The precise function assigned to this extensive plexus of arteries has not been as yet satisfactorily determined, although it is doubtless a receptacle wherein arterial blood is stored up during the long-continued submersion to which these animals are so frequently subjected.

(2339). As the Cetacea have no pelvic extremities, the aorta, instead of bifurcating into iliac arteries, is entirely appropriated to supply the enormous tail, beneath which it is continued, enclosed in a canal formed by the roots of the inferior spinous processes of the caudal vertebrae, that are here again developed as in fishes.

(2340). The venous system in the Cetacean order is equally remarkable for the plexuses formed by it in different parts of the body; of these, the most important communicates with the abdominal cava, and is of immense extent. The veins of these creatures, moreover, are almost entirely deprived of valves; so that every possible arrangement has been made to delay the course of the circulating blood during the temporary suspension of respiration that occurs whenever the animal plunges beneath the surface of the water.

(2341). In other aquatic Mammals that dive, and are thus subjected to prolonged immersion, large dilatations are found connected with the principal trunks of the venous system in the neighbourhood of the heart, in order to prevent a dangerous distension of these veins while the circulation is impeded and respiration put a stop to. This is particularly remarkable in the Seal tribe; and in these Carnivora we are assured by good authorities that it is not uncommon to find the foramen ovale of the heart, and the ductus arteriosus, which in the foetus allows blood to pass from the pulmonary artery directly to the aorta, still open even in the adult animal; but this arrangement, as we are well satisfied, is by no means to be regarded as the normal structure of the heart in a Seal.

* Hunter, ut supra, p. 365.

(2342). In many of the long-necked herbivorous quadrupeds a peculiar provision has been made in the disposition of the internal carotid arteries, apparently intended to equalize the force of the blood supplied to the brain in different positions of the head: for this purpose the arteries referred to, just as they enter the skull, divide into several branches, which again unite so as to assume a kind of plexiform arrangement, forming what is called the rete rnirabile of old authors. The effect of this subdivision of the main trunk into so many smaller channels will evidently be to moderate the rapidity with which the blood would otherwise enter the cranium, and thus preserve the brain from those sudden influxions to which it would otherwise be constantly liable.

(2343). We must likewise notice a structure, in some respects similar to the above, that exists in the arteries both of the anterior and posterior extremities of the Sloth (Bradypus.) In these slow-moving animals, the axillary and iliac arteries, just before entering the limbs to which they are respectively destined, suddenly divide into numerous small channels, which again unite into one trunk before the arteries of the member are given off. No doubt such an arrangement will very materially retard the course of the blood as it flows through these multiplied canals, and perhaps is materially connected with the long-enduring strength of muscle that enables these creatures to cling without fatigue to the branches whereby they suspend themselves.

(2344). Innumerable other minor differences in the course and distribution of the blood-vessels might of course be pointed out, a few of which may require notice elsewhere; but, generally speaking, the arrangement of the vascular system in all quadrupeds is so similar, that the anatomical student who may push his researches thus far will never be at a loss in identifying the different vessels, and comparing them with those found in the human body.

(2345). Although the respiration of Mammalia is inferior, as regards the extent to which their blood is exposed to the influence of the atmosphere, to the perfection of this process in Birds, nevertheless such is the elevated temperature of the body in these hot-blooded animals, that a warm covering of some non-conducting material is here absolutely requisite to retain the vital warmth, and defend them against the ther-mometrical changes of the element they inhabit. Their skin is generally, therefore, clothed with a warm covering of hair, a cuticular structure the nature and growth of which it behoves us now to examine. We must first, however, notice the organization of the skin itself; and then the nature of the various structures employed to defend it will be readily understood.

(2346). The skin of all Mammals, like that of the human body, consists of the cutis or vascular true skin, of the epidermis or cuticle, and of a thin layer of pigment interposed between the two, which is a diversely-coloured secretion deposited, like the cuticle, upon the surface of the cutis.

(2347). The hairs that cover the quadruped, whatever be their form or thickness, are cylinders of horny or cuticular substance, that grow upon so many minute vascular pulps, from the surface of which the corneous material is perpetually secreted. Some kinds of hair are permanent, and, if constantly cut, will continue to grow during the whole life of the animal; such is the hair of Man, and that which forms the mane and tail of the Horse: but generally the hair is shed at stated periods, to be replaced by a fresh growth. For the most part these structures are so minute, that the apparatus employed in forming them escapes observation; but in very large hairs, such as those that compose the whiskers of the Seal, or of the Lion, it is not difficult to display the organs by which they are secreted. The appended figure, taken from one of the drawings in the Hunterian collection, represents a section of the lip of a young Lion; and in it all the parts connected with the growth of the larger hairs are beautifully displayed.