81 



of life from crying, during which, the jaw is depressed beyond measure, 

 or when not knowing the just proportion between the capacity of the 

 mouth, and the size of the bodies they would put into it, children endea- 

 vour to introduce those which it cannot receive. The lower jaw forms 

 a double bended lever of the third kind, in which the power, represented 

 by the temporal, masseter and internal pterygoid muscles, lies between 

 the fulcrum and the resistance, at a smaller or greater distance from 

 the chin. 



The mode of articulation of the jaw to the temporal bones, allows it 

 only a motion upwards and downwards, in which the teeth of both jaws 

 meet like the blades of scissars, and a lateral motion* in which the teeth 

 glide on each other, producing a friction well calculated to grind the food, 

 which in the first part of the act of mastication was torn or divided. 



VIII. In carnivorous animals, the levator muscles of the under jaw, 

 especially the temporals and massetcrs, are prodigiously large and pow- 

 erful. In them, the coronoid processes, to which the temporal muscles 

 are attached, are very prominent; the condyles are received into a very 

 deep cavity ; while in herbivorous animals, on the contrary, they are less 

 strong and bulky, and the pterygoid muscles, by whose action the lateral 

 or grinding motion is performed, are stronger and more marked. The 

 glenoid cavities are also in them wide but shallow, so that they allow the 

 condyles to move freely on their surface. The comparative power of the 

 levator and abductor muscles of the lower jaw, may be easily appreciated, 

 by viewing the temporal and zygomatic fossse. Their depth is always in 

 an inverse ratio, and proportioned in the bulk of the muscles which they 

 contain. In carnivorous animals, the zygomatic arch, to which the mas- 

 seter is attached, is depressed, and seems to have yielded to the effort of 

 the muscle. In the point of view which we have just taken, man holds a 

 middle station between carnivorous animals and those which feed on ve- 

 getable substances ; nothing, however, determines his nature better than 

 the composition of his dental arches. 



IX. The small white and hard bones which form the dental arches, are 

 not alike in the animals whose jaws are furnished with them. All have 

 not, as man, three kinds of teeth. The laniary* teeth are not to be met 

 with, in the numerous class of rodentia, Some are without incisors; the 

 former appear more fitted to tear fibrous tissues which offer much resist- 

 ance. In carnivorous animals, they are likewise very long, and bent like 



* After the example of several naturalists, I have thought it right to give that name to 

 the canine teeth ; in the first place, because their principal use being to lacerate or 

 tear fibrous tissues, it is fit that they should have a name from their manner of acting on 

 the food, as is the case with the incisors and molares ; in the second place, because thq 

 word canine may lead to an erroneous conception, by leading to a belief that this kind 

 of tooth belongs only to one kind of carnivorous animals, while they are stronger and 

 more distinct in the lion, the tyger, Sec. 



The teeth differ essentially from the other bones, by the acute sensibility with which 

 they are endowed ; 2dly, by the nerves which may be traced into them, while they seem 

 to be wanting in every other part of the osseous system ; 3dly, by the mode of distribu- 

 tion of the blood-vessels : these penetrate into them at an aperture which is seen at the 

 extremity of their root, and they expand in the mucous membrane contained in the 

 tooth, and which forms the most essential part of the bone ; 4thly, by their not under- 

 going any change from exposure to the air, a property which they owe to the elmmel 



