COMPARATIVE ANATOMY. 



the mouth is shut, but are brought o 

 conti.i bv he- free lateral motion which 

 takes |)l..-'e 11, rumination. 



As the motions of the lower jaw must 

 be material^ influenced by the ibrm of 

 its coiKi}les, uicl u\ the manner in which 

 those processes are connected to the ar- 

 ticular cavity of the tt-mporul bone, we 

 shall find, as might have be* n expected, 

 a close relation between these circum- 

 stances and the kind of food by which an 

 animal is nourished. Thus, the lower jaw 

 of the carnivora can only move upwards 

 and downwards, and is completely inca- 

 pable of that horizontal motion which 

 constitutes genuine mastication. Hence 

 these animals cut and tear their food in 

 a rude and coarse manner, and swallow it 

 in large portions, which are afterwards 

 reduce'! by the solvent properties of the 

 gastric juice. Such mammalia, on the 

 contrary, as live on vegetables, have, in 

 addition to this motion, a power of mov- 

 ing the lower jaw backwards and for- 

 wards, and to either side, so as to pro- 

 duce a grinding effect, which is necessa- 

 ry for bruising and triturating grass, and 

 for pulverising and comminuting grains. 

 In all these, therefore, the form of the 

 condyle, and of its articular cavity, allows 

 of free motion in aJmost every direction. 

 The teeth may be compared, in the for- 

 mer case, to scissars ; in the latter, to the 

 stones of a mill. 



THE TEETH. 



The jaws of the mammalia, with a very 

 few exceptions, contain teeth. The pro- 

 per whales (balaena,) the pangolin (ma- 

 nis,) and the American ant-eaters, are 

 the only genera entirely destitute of these 

 organs. 



Animals of the genus balsena (the pro- 

 per whales) have, instead of teeth, the 

 peculiar substance called whalebone, co- 

 vering the palatine surface of the upper 

 jaw : this resembles in its composition 

 hair, horn, and such matters. 



Tiio lower surface of the upper jaw 

 forms two inclined planes, which may be 

 compared to the roof of a house reversed ; 

 but the two surfaces are concave. Both 

 these are covered with plates of the 

 whalebone, placed across the jaws, and 

 df scending vertically into the mouth. 

 They are parallel to each other, and exist 

 to the number of two or three hundred on 

 each of ihe surfaces. They are connect- 

 ed to the bone by the intervention of a 

 white ligamentous substance, from which 

 they grow ; but their opposite edge, 

 which is turned towards the cavity of the 



mouth, lias its texture loosened into a 

 kind of fringe, composed of long and 

 slender fibres of the horny substance, 

 which therefore covers the whole surface 

 of the jaw. This structure probably 

 serves the animal in retaining and con- 

 fining the mollusca, which constitute its 

 food. 



The teeth of the ornithorhynchus para- 

 doxus and hystrix deviate very consider- 

 ably from those of other mammalia. In 

 the former animal there is one on each 

 side of the two jaws : it is oblong, flat- 

 tened on its surface, and consists of a 

 horny substance adhering to the gum. 

 There are likewise two horny processes 

 on the back of the tongue : these point 

 forvvards,and are supposed by Mr. Home 

 to prevent the food from passing into 

 the fauces before it has been sufficiently 

 masticated. In the ornithorhynchus hys- 

 trix there are six transverse rows of point- 

 ed horny processes at the back of the 

 palate, and about twenty similar horny 

 teeth on the corresponding part of the 

 tongue. 



The teeth of the human subject seem 

 to be designed for the single purpose of 

 mastication, and hence an erroneous con- 

 clusion might be drawn, that they serve 

 the same office in other animals. Many 

 exceptions must, however, be made to 

 this general rule. Some mammalia, which 

 have teeth for the office of mastication, 

 have others, which can only be consider- 

 ed as weapons of offence and defence ; 

 viz. the tusks of the elephant, hippopo- 

 tamus, walrus, and manati. The large and 

 long canine teeth of the carnivora, as the 

 lion, tiger, dog, cat, &c. not only serve 

 as natural weapons lo the animal, but en- 

 able it to seize and hold its prey, and 

 assist in the rude laceration which the 

 food undergoes previous to deglutition. 

 The seal, the porpoise, and other cetacea, 

 as the cachalot, have all the teeth of one 

 and the same form, and that obviously not 

 calculated for mastication. They can only 

 assist in securing the prey which forms 

 the animal's food. 



As the number and arrangement of the 

 teeth was made by Linnaeus the basis of 

 his classification of animals, it may be 

 worth while to mention, that this anato- 

 mist gives the name of primores to the 

 front, or incisor teeth ; and of laniarii to 

 the canine or cuspidati. The term of 

 tusks is applied to such teeth as extend 

 out of the cavity of the mouth. 



Certain classes of the teeth are entirely 

 wanting in some orders, classes, and ge- 

 nera of quadrupeds ; and in other in- 

 stances, the different descriptions of teeth, 



