764 MAMMALIA. 



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 arrange- 

 ment is, no doubt, intended to prevent the stunning noises that would 

 else be conveyed from every side to the ear, by cutting off all imme- 

 diate communication between the auditory apparatus and the osseous 

 framework of the head. 



(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 corre- 

 sponding vertebras. 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 



