

NERVE OF MASTICATION. 549 



motor filaments only, to the external pterygoid and the temporal, its final 

 branches of distribution being sensory and going to integument and to 

 mucous membrane. 



In treating of the physiology of digestion, a table has been given of the 

 muscles of mastication, with a description of their action. It will be seen by 

 reference to this table that the following muscles depress the lower jaw ; viz., 

 the anterior belly of the digastric, the mylo-hyoid, the genio-hyoid and the 

 platysma myoides. Of these the digastric and the mylo-hyoid are animated 

 by the motor root of the fifth ; the genio-hyoid is supplied by filaments from 

 the sublingual ; and the platysma myoides, by branches from the facial and 

 from the cervical plexus. All of the muscles which elevate the lower jaw 

 and move it laterally and antero-posteriorly ; viz., the temporal, masseter, and 

 the internal and external pterygoids the muscles most actively concerned in 

 mastication are animated by the motor root of the fifth. 



Properties and Uses of the Nerve of Mastication. The anatomical dis- 

 tribution of the small root of the fifth nerve points at once to its uses. 

 Charles Bell, whose ideas of the nerves were derived almost entirely from 

 their anatomy, called it the nerve of mastication, in 1821, although he did 

 not state that any experiments were made with regard to its action. All ana- 

 tomical and physiological writers since that time have adopted this view. It 

 would be difficult if not impossible to stimulate the root in the cranial cav- 

 ity in a living animal ; but its Faradization in animals just killed determines 

 very marked movements of the lower jaw. Experiments have demonstrated 

 the physiological properties of the small root, which is without doubt solely 

 a nerve of motion. 



The observations upon section of the fifth pair in the cranial cavity 

 are most important in connection with the uses of its sensory branches and 

 will be referred to in detail in treating of 

 the properties of the large root. In addi- 

 tion to the loss of -sensibility following sec- 

 tion of the entire nerve, Bernard noted 

 the effects of division of the small root, 

 which can not be avoided in the operation. 

 In rabbits the paralysis of the muscles of 

 mastication upon one side, and the con- 

 sequent action of the muscles upon the 

 unaffected side only, produce, a few days 



PI iv i- iv.li FIG. 198. Incisors of the rabbit, before and 



alter the operation, a remarkable change after section of the nerve of mastication 

 iu the appearance of the incisor teeth. A ^ ormal condition. 



As the teeth in these animals are gradu- B ' incisors, seven days after section of the 



nerve on one side. 



ally worn away in mastication and repro- 

 duced, the lower jaw being deviated by the action of the muscles of the 

 sound side, the upper incisor of one side and the lower incisor of the other 

 touch each other but slightly and the teeth are worn unevenly. This makes 

 the line of contact between the four incisors, when the jaws are closed, ob- 

 lique instead of horizontal. 



