(2192). The lowest order of Placental Mammalia comprises those forms which, although they breathe air by means of lungs, and have hot blood like ourselves, are appointed to inhabit the waters of the ocean, wherein they pass their lives, and even bring forth and suckle their young. In order to live under such circumstances as these, the Cetacea must necessarily, in many points of their structure, be organized after the model of fishes ; and we cannot be surprised if, in their outward form, and even in the disposition of their limbs, they strikingly resemble the finny tribes. Their head is large - frequently, indeed, of enormous proportions; there is no neck apparent externally, - the head and trunk, as in fishes, appearing continuous. The anterior extremities are converted into broad fins, or paddles; whilst the pelvic extremities are absolutely wanting: posteriorly, the body tapers off towards the tail, and terminates in a broad, horizontal fin, which latter, however, is not supported by bony rays, as in the fish, but is entirely of a cartilaginous and fleshy structure.

Frequently there is even a vertical dorsal fin; but this, too, is entirely soft and cartilaginous, so that in the skeleton no vestiges of it are apparent*.

(2193). In the Whalebone-Whale (Ba-loena mysticetus) the peculiarities of the Cetaceous skeleton are well exhibited. In this gigantic animal (fig. 385), which sometimes measures upwards of a hundred feet from the snout to the tail, the head forms nearly a fourth part of the entire length of its stupendous carcass; so enormously developed are the bones of the face that form the upper and the lower jaws. The cranial cavity, wherein the brain is lodged, of course does not participate in this excessive dilatation, but corresponds to the size of the brain lodged within it. It, however, presents one point of physiological interest, serving to prove still more demonstratively that the temporal bone is merely an adjunct to, and not essentially a constituent part of, the cranium; for here the petrous portion of the temporal bone, wherein is lodged the organ of hearing, is entirely detached from the skull, to which it is only fastened by a ligamentous connexion. This remarkable arrangement is, no doubt, intended to prevent the stunning noises that would else be conveyed from every side to the ear, by cutting off all immediate communication between the auditory apparatus and the osseous framework of the head.

Skeleton of the Whalebone Whale.

Fig. 385. Skeleton of the Whalebone Whale.

* It is interesting to see these fins still formed by the skin (exoskeleton), where the osseous system could not enter into their composition without deviating altogether from the Mammiferous type.

(2194). The cervical vertebrae, in conformity with the shortness of the neck, are exceedingly thin; and some of them are not unfrequently anchylosed into one piece.

(2195). The thorax is composed in the ordinary manner; but the posterior ribs are only fixed to the transverse processes of the corresponding vertebrae. Behind the thorax the whole spine is flexible, its movements being untrammelled by any pelvic framework or posterior extremity; so that, as in fishes, the broadly-expanded tail is the great agent in locomotion; and from the horizontal position of this mighty oar it is better adapted to enable the animal to plunge headlong into the depth, and to rise again to the surface, with all expedition, than if it had been placed vertically, as it is in fishes.

(2196). The only vestiges of a pelvis met with in the Whale are, the rudimentary ossa pubis represented in the figure. These are imbedded in the abdominal muscles, and serve to support the external organs of generation: the caudal vetebrae, however, are distinguishable by the inferior spinous processes developed from their under surfaces. As to the construction of the anterior extremity, the shoulder is composed of the scapula alone. The arm and fore-arm are much stunted, and are not moveable at the elbow; therefore the muscles for pronating and supinating the arm do not exist, but are represented by aponeurotic expansions spread over the surfaces of the bones. The bones of the carpus are flattened, and more or less consolidated together. The fingers, likewise, are flat; and the whole limb so covered with tendinous bands, and enveloped in skin, as to form merely a fin whereby the creature guides its course through the water.

(2197). In the Herbivorous Cetacea, as the Manatus and Dugong, the head is smaller in proportion to the sides of the body, and the hands better developed, so as to be useful in creeping on land, or in carrying their young. These genera inhabit the mouths of tropical rivers.

(2198). The relationship between the Cetacea and the next order that offers itself to our notice is too evident not to be immediately appreciated. The thick and naked skin, the gigantic body, the massive bones, the bulky head, and even the variable and irregular teeth that arm the ponderous jaws are all again conspicuous in the Pachydermata; and the river and the marsh, the localities frequented by the latter, as obviously indicate the intermediate position which these animals occupy between the aquatic and the terrestrial Mammalia.

(2199). The skeleton of the Hippopotamus (fig. 386) offers a good example of the general disposition of the osseous system in the Pachydermata. The spinous processes of the last cervical and anterior dorsal vertebrae are necessarily of prodigious strength, giving origin as they do to the muscles that support the weighty skull; the ribs are numerous, broad, and flat; they extend nearly along the entire length of the trunk, and thus assist in sustaining the bulky viscera of the abdomen. The pelvis is massive, in proportion to the weight of the body; and both the thoracic and pelvic extremities, short, thick, and strong, form, as it were, pillars upon which the trunk is raised.