MAMMALIA 443 



numerous, simple, and very much alike, but in the majority 

 of mammals there are at least three types of teeth. In the front 

 of the upper jaw (on the premaxillary bones) are simple, chisel- 

 shaped teeth, the incisors; behind these (the first tooth on the 

 maxilla) is the canine tooth, usually pointed and adapted for 

 tearing; posterior to the canines are the grinders or molars. 

 Those grinders which replace milk teeth are sometimes called 

 premolars. The true molars do not have any representatives 

 in the milk set. The corresponding teeth in the lower jaw are 

 similarly named. The typical number of teeth is forty-four, 

 eleven in each half -jaw. This may be shown by a formula in 

 which the numerator indicates the number of each kind in one- 

 half of the upper jaw and the denominator a similar portion 



of the lower: i. , c. -, p. -, m. , = 44. This means that there 



3*43 

 are three incisors, one canine, four premolars, and three molars 



in each half jaw, both above and below. The dental formula 



2X2 3 



for adult man is: i. , c. , p. , m. , = 32. This may be written 



2 I 2 "3 



more simply '. The numbers are not always the same in 

 the upper and lower jaw of mammals. 



456. Supplementary Studies. Let the student determine by examination, 

 and write the dental formulae of the cat, dog, horse, cow; milk set in man. 



Compare the molars of some carnivorous animals with those of some herbivo- 

 rous; similarly the canines. Describe the action of the jaws in the act of chewing in 

 the dog, cow, rabbit, horse. 



457. The Digestive Organs present the same regions and 

 general arrangement found in the typical vertebrates. There 

 are usually fleshy and movable lips covering the teeth. Some- 

 times these are much extended and in connection with the nose 

 may become important organs (snout, proboscis) for the capture 

 of food. The stomach varies widely but is ordinarily a simple 

 sac with muscular walls. Sometimes it is partly separated into 

 chambers by folds (Figs. 234, 235). This reaches its greatest 

 complexity in the ruminants, in which four chambers occur 

 (Fig. 235). One of these the rumen becomes a temporary 

 receptacle for the food which is first swallowed without being 



