296 BACKBONED ANIMALS. 
set in separate sockets. The first set, or miik 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 cesophagus 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 Amede (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 aérates it. 
