DIGESTION. 155 



the lumen of the tubule, it is probable that there is a free circulation 

 of lymph from the pulp chamber through the dentine tubules into the 

 enamel clefts, into the interglobular spaces, and possibly into the 

 lacunae of the cementum. 



The peridental membrane is composed of connective-tissue fibers 

 abundantly supplied with blood-vessels and nerves. 



Movements of the Lower Jaw. The lower jaw is capable of a 

 downward and upward, an antero-posterior, and a lateral move- 

 ment, all dependent on the peculiar construction of the 



Temporo-maxillary Articulation. This articulation is formed by 

 the anterior portion of the glenoid cavity, the eminentia articularis, 

 and the condyle of the inferior maxilla, all of which are united by 

 means of ligaments. Situated between the glenoid cavity and the 

 condyle is a plate of fibro- cartilage oval in shape and biconcave. 

 This cartilage divides the joint into two cavities, one above, the 

 other below, each of which is provided with a synovial membrane. 

 The function of the cartilage is to present constantly an articulating 

 surface to the condyle in the various movements of the lower jaw, 

 which it is enabled to do by virtue of its mobility. 



In the downward movement of the lower jaw each condyle glides 

 forward, carrying with it the interarticular fib ro- cartilage the upper 

 concave surface of which is applied to the convex surface of the 

 eminentia articularis. In the upward movement of the jaw both the 

 condyles and the cartilages pass backward and resume their normal 

 position. The movements of depression and elevation are made 

 possible by the transverse direction of the condyle. In the carnivor- 

 ous animals, whose food requires considerable cutting, these move- 

 ments are especially well developed. In the antero-posterior move- 

 ment the jaw moves in a horizontal direction and the condyles and 

 the articular cartilages glide forward and backward in the glenoid 

 fossae. In the rodent animals the long axis of the condyle runs 

 in the antero-posterior direction, which allows of a considerable 

 gliding movement. When the jaw performs a lateral movement, the 

 condyle and cartilage of one side remain in their normal position, 

 while the opposite condyle and cartilage glide forward in the glenoid 

 fossa, directing the symphysis of the jaw to the opposite side of the 

 median line. The lateral movements are well exhibited by the 

 herbivorous animals, in which they are quite extensive, and made 

 possible by the small size of the condyle and the large extent of 

 articulating surface. In man the structure of the joint is such as to 

 admit of all these possibilities, and the lower jaw acquires in conse- 

 quence great freedom of movement. 



The Functions of the Muscles of Mastication. The move- 

 ments of the lower jaw are caused by the action of numerous 

 muscles, which, having a fixed point of origin, are attached to various 



