CETACEA. 585 



persist near the mouth. The thick layer of fat or blubber 

 beneath the skin serves to retain the warmth of the body, and 

 thus compensates for the absence of hair. 



The general shape, the absence of external ears, the 

 absence of an eye-cleansing nictitating membrane, the dorsal 

 position and valvular aperture of the single or double nostril, 

 the sponginess of the bones, the networks or retia mirabilia 

 of blood-vessels in different parts of the body, may be asso- 

 ciated with the aquatic life of these mammals. 



The cervical vertebrae are thin and more or less fused. 

 There is no union of vertebrae to form a sacrum, for the 

 hind-limbs are at most very rudimentary. Under the caudal 

 vertebrae there are wedge-shaped chevron bones. 



The brain-case is almost spherical ; the supra-occipital 

 meets the frontals and shuts out the parietals from the roof 

 of the skull ; the frontals arch over the orbit ; the snout or 

 rostrum of the skull is composed of premaxillae, maxillae, 

 and vomer, and of the mesethmoid cartilage. 



There is only one set of teeth, of uniform pattern, and in 

 the baleen whales they are shed before birth, being in 

 some measure replaced in adult life by horny baleen plates 

 developed on the palate. No clavicles are developed. 

 Excepting the humerus, the bones of the fore-limb are stiffly 

 jointed and flattened. There are four or five digits, of 

 which the second and third have more than the usual 

 number of phalanges. 



The pelvis consists of two rudimentary ischia lying freely, 

 and to these small rudiments of a limb are sometimes 

 attached. 



The rounded brain is relatively large, with well-convoluted 

 cerebral hemispheres. 



As to the alimentary system, salivary glands are rudi- 

 mentary or absent, the stomach is chambered, the intestine 

 has rarely a csecum, the liver is but slightly lobed, there is 

 no gall-bladder. 



The heart is often cleft between the ventricles. Both 

 arteries and veins tend to form plexuses or retia mirabilia. 



The larynx is elongated so that it meets the posterior 

 nares, and forms a continuous canal down which air passes 

 from nostrils to lungs. Cetaceans must, of course, rise to 

 the surface to inspire, and the expiration occurs at longer 



