DIGESTION. 157 



Nerve Mechanism of Mastication. The movements of masti- 

 cation, though originating in efforts of the will and under its control, 

 are for the most part of an automatic or reflex character ; for when 

 once initiated by a voluntary effort they continue Jor an indefinite 

 period so long, in fact, as the impressions which the food makes 

 upon the afferent nerves are received by the nerve-centers which 

 regulate and control them. That the masticatory movements are of 

 this reflex nature is shown by the fact that they will be maintained 

 even though the voluntary effort which called them forth has sub- 

 sided and the attention has been directed to some entirely different 

 subject. It would appear that all that is necessary under such con- 

 ditions is the exciting action of the food upon the periphery of the 

 afferent nerves distributed to the tongue and mouth. 



The nerves involved in this reflex are shown in the following table : 



Afferent or Excitor Nerves. 



1 . Lingual branch of fifth nerve. 



2. Glossopharyngeal. 



Efferent or Motor Nerves. 



1. Inferior maxillary division of fifth 



nerve. 



2. Hypoglossal or sublingual. 



3. Facial or portio dura. 



The nerve-center coordinating the movements of mastication is 

 situated in the medulla oblongata. The afferent or excitor nerves 

 which receive the impressions of the food are distributed largely to 

 the mucous membrane of the tongue. When these impressions are 

 received by the center in the medulla oblongata, it discharges nerve 

 impulses, which, passing outward through motor nerves, excite con- 

 traction in the masticatory muscles. The motor nerves innervating 

 the muscles are: (i) The small root of the fifth nerve, which, after 

 emerging from the cavity of the cranium through the foramen ovale, 

 joins the inferior maxillary division of the large-sensory root. It 

 then is distributed to the masseter, temporal, internal, and external 

 pterygoids, anterior belly of the digastric, and mylohyoid muscles, 

 and controls their movements. (2) The hypoglossal nerve, which, 

 after emerging through the anterior condyloid foramen, passes down- 

 ward and forward to be distributed to the extrinsic and intrinsic 

 muscles of the tongue. (3) The facial or portio dura, which, after 

 emerging from the stylomastoid foramen, is distributed to the mus- 

 cles of the face. Irritation of any one of these nerves produces con- 

 vulsive movements in the muscles to which it is distributed, while 

 their division is followed by paralysis of these muscles. The medulla 

 not only generates the impulses which are directly responsible for 

 the movements of mastication, but also coordinates them in such a 

 manner that the movements of mastication may be directed toward 

 the accomplishment of a definite purpose. 



