NERVE OF MASTICATION. 617 



are sent to the mylo-hyoid muscle and to the anterior belly of the digastric. In addition, 

 the motor branch of the fifth sends filaments to the tensor muscles of the velum palati. 



The above description shows, in general terms, the distribution of the nerve of masti- 

 cation, without taking into consideration its various anastomoses, the most important of 

 which are with the facial. Physiological experiments have shown that the buccinator 

 muscle receives no motor filaments from the fifth but is supplied entirely by the facial. 

 The buccal branch of the fifth sends motor filaments only, to the external pterygoid and 

 the temporal, its final branches of distribution being sensitive and going to integument 

 and to mucous membrane. 



In treating of the function of digestion, we have given a table of the muscles of mas- 

 tication, 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 inyoides. 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 ja\v 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 Functions of the Nerve of Mastication. The anatomical distribution 

 of the small root of the fifth nerve points at once to its function. Charles Bell, whose 

 ideas of the nerves were derived almost entirely from their anatomy, called it the nerve 

 of mastication, in 1821, although he does not state that any experiments were made with 

 regard to its function. All anatomical and physiological writers since that time have 

 adopted this view. It would be difficult, if not impossible, to galvanize the root in the 

 cranial cavity in a living animal ; but its galvanization in animals just killed determines 

 very marked movements of the lower jaw. Experiments have clearly demonstrated the 

 physiological properties of the small root, which is without doubt solely a nerve of motion. 



The observations upon the division of the fifth pair in the cranial cavity are most 

 interesting in connection with the functions of its sensitive branches, and will be 

 referred to in detail in treating of the properties of the large root. In addition to 

 the loss of sensibility following section of the en- 

 tire nerve, Bernard has carefully noted the effects 

 of division of the small root, which cannot be 

 avoided in the operation. In rabbits, the paraly- 

 sis of the muscles of mastication upon one side, 

 and the consequent action of the muscles upon the 

 unaffected side only, produce, a few days after 

 the operation, a remarkable change in the appear- 

 ance of the incisor teeth. As the teeth in these 

 animals are gradually worn away in mastication 

 and reproduced, the lower jaw being deviated by 

 the action of the muscles of the SOlind side, the ^\G. 200. Incisors of the, rabbit, before and 

 upper incisor of one side and the lower incisor of n^milf/ f *** ne 6 f mastication ' 

 the other touch each other but slightly and the A, incisors, normal condition, 

 teeth are worn unevenly. This makes the line B, indsors^ se^ven days after section of the nerve 



of contact between the four incisors, when the 



jaws are closed, oblique instead of horizontal. We have often divided the fifth pair in 

 the cranial cavity in rabbits, by the method employed by Magendie and Bernard, and 

 have repeatedly verified these observations. 



There is little left to say with regard to the functions of the motor root of the fifth 

 nerve, in addition to our description of the action of the muscles of mastication, contained 



