44 VERTEBRATA: MAMMALIA. 



Their nervous system, consisting of the brain composed 

 of two hemispheres the spinal cord and nerves, and nerve- 

 masses, is more fully developed than in any other animals. 



Their reproductive system is the most perfect that is 

 found in the whole Animal Kingdom. 



They are all endowed with vocal organs ; and they excel 

 all other animals in intelligence. 



As the Mammals are the highest class of the Vertebrates, 

 and consequently the highest class in the whole Animal 

 Kingdom, we should become specially familiar with their 

 structure, not only for the sake of the information which 

 we may thereby gain, but that we may better understand 

 the structure of the Vertebrates in general, and that we 

 may have a standard with which to compare the lower 

 Vertebrates. 



The general structure of the digestive system in Mam- 

 mals is shown in Fig. 54. 



The function of the digestive system, as has already 

 been stated, is to convert the food into blood. 



The acts by which the food is converted into blood, may now be 

 briefly stated. 



The food is seized by the animal and placed in the mouth, where it 

 is masticated by the teeth, and where it is mixed with a fluid called 

 saliva, which comes from cavities and glands situated in or near the 

 cavity of the mouth. 



After mastication it is pressed backwards by the tongue, and it then 

 passes into the pharynx, where swallowing commences, and whence 

 the food passes through the gullet into the stomach. 



By the action of a fluid called the gastric juice, the food, while in the 

 stomach, is changed into a substance called chyme. 



From the stomach the chyme passes into the small intestines, where 

 it mingles with a fluid secreted by its walls, and with the bile and pan- 

 creatic juice liquids furnished by the liver and pancreas and, during 

 the passage through the small intestines, digestion is completed, the 

 chyme being changed into a substance called chyle. 



The chyle is now absorbed by numerous vessels called the lacteals. 



