THE MAMMALS 207 
every case the food substances are acted on by a digestive 
system constructed on the same general plan as that in man, 
yet modified according to the specific work it is required to 
perform. The teeth especially afford a valuable indication 
of the animal’s feeding habits, and, as we may notice later, 
are also of much value in classification. They consist of 
incisors used in biting, canines for tearing, and premolars 
and molars for crushing and grinding. 
The remaining portions of the digestive tract, esopha- 
gus, stomach, and intestine, with their appended glands, are 
usually not unlike those possessed by the squirrel (Fig. 1). 
The chief differences are in the size of the various regions. 
The stomach, for example, may be long and slender or of 
great dimensions, and its surface may further be increased 
by several lobes, which are especially well developed in the 
ruminants or cud-chewers. The intestine, relatively longer 
in the mammals than in any other class of vertebrates, also 
exhibits great differences in length and size. In the flesh- 
eating species its length is about three or four times the 
length of the body, while in the ruminants it is ten or 
twelve times the length of the animal. 
214. Nervous system and sense-organs— As before noted, 
the nervous system of mammals is characterized by the 
large size and great complexity of the brain. Even in the 
simpler species the cerebral hemispheres (large front lobes 
of the brain, Fig. 1) are well developed, and in the higher 
forms of the ascending series they form by far the larger 
part of the brain. The sense-organs also are highly de- 
veloped, and are constructed and located much as they are 
in man. The greatest variations occur in the eyes. In 
some of the burrowing animals they are usually small, and 
in some of the moles and mice may even be buried beneath 
the skin and very rudimentary. On the other hand, they 
are large and highly organized in nocturnal animals; more 
so, usually, than in those which hunt their prey by day. 
The ears also have different grades of perfection, which 
