MASTICATION. 



201 



The canines are more conical and pointed than the incisors and have longer and 

 larger roots, especially those in the upper jaw. Their roots are single. They are used 

 to some extent, in connection with the incisors, in dividing the food ; but they have no 

 prominent function in tearing the food, as in the carnivora, in which they are extraor- 

 dinarily developed. The permanent canines make their appearance from the eleventh 

 to the twelfth year. 



The bicuspid teeth are shorter and thicker than the canines. Their opposed surfaces 

 are rather broad and are marked by two eminences. The upper bicuspids are somewhat 

 larger than the lower. The roots are single, but in the upper jaw they are slightly bifur- 

 cated at their extremities. They are used, with the true molars, in triturating the food. 

 The permanent bicuspids make their appearance from the ninth to the tenth year. 



The molar teeth, called respectively counting from before backward the first, sec- 

 ond, and third molars, are the largest of all and are, par excellence, the teeth used in 

 mastication. Their form is that of a cube, rounded laterally and provided with four or 

 five eminences on their opposed surfaces. The first molars are the largest. They have 

 generally three roots in the upper jaw and two in the lower, although they sometimes 

 have four or even five roots. The second molars are but little smaller than the first 

 and resemble them in nearly every particular. The third molars, called frequently the 

 wisdom-teeth, are much smaller than the others and are by no means so useful in masti- 

 cation. In the upper jaw the root is grooved or imperfectly divided into three branches; 

 but in the lower jaw it generally has two distinct branches. The first molars are the 

 first of the permanent teeth, making their appearance between the sixth and the seventh 

 year. The second molars appear from the twelfth to the thirteenth year ; and the third 

 molars, from the seventeenth to the twenty-first year, and sometimes even much later. 

 In some instances the third molars are never developed. 



The upper jaw has ordinarily a somewhat longer and broader arch than the lower ; 

 so that when the mouth is closed the teeth are not brought into exact apposition, but the 

 upper teeth overlap the lower teeth both in front and laterally. The lower teeth are all 

 somewhat smaller than the corresponding teeth 

 in the upper jaw and generally make their ap- 

 pearance a little earlier. 



The physiological anatomy of the maxillary 

 bones and of the temporo-maxillary articulation 

 necessarily precedes the study of the muscles of 

 mastication and the mechanism of their action. 



The superior maxillary bones are immovably 

 articulated with the other bones of the head and 

 do not usually take any active part in mastica- 

 tion ; but their inferior borders, with the upper 

 teeth embedded in the alveolar cavities, present 

 fixed surfaces against which the food is pressed 

 by the action of the muscles which move the lower 

 jaw. 



The inferior maxilla is a single bone. Its 

 body is horizontal, of a horseshoe shape, and, in 

 the alveolar cavities in its superior border, are 

 embedded the lower teeth. Below the teeth, 

 both externally and internally, are surfaces for the attachments of the muscles concerned 

 in the various movements of the jaw, and for one of the muscles of the tongue. 



Behind the body of the inferior maxilla, on either side, is a vertical portion called the 

 minus. In the adult, this forms nearly a right angle with the body, making what is called 

 the angle of the jaw. Superiorly, the ramus terminates in two processes, separated by a 

 deep groove called the sigmoid notch. The posterior process is the condyle, or condyloid 



Fio. 50. Inferior maxilla. (Sappey.) 

 1, body; 2, ramus; 8, symphysis; 4, incisive 

 fossa; 5, mental foramen; 6, attachment of 

 the digastric muscle , 7, depression at the 

 site of the facial artery ; 8, ansr'.e;. 9, attach- 

 ment of the superior constrictor of the 

 pharynx; 10, coronoid process; 11, condyle; 

 12, siprmoid notch ; 18, opening of the inferior 

 dental ranal ; 14, groove for the rnylo-hyoid 

 muscle; 15, alveolar border; i, incisor teeth: 

 c, canine teeth ; b, bicuspid teeth ; m, molars. 



