MAMMALS. 



385 



later, three-toed forms as large as a donkey. In domesti- 

 cation horses vary extremely in size as in other respects. 



Lowest of the artiodactyls, or even-toed ungulates, come 

 the two species of hippopotamus, in which there are four 

 toes, large canine teeth, and a huge, clumsy body, some- 

 times fourteen feet in length. In the pigs the canines 

 are still large, and the toes are four in number, but the 

 outer ones are lifted above the ground so that they are 

 useless as organs of locomotion. Our domestic swine have 

 descended from the wild boars of Europe. In the warmer 

 parts of America the peccaries represent the group. 



The hippopotamus and the pigs have the axis of the 

 foot passing up between the middle toes ; in other words, 

 they have cloven hoofs. In 

 all other artiodactvls the 



*/ 



cloven hoof occurs, and be- 

 sides, they chew the cud, 

 and hence they are asso- 

 ciated as a group of rumi- 

 nants. The stomach is di- 

 vided into four chambers, 

 and when a cow, for instance, 

 feeds, it swallows the grass 

 without chewing it. It 

 passes down to the first 



stomach and thence to the second. In these it becomes 

 mixed with digestive fluids and softened. It is then 

 brought up in the mouth, thoroughly chewed, and again 

 swallowed. This time it passes into the third stomach, 

 and from this into the fourth, and so into the intestine. 



To the ruminants belong the most valuable domesti- 

 cated animals. In South America are found the llamas 

 and alpacas, which were the cattle and beasts of burden 



FIG. 171. Diagram of the stomach 

 of a ruminant. The dotted line 

 shows the course of the food. 



