550 NERVOUS SYSTEM. 



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

 fifth nerve, in addition to the description of the action of the muscles of mas- 

 tication, contained in the chapters on digestion, except as regards the action 

 of the filaments sent to the muscles of the velum palati. In deglutition the 

 muscles of mastication are indirectly involved. This act can not be well 

 performed unless the mouth be closed by these muscles. AVhen the food is 

 brought in contact with the velum palati, muscles are brought into action 

 which render this membrane tense, so that the opening is adapted to the 

 size of the alimentary bolus. These muscles are animated by the motor root 

 of the fifth. This nerve, then, is not only the nerve of mastication, animat- 

 ing all of the muscles concerned in this act, except two of the most unimpor- 

 tant depressors of the lower jaw (the genio-hyoid and the platysma myoides), 

 but it is concerned indirectly in deglutition. 



FACIAL, OR KERVE OF EXPRESSION (SEVEXTH NERVE). 



The anatomical relations of the facial nerve are quite intricate and it 

 communicates freely with other nerves. As far as can be determined by exper- 

 iments upon living animals, this nerve is exclusively motor at its origin ; but 

 in its course it presents anastomoses with the sympathetic, with branches of 

 the fifth and with the cervical nerves, undoubtedly receiving sensory filaments. 



Physiological Anatomy. The facial nerve has its apparent origin from 

 the lateral portion of the medulla oblongata, in the groove between the oli- 

 vary and restiform bodies, just below the border of the pons Varolii, its 

 trunk being internal to the trunk of the auditory nerve. It is separated from 

 the auditory by the two filaments constituting what is known as the interme- 

 diary nerve of Wrisberg, or the portio inter duram et mollem. As this little 

 nerve joins the facial, it is usually included in its root. 



Many anatomists have endeavored to trace the fibres of the facial from 

 their point of emergence from the encephalon to their true origin, but with 

 results not entirely satisfactory. Its fibres pass inward, with one or two de- 

 viations from a straight course, to the floor of the fourth ventricle, where 

 they spread out and become fan-shaped. In the floor of the fourth ventricle 

 certain of the fibres have been thought to terminate in the cells of the gray 

 substance, and others have been traced to the median line, where they decus- 

 sate ; the course of most of the fibres, however, has not been satisfactorily 

 established. The fibres of origin of the intermediary nerve of Wrisberg have 

 been traced to the nucleus of the glosso-pharyngeal. 



It is evident from physiological experiments, that the decussation of the 

 fibres in the floor of the fourth ventricle itself is not very important. Yul- 

 pian made, in dogs and rabbits, a longitudinal section in the middle line of 

 the ventricle, which would necessarily have divided the fibres passing from 

 one side to the other, without producing notable paralysis of the facial nerves 

 upon either side. This single fact is sufficient to show that the main decus- 

 sation of the fibres animating the muscles of the face takes place, if at all, at 

 some other point. 



The pathological facts bearing upon the question of decussation of the 



