THE DIAPHRAGM OR MIDRIFF 



animals, it is not always possible to decide whether a 

 given fragment is really of a mammal or a reptile. There 

 is a famous fossil, the fragment of a skull of an animal, 

 which has been named Tritylodon ; it is not fully agreed 

 whether this creature was a mammal or a reptile. 

 In the case of such a fragment as a bit of a long bone or 

 a finger bone, the problem would be still harder, and 

 often insoluble. 



With living mammals and living reptiles there is no 

 difficulty at all in distinguishing them by bony and other 

 characters. With a skull or the entire skeleton before 

 him, no one could fail to recognize the few features to 

 which attention has been directed. And there are 

 of course many others, to enumerate which would be 

 too long a task here. 



The internal anatomy of a mammal is constructed upon 

 the same general plan as is that of a reptile or an am- 

 phibian, as has been already set forth in our general 

 description of the vertebra ta. But there are important 

 differences of minor weight which differentiate the 

 mammal from all other vertebrates. The most important 

 of these is the fact that in the mammal the heart and 

 lungs lie in a separate chest cavity, divided off from the 

 abdominal cavity, which contains the liver, intestines, 

 kidneys, and so forth, by a muscular partition, known 

 as the diaphragm. In all mammals this character 

 holds good ; and in no animal which is not a mammal is 

 there any structure exactly comparable to this dia- 

 phragm. 



CLASSIFICATION OF MAMMALS 



Having now got at a notion of what a mammal is as 

 compared with other vertebrates, it is requisite to see 

 how far the mammalia can be conveniently subdivided 

 among themselves ; for it will be plain to the least 



13 



