Of the digestive organs of the Primates the teeth present very important characters, from the point of view of the classification of the Order. They differ in form and number, and have distinct functions to perform. The teeth situated in front are the _incisors_ and _canines_, sharp and pointed, for seizing, cutting, and holding the food. Behind them come the {6}_pre-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_,[2] in contradistinction to forms like the Dolphins and Whales, which are termed _Homodont_,[3] because the whole of these teeth are of the same pattern. The Primates are _Diphyodont_[4] 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 I2/2, C1/1, P3/3, M3/3 = 36, indicates that in _one half of the mouth_, above and below, there are 2 incisors, 1 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 intestines. 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 _cæcum_. 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. {7}The upper part of the windpipe is, in all Primates, modified to form the larynx, or organ of voice, constituted by fibrous strings stretched across its orifice, where they may be set in vibration by the air, in its passage to and from the lungs.