It is in consequence of this great aggre gation of muscles, -which are developed in unexampled proportions as compared with other Mammals, that the tail of the Cetaceans acquires the prodigious strength which it pos sesses, and by means of which these gigantic animals propel themselves with so much faci lity and impetuosity through the water, and so readily ascend to the surface to respire, and again seek protection in the deep abysses of the ocean.
The sternum (g,lig. 246) is short and large. In the Dugong it is composed of five pieces ; in the Dolphin, the Porpesse, and the Pla tanist, it is generally composed of only three ; in the Whales it consists of but one. In the subjoined figure (fig. 254) from the Bake noptera Boops, the sternum is deeply notched behind, and has a large ridge on its exterior or under surface.
The ribs of the Cetaceans are ch efly remarkable for their great curvature, but differ in their rela tive length, thickness, and mode of connection.
Their thickness and the density of their tex ture is most remarkable in the Herbivorous species, especially in the Manatee. In the Dugong, which has eighteen pairs of ribs (r, 7,, fig. 246), only the first three have car tilages which join the sternum. In the Del phinidee the first pair of ribs are articulated at their sternal extremities to the anterior angles of the first bone of the sternum ; the second pair join the sternum between the first and second bones ; the third between the second and third, and the fourth, fifth, and in some species the sixth pairs of ribs are joined to the third bone of the sternum the sternal portions of these ribs are ossified. The anterior ribs are articulated at first by a head to the ver-. tebral centres, and by a tubercle to the trans verse processes ; but as they extend backwards the head disappears, and the ribs are attached only to the extfemities of the transverse pro cesses.
In the Bakenida the first pair of ribs are remarkable for their great breadth, •especially at the sternal extremity, and these alone join the sternum. In the Edema Capensis the two first, as well as the four last pairs of ribs are joined only to the transverse processes of? the vertebrm.
The depressors and elevators of the ribs ap pear to possess nothinq particular, and the same may be said of the diaphragm and the muscles of the abdomen ;. but in regard to the movements of these parts, we must remember what M. Mayer says of the muscular fibres, which encircle closely the lungs, and which take part in the actions of inspiration and expiration.
[Mr. Hunter observes that, " as the ribs in this tribe do not completely form the cavity of thd thorax, the diaphragm has not the same attachments as in the Quadruped, but is con nected forwards to the abdominal muscles, which are very strong, being a mixture of muscular and tendinous parts. The position of the diaphragm is less transverse than in the Quadruped, passing more obliquely back ward and coming very low on the spine, and high up before, which makes the chest longest in the direction of the animal at the back, and gives room for the lungs to be con tinued along the spine."] The anterior members in the Cetaceans do not essentially differ from those of the other Mammalia, but they undergo, in these animals, very great modifications.
In the shoulder they are entirely devoid of clavicles. Their scapula is very large in general, but varies in this respect according to the species. In the Herbivorous Cetaceans, as the Dugong (s, fig. 246), the anterior angle is rounded, the posterior is extended backvvards, and the posterior margin or costa is concave. The spine is prominent, and so placed as to divide the dorsum of the scapula into a supra-spinal and infra-spinal depression. The acromion is pointed, but much less elongated in the Dugong than in the Manatee. The coracoid process is also more pointed in the Dugong,.
In the Zoophagous Cetaceans the spine of the scapula does not project much. The supra-spinal fossa is reduced to a mere groove in the common Dolphin, and entirely dis appears in the Gangetic species (Platanista) ; the coraco'id process does not exist in this last dolphin ; and the same absence is found in the Bakenidee, whilst it is seen in the common Dolphin and the Cachalot. Lastly, the acro mion appears always to exist, but with a different development, in different species. In the scapula of the Whalebone Whale (A, fi, 255) it is marked a. The articular or b• glenoid cavity (b) is proportionally larger in this species than in the Spermaceti Whale. The muscles of this part of the anterior mem ber present some remarkable modifications, but with which we are only acquainted as they exist in the common Dolphin. Thus the serratus magnus does not extend as far as the cervical vertebrce, and ends at the ribs ; the pectoralts minor, instead of descending on the ribs, is directed towards the anterior extremity of the sternum.