MAMMALIA. 307 
5. The respiratory organs are in the form of two lungs, placed 
in the chest, and never communicating with air-receptacles 
situated in different parts of the body. In no case and at no 
period of life are gills or branchize present. 
As regards the skeleton of the Zamma/ia it is not necessary 
to add much to what was said in speaking of the Vertebrata 
generally. With few exceptions, the spinal column is divisible 
into the same regions as in man—namely, the neck or cervica] 
region, the back or dorsal region, the loins or lumbar region, 
the sacral region, and the tail or caudal region (see fig. 117). In 
spite of the great differences observable in the length of the 
neck in different Mammals, the number of vertebrze which 
form the cervical region is extraordinarily constant, being 
almost invariably seven. In this respect the Giraffe, which is 
one of the longest-necked of Mammals, agrees with the Whale, 
which can hardly be said to have aneck at all. The vertebra 
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 vertebrz are 
usually six or seven in number ; five in man; rarely less than 
four. The sacral vertebra are usually amalgamated to form a 
single bone—the sacrum—but this is wanting in the Whales. 
The number of vertebrze 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 dorsal vertebree. 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 Quadrupeds. 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. 
