310 VEETEBRATE ANIMALS. 



As regards the skeleton of the Mdinmalia, it is not necessary to 

 add much to what was said in speaking of the Yertebrata generally. 

 "With few e.xceptions, the spinal column is divisible into the same 

 regions as in man — namely, the neck or cervical I'egion, the back 

 or dorsal region, the loins or lumbar region, the sacral region, and 

 the tail or caudal region (see fig. 159). In spite of the great differ- 

 ences observable in the length of the Jieck in different Mammals, 

 the number of vertebr;^ which form the cervical region is extra 

 ordinarily constant, being almost invariably seven. In this res- 

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

 with 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 vertebrfe 

 are usually six or .seven in number ; five in Man ; rarely less than 

 four. The sacral vertebr;e are usually amalgamated to form a 

 .single bone — the sacrum — but this is wanting in the Whales. The 

 number of vertebrse in the tail or caudal region varies from four 

 to as many as flve-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 dorsal vertebrae. As a rule, the ribs are 

 united to the breast-bone or sternum in front, not by bony pieces, 

 as in bin Is, but by cartilages. Only the front ribs I'each the ster- 

 num, and these are called the "true" ribs; the hinder ribs fall 

 short of the breast -bone, and are called the " false " ribs. The ster- 

 mmi 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 Bii-ds. The I'egular number of limbs in the Mam- 

 mals is four, two anterior and two posteiior ; and for this i-eason 

 the Mammals are often spoken of as Quadi-upeds. Some Mammals, 

 however, such as the Whales and Dolphins, have only the anterior 

 limbs, and many of the Amphibia and Reptiles walk upon four legs. 

 As regards the structure of the fore -limbs (fig. 160), the general 

 conformation of the limb has been ])reviously noted. The shoulder- 

 blade or sfiipida is never wanting, and is generally broad and flat. 

 The coracdid bones, Avhich form such a marked feature in the pec- 

 toral arch of the Birds, are incomplete, and are fused with the 

 scapula;, of which they form the so-called "coracoid process." In 

 the Monotrematous Mammals klone are the coracoids distinct bones, 

 sufficiently developed to articulate with the sternum. The collar- 

 bones or clavicles are often rudimentaiy or absent, and, when pres- 

 ent, are never united into a single bone. The humerus presents 



