layer of the dura mater, carries the blood from the 'rete' to the brain, supplying principally the prosencephalon. Another instance of plexiform disposition in the arteries of Artiodactyles has been observed in the gastric branch of the cœliac axis of the Hog, prior to its ramification in the dorsal parietes of the stomach.'

The Perissodactyles have a recognisable entocarotid which, however, has no proper bony canal, but enters the cranium by the posterior fissura lacera: it forms strong bends as it converges towards its fellow, with which it is united, behind the sella,' by an external flexuous ramus communicans'; then, piercing the dura mater, the entocarotids are again united by the internal ramus communicans,' which completes the circle of Willis behind and also receives the arteria basilaris.' A rudiment of the 'rete mirabile' of Artiodactyles is represented by small plexiform vessels given off from the hinder part of Willis's circle.'

The entocarotid deeply grooves the apex of the petrosal in the Elephant. Branches of the ectocarotid form a remarkable plexus internal to the cheek-gland which opens between the eye and ear in this animal.

In most Carnivora the entocarotid traverses a curved canal in the petrosal and makes a bend on emerging, which protrudes at the foramen lacerum before entering the cranial cavity: the bend, so exposed externally, receives in Canide and Viverride (Herpestes) a branch of the ectocarotid; and, as it advances alongside the sella,' also anastomoses with branches of the internal maxillary and ophthalmic arteries. In Urside a smaller portion of the loop projects into the cartilage occupying the 'foramen lacerum.' The entocarotid and its bony canal are smallest in Felines and the Hyæna. In these a maxillary plexus is formed which supplies the internal maxillary, ciliary, ethmoidal, ophthalmic, and anterior meningeal arteries.

The condition of the mesenteric arteries in Man renders them, in a degree, reservoirs as well as conveyers of blood: it facilitates a more continuous or less interrupted supply to the intestinal membrane. The fewer anastomotic arches in the Carnivora favour a more rapid and direct supply of blood when the presence of chyme in the intestines gives stimulus to such supply. The human condition relates to the more regular and frequently repeated supplies of food; to the more constant and continuous presence of chyme upon the villous surface of the gut. In the Dog, and more especially in wild Carnivora, the gorging of prey

ccr". p. 614, pl. 28, fig. 4.

is followed by fasting, by long intermission of the supply of chyme; consequently the provision for the arterial supply is simplified, and at the same time adapted to the more rapid assimilation which the hungry or famished frame requires.

Modifications of the arterial supply of the mammary glands, exemplified by large epigastric and subcutaneous abdominal branches, anastomosing with the internal and thoracic-mammary arteries, accompany the position and extension of the mammary glands from the thoracic to the inguinal regions in many quadrupeds.

The ultimate capillaries of the arterial ramifications either open directly into venous capillaries, or into sinuses, as in erectile and uterine structures, whence the venous capillaries begin. In exceptional cases, as in the Bat's wing, arteries of the second and third order of

424

4

100

Epithelium from vena cava of Sheep. cci".

branches have been observed to pass into veins of corresponding

size, without the intermedium of capillaries.'

§ 349. Veins of Mammalia.-The delicate structureless coat of the capillaries present, as the venules enlarge, an epithelial lining of flat, usually rhomboid, nucleate scales, fig. 424. The middle tissue of the vein includes sparing delicate unstriped fibres in an abundant bed of connective or areolar tissue, in which may be distinguished an internal stratum of wavy longitudinal fibres, fig. 425 a, a middle stratum of intermixed circular and

425

longitudinal fibres of elastic Longitudinal vertical section of wall of subclavian vein tissue, imbedded in a nidus

of white or contractile fibres, b, and these degenerating into an

' cxcvII". p. 968.

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 b and 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

426

a

6

the major part of the venous 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 to 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,
Human. CCVI".

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."

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.

2

X

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' trunk 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. 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 posteaval, 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