604 HANDBOOK OF PHYSIOLOGY. 



they are purposely constructed and supplied with special nerves besides 

 the fifth. The facts observed in these cases can, perhaps, be only ex- 

 plained by the influence which the fifth nerve exercises on the nutritive 

 processes in the organs of the special senses. It is not unreasonable to 

 believe, that, in paralysis of the fifth nerve, their tissues may be the 

 seats of such changes as are seen in the laxity, the vascular congestion, 

 oedema, and other affections of the skin of the face and other tegumen- 

 tary parts which also accompany the paralysis; and that these changes, 

 which may appear unimportant when they affect external parts, are 

 sufficient to destroy that refinement of structure by which the organs of 

 the special senses are adapted to their functions. 



The Vlth Nerve (AMucem). 



Origin. The Vlth nerve arises from a compact oval nucleus, situ- 

 ated somewhat deeply at the back part of the pons near the middle of 

 the floor of the fourth ventricle. The eminentia teres marks its posi- 

 tion. It contains moderately large nerve-cells with distinct axis cylin- 

 der processes. It is connected (fig. 364) with the nuclei of the third, 

 fourth, and seventh nerves. It is nearer the middle line than the nuclei 

 of the fifth and seventh. The root is thin, and passes ventrally and 

 laterally through the reticular formation, to the surface, which it reaches 

 at the hind end of the pons opposite the front end of anterior pyramid. 



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

 the rectus externus muscle of the eye. 



The rectus externus is convulsed, and the eye is turned outward, 

 when the sixth nerve is irritated ; and the muscle is paralyzed when the 

 nerve is divided. In all such cases of paralysis, the eye squints inward, 

 and cannot be moved outward. 



In its course through the cavernous sinus, the sixth nerve forms 

 larger communications with the sympathetic nerve than any other nerve 

 within the cavity of the skull does. But the import of these communi- 

 cations with the sympathetic, and the subsequent distribution of its 

 filaments after joining the sixth nerve, are quite unknown. 



The Vllth Nerve (Facial). 



Origin. The facial, or portio dura of the 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 cells with well marked axis 

 cylinder-processes, which are gathered up at the dorsal surface of the 

 nucleus to form a root. The root describes a loop round the nucleus of 



