ORGANS OF INSALIVATION. 



191 



termine the particular group to which it is related ; thus, those 

 beasts of prey which feed on insects, and which, on that ac- 

 count, have been called in- 

 sectivora, such as the moles 

 and bats, have the molars 

 terminated by several sharp, 

 conical points, so arranged 

 that the elevations of one 

 tooth fit exactly into the de- 

 pressions of the tooth oppo- 

 site to it. In the true car- 

 nivora (fig. 207), on the con- 



Fig. 207. The skull of a tiger. 



trary, the molars are compressed laterally, so as to have 'sharp- 

 cutting edges, as in the cats, and shut by the side of each 

 other, like the blades of scissors, thereby dividing the food 

 with great facility. 



343. The same adaptation is observed in the teeth of her- 

 bivorous animals. Those which chew the cud (ruminants), 

 many of the thick-skinned animals (pachydermata), (fig. 205), 

 like the horse and the elephant, and some of the gnawers (ro- 

 dentia), like the squirrel (fig. 206), have the summits of the 

 molars flat, like mill-stones, with more or less prominent 

 ridges, for grinding the grass and leaves on which they sub- 

 sist ; finally, the omnivora, those which feed on both flesh and 

 fruit, like man and the monkeys, have the molars terminating 

 in several rounded tubercles (fig. 102), being thus adapted to 

 the mixed nature of their food. 



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

 with the canines and incisors furnishes excellent means for cha- 

 racterizing families and genera ; even the internal structure of 

 the teeth is so peculiar in each group, and yet subject to such 

 invariable rules, that it is possible to determine with precision 

 the general structure of an animal, merely by investigating 

 fragments of its teeth under a microscope. 



345. Another process, subsidiary to digestion, is called 

 insalivation. Animals which masticate their food have glands, 

 in the neighbourhood of the mouth, for secreting 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 generally wanting, or rudimentary or otherwise 

 modified, in animals which swallow their food without masti- 



