MAMMALIA. 281 



5. The respiratory organs are in the form of two lungs, 

 placed in the chest, and never communicating with air-recep- 

 tacles situated in different parts of the body. In no case and 

 at no period of life are gills or branchias present. 



As regards the skeleton of the Mammalia it is not neces- 

 sary to add much to what was said in speaking of the Vert&- 

 hrata generally. With few exceptions, the spinal column is 

 divisible into the same regions as in man — namely, the neck 

 or cervical region, the back or dorsal region, the loins or lum- 

 bar region, the sacral region, and the tail or caudal region {see 

 Fig. 95). In spite of the great differences observable in the 

 length of the neck in different Mammals, the number of ver- 

 tebrae which form the cervical region is extraordinarily con- 

 stant, being almost invariably seven. In this respect the 

 Giraffe, which is the longest-necked of Mammals, agrees witli 

 the Whale, which can hardly be said to have a neck at all. 

 The vertebrae of the back or dorsal region are mostly thirteen 

 in number, but are often more. In man there are only twelve ; 

 and in some cases there are only eleven or ten. The lumbar 

 vertebrae are usually six or seven in number; five in man; 

 rarely less than four. The sacral vertebrae are usually amalga- 

 mated to form a single bone — the sacrum — but this is wanting 

 in the Whales. The number of vertebrae in the tail or caudal 

 region varies from four to as many as five-and-forty, and they 

 are usually freely movable upon one another. The thoracic 

 cavity or chest in Mammals is always enclosed by a series of 

 ribs ; the number of which varies with the number of the dor- 

 sal vertebrae. As a rule, the ribs are united to the breastbone 

 or sternum in front, not by bony pieces, as in birds, but by 

 cartilages. Only the front ribs reach the sternum, and these 

 are called the " true " ribs ; the hinder ribs fall short of the 

 breastbone, and are called the " false " ribs. The sternum is 

 composed of several pieces, placed one behind the other, but 

 usually amalgamated to form a single bone. It is usually long 

 and narrow in shape, and is only rarely furnished with any 

 ridge or keel, as it is in birds. The regular number of limbs 

 in the Mammals is four, two anterior and two posterior ; and 

 for this reason the Mammals are often -spoken of as Quadru- 

 peds. Some Mammals, however, such as the Whales and 

 Dolphins, have only the anterior hmbs, and many of the Am- 

 phibia and Reptiles walk upon four legs. As regards the 

 structure of the fore-limbs (Fig. 96), the general plan of con- 

 formation is the same as described in treating of the Verte- 



