi-azygos.' With this change goes on an enlargement of an anastomosing vein between the right and left precavals at the upper and fore part of the chest, ultimately diverting the blood from the left precaval into the right, as the blood of the left cardinal had been attracted to the right cardinal vein. This is accompanied by obliteration of the left precaval trunk, of which a 1 CCIV". ་ 2 Called by some ductus Cuvieri.' remnant, recognisable by the developmental anatomist, becomes the coronary sinus' of Anthropotomy, or the dilated portion of the heart-vein, with well-marked fibrous tunic, receiving the 'oblique auricular vein' representing the left ductus Cuvieri,' and terminating by the wide valvular opening in the right auricle.' In the Monotremes each precaval, fig. 308, e, f, receives the azygos vein of its respective side: they are united by the characteristically mammalian transverse canal, g, which becomes the ' vena innominata' in Man. The postcaval has a long course in the thorax; before entering which it is greatly dilated, within the liver, in the Ornithorhynchus, as it is in the placental divers. The vena portæ is constituted as in other Mammalia. The veins of the kidney are continued from the renal artery, and communicate solely with the postcaval. In the Marsupials, also, the iliac veins combine to form the postcaval trunk, as in the rest of the Mammalia, without conveying any part of their blood to the kidneys: in the Kangaroo they both pass on the central aspect of the iliac arteries. The renal veins, in like manner, directly communicate with the abdominal cava, and do not contribute any share in the formation of the portal vein. This great secerning trunk of the hepatic organ presents the strictly mammalian condition, being formed by the reunion of the gastric, intestinal, pancreatic, and splenic veins. The primitive veins of the animal system of organs, commonly called "azygos," retain their original separation and symmetry; the left "azygos" bends over the left bronchus to communicate with the left precaval, and the right azygos over the right bronchus to join the right precaval.'2 This vein, b, returns the blood from the right side of the head and the right anterior extremity; the corresponding vein on the left side, c, passes down in all the Marsupials, in front of the root of the left lung, as in Birds and Reptiles, behind the left auricle, and, after receiving the coronary vein, joins the postcaval, d, immediately before its expansion into the auricle. The anterior anastomosing vein between the two precavals exists. Where the pelvic extremities are less or not larger than the pectoral ones, as in the Ursine Dasyure and Wombat, the postcaval is somewhat less than the left precaval, figs. 402 and 403, and they appear to terminate by separate apertures in the auricle; but in the Kangaroo, fig. 401, the proportions of the two veins are reversed, and the postcaval more obviously receives the left pre caval before it terminates: these two veins meet at a very acute angle, and are separated by a crescentic ridge similar to, but thinner than, that which divides their common orifice from the orifice of the right precaval. In most Lissencephala the left precaval descends behind the auricle to terminate with or near to the postcaval: the left vena azygos communicates with the left precaval in the Hedgehog and many others, and is larger than the right: opposite proportions prevail in Leporide and some other Rodents, as in Squirrels, the left azygos being small or wanting. In the Bats the venules of the wing-membrane, owing to their comparatively wide communications with the arterioles, manifest the impulse of the heart's action. In Cetacea the left cardinal and precaval veins are reduced to the condition of a coronary sinus which is large; the blood from the head and fins is returned by the right precaval trunk to the auricle. The definition of any distinct right or left azygos trunks is obscured by the characteristic expansion and plexiform multiplication of the veins at the back of the thoracic-abdominal cavity, fig. 427; and indeed the precaval system is chiefly brought into communication with the postcaval one by the continuity of the vast venous sinuses surrounding the neural axis and receiving the intercostal and lumbar veins, ultimately opening into the precaval by a short trunk which penetrates the posterior and right part of the chest. In fig. 427, the postcaval vein is cut across at p, where it lies in the interspace of the two masses of depressor muscles of the tail. Veins, m, m, which seem to answer to the iliacs of quadrupeds, return from the side-muscles of the tail; the caudal vein is represented by a plexus, f, and occupies much of the hæmapophysial canal. A plexus from the intestine, G, terminates in the right iliac vein, and thus establishes a communication between it and the portal system. A hypogastric plexus terminates at i. The peritoneal plexus is shown at 7; it becomes more considerable at the season of sexual excitement: but the chief abdominal reservoir of venous blood is formed by the vast psoadic plexus, k, which extends from behind the hinder end of the kidney, E, to the hinder end of the abdomen. In the Porpoise it forms a mass of reticulate veins upwards of an inch in thickness: it is fed by the caudal veins, a, b, c, d, behind, and laterally by from five or seven veins which, returning blood from the dorsal and lateral parietes of the abdomen, pierce the lateral abdominal muscles to join the plexus at e, e. At the mesial margin the psoadic plexus communicates by many and wide apertures with the iliac vein; and anteriorly with veins of the diaphragm or phrenic plexuses' which con |