296 



BACKBONED ANIMALS. 



set in separate sockets. The first set, or milk teeth, are 

 finally discarded and a permanent set attained, general- 

 ly of four distinct kinds, adapted for various purposes : 

 incisors, canines, premolars, and molars. With these, 

 which differ much in different animals, the food is ground 

 up or torn, and rudely prepared, mixed with saliva and 

 swallowed, passing down the oesophagus into the stomach. 

 Here it is mixed with a secretion known as gastric juice, 

 and converted into chyme, finally passing into the smaller 

 intestine, where it is brought in contact with various secre- 

 tions, as bile, pancreatic juice, etc., and is known as chyle, 

 then passing to the blood-vessels through the lacteal tubes ; 

 thus a part of everything eaten is so much fuel for the 

 system. From the small intestine follows a larger one 

 through which all rejected matter passes. 



Circulation. The heart of mammals is four-chambered, 

 comprising two auricles and two ventricles. The blood 

 is hot, red, and contains two kinds of corpuscles, red. and 

 colorless. The latter have a nucleus, are spherical, and ex- 

 hibit movements similar to those of the Amosbce (Fig. 2). 

 The red corpuscles are the most abundant, and are nearly 

 circular. The impure blood from the body pours into the 

 right auricle, from where it passes to the right ventricle, 

 and thence to the lungs. Here it is changed into arterial 

 blood by the oxygen of the air and passes back to the 

 left auricle, then to the left ventricle, and finally is driven 

 through the great aorta and sent flowing through innumer- 

 able branches all over the body. 



Respiration. The mammals breathe by lungs, two 

 elastic, spongy bodies permeated with air-cells, each in- 

 closed in a membranous sac called the pleura. They hang 

 free in the cavity of the thorax. Air is taken in at the 

 mouth and nostrils, and passes down the windpipe into 

 the branches or bronchi, that do not connect with air-sacs 

 in the body as in the birds. In this way the oxygen is 

 brought in contact with the blood and aerates it^ 



