6 ALLEN’S NATURALIST’S LIBRARY. 
molars, and still further back the molars, both with broad 
crowns of complicated tubercles and ridges for milling the hard 
portions contained in the food. Animals provided—as all the 
Primates are—with these different sorts of teeth, are said to be 
FTeterodont,* in contradistinction to forms like the Dolphins 
and Whales, which are termed Homodont,t because the whole 
of these teeth are of the same pattern. The Primates are 
Diphyodontt as well, because many of their permanent teeth 
are preceded by another set, commonly known as the m/k- 
teeth. In order to present to the eye at a glance the number of 
each sort that any species possesses, a dental formula has been 
adopted by naturalists. Such a formula as 12, C+, P?, M3= 36, 
indicates that in oxe half of the mouth, above and below, there 
are 2 incisors, I canine, 3 pre-molars, and 3 molars=18; and 
therefore in the ¢wo halves of the mouth together there are 36 
teeth in all. 
The masticated food, partially digested by the saliva of the 
mouth, descends the gullet by the muscular contractions of its 
walls to the simple, sac-like, stomach, and thence to the intes- 
tines. These latter consist of two portions, one smaller and 
narrower, nearer to the stomach, and a second portion further 
down, larger and wider ; the junction of the two portions being 
marked by a process of varying length, the cecum. The 
stomach and intestines, with other important structures, such 
as the liver, kidneys and generative organs, are contained in a 
lower cavity, separated by a muscular midriff, the diaphragm, 
from the upper part or thorax, containing the blood-purifying 
and pumping organs, the lungs and the heart. 
* repos, different, 65ovs, a tooth. t+ duds, the same, ddovs, a tooth. 
t Sipujs, double, ddovs, a tooth. 
