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 

 Heterodont* in contradistinction to forms like the Dolphins 

 and Whales, which are termed Homodont,\ because the whole 

 of these teeth are of the same pattern. The Primates are 

 Diphyodont\ as well, because many of their permanent teeth 

 are preceded by another set, commonly known as the milk- 

 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 If, Cy, Pf , Mf = 36, 

 indicates that in one half of the mouth, above and below, there 

 are 2 incisors, i canine, 3 pre-molars, and 3 molars =18; and 

 therefore in the two 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 cacnm. 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. 



tpos, different, oSous, a tooth. f <W* the samey oSovs, a tooth. 

 J Si<pvf)s, double, bSovs, a tooth. 



