THE SEVENTH NERVE, OR FACIAL 615 



Trophic Influence. The morbid effects which division of the fifth nerve 

 produces in the organs of special sense make it probable that the fifth nerve 

 exercises some special or trophic influence on the nutrition of all these organs, 

 although the effects may in part be due to the loss of sensibility which is the 

 natural protective safeguard. Thus, after such division and within a period 

 varying from twenty-four hours to a week, the cornea begins to be opaque 

 and later it grows completely white. A low destructive inflammatory process 

 ensues in the conjunctiva, sclerotic coat, and in the interior parts of the eye. 

 The sense of smell may be at the same time lost or gravely impaired. Com- 

 monly, whenever the fifth nerve is paralyzed, the tongue loses the sense 

 of taste in its anterior and lateral parts, and according to Gowers in the 

 posterior part as well. 



In Relation to Taste. The tactile sensibility of the tongue is due to the lin- 

 gual branch of the fifth nerve, which supplies the anterior and lateral parts of 

 the tongue. The sense of taste in the lateral and anterior portions of the tongue 

 has recently been traced back to the pars intermedia and chorda tympani 

 of the seventh, figures 412 and 413. It forms also one chief sensory link 

 in the nervous circle for reflex action in the secretion of saliva. But, de- 

 ferring this question until the glosso-pharyngeal nerve is to be considered, 

 it may be observed that in some brief time after complete paralysis or division 

 of the fifth nerve, the power of all the organs of the special senses may be im- 

 paired. They may lose not merely their sensibility to common impressions, 

 for which they all depend directly on the fifth nerve, but also their sensibility 

 to the special stimuli to which they are adapted. 



The Sixth Nerve, the Abducens. Origin. The sixth nerve arises 

 from a compact oval nucleus, situated somewhat deeply at the back part of 

 the pons near the middle of the floor of the fourth ventricle. The eminentia 

 teres marks its position. It contains moderately large cells with large nerve 

 axis-cylinder processes. It is connected, figure 375, with the nuclei of the 

 third, fourth, and seventh nerves, and with reflex centers of the optic tracts, 

 as previously mentioned. The root is thin, and passes ventrally and laterally 

 through the reticular formation, to the surface, which it reaches at the lower 

 edge of the pons, opposite the front end of the pyramid. 



Functions. The sixth nerve is exclusively motor, and supplies only the 

 rectus externus muscle of the eye. The muscle is paralyzed when the nerve 

 is divided. In all such cases of paralysis the eye squints inward and cannot 

 be moved outward. 



The Seventh Nerve, or Facial. Origin. The facial or seventh pair of 

 nerves arises from the floor of the central part of the fourth ventricle, behind 

 and in line with the motor nucleus of the fifth, to the outside of and deeper 

 down than the nucleus of the sixth. The nucleus is narrower in front than 

 behind, and consists of large motor cells with well-marked axis-cylinder proc- 

 esses, which are gathered up at the dorsal surface of the nucleus to form a 



