THE MAMMALS 227 



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 these 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 



