DIGESTION 145 



traction of the elevator muscles of the lower jaw. When the occasion arises 

 for the introduction of food, the mouth is opened by the depressor muscles; 

 after the food is introduced into the mouth it is again closed and that 

 combination of muscle contractions initiated which when continued results 

 in the mechanical division of the food. 



The nerves and nerve centers constituting the nerve mechanism for 

 mastication are shown in the following table : 



Afferent Nerves. Nerve-center. Efferent Nerves. 



1. Lingual and buccal branches of Medulla oblongata. i. Small root of the trigeminal 

 the trigeminal nerve. nerve. 



2. Glosso-pharyngeal. 2. Hypoglossal. 



3. Facial or portio dura. 



The Efferent Nerves. The efferent nerves that transmit nerve im- 

 pulses to the various muscles of mastication are the small root of the tri- 

 geminal, the hypoglossal, and the facial. 



The small root of the trigeminal nerve after emerging from the cavity 

 of the cranium through the foramen ovale joins the inferior maxillary divi- 

 sion of the large sensor root. After a short course the efferent fibers sepa- 

 rate into two groups, an upper and a lower; the upper group is distributed to 

 the masseter, temporal, internal and external pterygoid muscles, the lower 

 group is distributed to the mylohyoid and anterior belly of the digastric 

 muscles. The hypoglossal nerve, after emerging from the cranium through 

 the anterior condyloid foramen, passes downward and forward to be dis- 

 tributed to the intrinsic and extrinsic muscles of the tongue. The facial 

 or portio dura after emerging from the stylo-mastoid foramen is distributed 

 to the superficial muscles of the face. 



Stimulation of any one of these nerves with induced electric currents 

 gives rise to convulsive movements in the muscles to which it is distributed 

 while its division is followed by paralysis of the muscles. 



The Central Mechanism. The central mechanism that excites and 

 coordinates the action of the nerve-cells from which these nerves emerge, may 

 be excited to activity (i) by nerve impulses descending from the cerebrum as 

 a result of volitional efforts; and (2) by nerve impulses transmitted through 

 afferent nerves from the mouth. Though movements of mastication are pri- 

 marily volitional and may so continue, nevertheless when once initiated they 

 continue for an indefinite period, so long in fact as the nerve impulses which 

 the food develops in afferent nerves are received by the central mechanism, 

 thus falling into the category of secondary or acquired reflex acts. That 

 the masticatory movements are of this reflex character is indicated by the 

 fact that they will be maintained, even though the volitional effort that 

 called them forth has subsided and the attention has been directed to some 

 entirely different subject. It would appear that all that is necessary under 

 such circumstances is the stimulating action of the food upon the peripheral 

 terminations of the afferent nerves distributed to the mucous membrane 

 of the tongue and mouth. 



The Afferent Nerves. The afferent nerves, stimulation of which 

 excites the central mechanism, are the lingual and buccal branches of the 

 superior and inferior maxillary divisions of the trigeminal nerve, the lingual 

 branches of the glosso-pharyngeal, and in all probability the gustatory fibers 

 of the chorda tympani. The introduction of food into the mouth develops 



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