DIGESTION 143 



Each cell presents on its inner surface short processes which pass into the 

 pulp; on its outer surface it presents a long process which enters a dentine 

 tubule and extends as far as its ultimate terminations. Collectively these 

 processes are known as the dentine fibers. Inasmuch as the fibers do not 

 completely occupy 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 abun- 

 dantly supplied with blood-vessels and nerves. 



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

 ward and upward, an antero-posterior, and a lateral movement, all depend- 

 ent on the peculiar construction of the joint. 



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 liga- 

 ments. Situated between the glenoid cavity and the condyle is a plate of 

 nbro-cartilage oval in shape and biconcave. This cartilage divides the 

 joint into two cavities one above, the other below each of which is pro- 

 vided 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 for- 

 ward, carrying with it the interarticular nbro-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 depres- 

 sion and elevation are made possible by the slightly oblique direction of the 

 condyle. In the carnivorous animals, whose food requires considerable 

 cutting, these movements are especially well developed. In these animals 

 the condyles are transversely arranged and at right angles to the long axis 

 of the jaw. In the antero-posterior movement the jaw moves in a hori- 

 zontal 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 may remain in their natural 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 consequence great freedom of movement. 



The Functions of the Muscles of Mastication. The movements of 

 the lower jaw are caused by the action of numerous muscles, which, having 

 fixed points of origin, are attached to various points on its surface. The 

 muscles concerned in the movements of mastication are presented in the 

 following table: 



