A closer resemblance to the plexiform limb-arteries of the Sloths is shown by the Slow Lemurs, as exemplified in the arm of Stenops tardigradus in fig. 423, c.1

Cuvier first indicated the analogous division into numerous branches of the brachial artery of the Porpoise; the plexuses afterwards receiving a more detailed account, with a figure, from V. Baer,3 who also found a similar arrangement in the pectoral fin of the Manatee, and, with a minor degree of subdivision, in that of the Walrus.5 V. Baer associates the speedy subdivision of the main artery with the restricted degree of movement of fin-shaped forelimbs but the extent and freedom of motion of the long prehensile limbs of the Sloths and Lemurs point to other conditions.

The extreme degree of plexiform multiplication of the arterial system in the intercostal and other vertebral branches, in the Cetacea, more plainly relates

1 Vrolik remarks of these plexuses, 'they consist not only of arteries, but also of veins; and that, by dividing in branches, these ramifications become smaller and smaller, and composed of a less number of vessels.' LXIX. p. 219.

2 XII. (1805, tom. 4).

3 CXCVI". fig. 1.

Ib. fig. II.
5 Ib. fig. III.

VOL. III.

423

LXIX'.

N N

to the provision of a reservoir of arterial blood, especially in behoof of the brain.

In the Porpoise the aorta, after giving off the two coronary arteries, forms the usual arch from the convexity of which are sent off three primary trunks: the first is the largest, and gives off, first, the posterior thoracic,' then the right carotid, and afterwards divides into the right brachial and internal mammary arteries. The second trunk sends off the left carotid, left brachial, and internal mammary arteries. The third primary branch is the left posterior thoracic artery. The common carotid sends off an inferior thyroid before dividing into external and internal carotids: the subordinate branches of both these vessels form the plexuses in various parts of the head, especially at the basis cranii and around the optic nerve. The posterior thoracic, on each side, bends down and gives off branches to the five anterior intercostal spaces: the succeeding ones derive their arteries directly from the thoracic aorta, and some of them by a short common trunk, which bifurcates. The intercostals send off the dorsal branch, and that which accompanies the rib: but they also, and chiefly, divide into a vast number of branches, forming by their close tortuous interlacement a thick substance, compared by Tyson to a gland,2 and, more truly, by Hunter, to the plexiform mass of the spermatic artery in the bull. This arterial structure lines the sides of the thorax from the ninth or tenth pair of ribs, forwards, penetrating between the ribs near their joints and behind the costal ligaments, and there anastomosing with corresponding productions from contiguous intercostal spaces: branches pass therefrom into the neural canal, surrounding the myelon with a similar plexus, increasing in thickness near the skull and about the macromyelon, and anastomosing freely with the myelonal meningeal arteries. Thus the neural axis can receive its appropriate stimulus of oxygenated blood during the periods of long submersion and consequent interruption of respiration, to which the Cetacea are subject. Any convoluted intercostal artery, contributing to this reservoir, can be unravelled and traced to a great length, without sending off branches or changing its calibre. In the Piked Whale (Balanoptera), the external thoracics and internal mammaries combine with the intercostals in supplying this huge and singular plexiform reservoir, The brachial trunk gives off an external thoracic to the pectoral muscles, a subscapular artery, one to the supraspinal fossa and a circumflex branch: these supply the muscles of the fin. The trunk then divides into two, each of which subdivides, and forms

1

XCIV. p. 365, note (1837). See also cxcvin". (1841), p. 383.

2

XCIV. p. 365.

XCIV. p. 365.

plexuses upon the humerus, mainly expended in nourishing the bones, their ligaments, and the enveloping integument. The caudal prolongation of the aorta is surrounded by layers of plexiform arterioles, as by a sheath, in its course along the wide hæmal canal.2

The littoral and herbivorous Sirenia, which never go so deep or stay so long submerged as the Whales, seem not to possess the intercostal and myelonal arterial plexuses: at least I found them not in the Dugong.3

Amongst the minor degrees of plexiform multiplication of arterial canals is that oldest recorded instance of the intracranial 'rete mirabile' at the base of the skull in Ruminants: it is large in grazers, is less in browsers, and least in the Giraffe which habitually feeds with the head raised. In Bovide, where the 'rete' is most extensive, it is situated at the sides and back of the sella turcica, in the sclerous venous receptacle called cavernous sinus.' There is no definite bifurcation of the cephalic arterial trunk into an ecto- and ento-carotid, in Ruminants: a small branch of the carotid, perforating the cranium at, or behind, the foramen ovale, represents the middle meningeal artery,' and joins the rete mirabile'; but this is mainly formed, in advance, by branches of the internal maxillary, which enter the fissura lacera anterior, subdivide and anastomose reticularly, and are continued backward, on each side of the 'sella,' as a 'rete mirabile': three or four transverse portions bring the main lateral retia' into union with each other; while, posteriorly, are sent off a median and two lateral narrow plexiform extensions: the latter diverge to anastomose with the precondyloid arteries; then the middle portion is continued to join the converging cerebral branches of the vertebral arteries. These, in the Ox, enter the neural canal between the axis and third cervical, are united together by oblique cross-branches, as they advance; each bifurcates in the neural canal of the atlas, sending one branch out through the anterior perforation of the neurapophysis, while the other converges towards its fellow, to terminate by anastomosing with the median production of the 'rete' from the circle of Willis,' and also with a division of each precondyloid artery.

In the Hog (Sus scrofa), a larger proportion of the comparatively small 'rete mirabile' is formed by the vessel entering the cranium through the posterior fissura lacera,' representing anentocarotid.' A common efferent vessel, piercing the inner

XCIV. p. 366 (note).

2 CXCVIII".

3 cxvir". p. 35. The trace observed by Stannius in the Manatee does not support a functional inference, as in the true whales. 4 cc",

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

1 cci". p. 614, pl. 28, fig. 4.

prey

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

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 an Ox: magn. 200 diam. CCIII".

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

1 CXCVII".

p. 968.