OF DIGESTION. 83 



like the elephant, and some of the gnawers (rodentia), 

 like the hare (Fig. 76), have the summits of the molars 

 flat, like millstones, for grinding the grass and leaves 

 on which they subsist. Finally, the omnivora, those which 

 feed on both flesh and fruit, like man and the monkeys, 

 have the molars terminating in several rounded tubercles, 

 being thus adapted to the mixed nature of their food. 



221. Again, the mode in which the molars are combined 

 with the canines and incisors furnishes excellent means of 

 characterizing families and genera. Even the minute struc- 

 ture of a tooth is so peculiar in each group of animals, 

 and yet is subject to such invariable rules, that it is possible 

 to decide positively the structure of an animal, merely by 

 the inspection of the fragment of a tooth under the micro- 

 scope. 



222. Another process, subsidiary to digestion, is called 

 insalivation. Animals which masticate their food have 

 glands, in the neighborhood of the mouth, which secrete a 

 fluid called saliva. This fluid mingles with the food as it is 

 chewed, and prepares it also to be more readily swallowed. 

 The salivary glands are wanting in all animals which swal- 

 low their food without mastication. When the food is mas- 

 ticated and mingled with saliva, it is carried back by 

 the tongue, and passes down a tube, the oesophagus, into 

 the stomach. This act is called deglutition or swal- 

 lowing. 



223. The wisdom and skill of the Creator is strikingly illus- 

 trated in the means he has afforded to every creature for se- 

 curing the means of its subsistence. Some animals have no 

 ability to move from place to place, but are fixed to the soil ; 

 as the oyster, the polypi, &c. These are dependent for sub- 

 sistence upon such food as may stray or float near, and they 

 have the means of securing it when it comes within their 

 reach. The oyster closes its shell, and thus secures its prey ; 



