PHYSIOLOGY OF THE CRANIAL NERVES. 537 



ritation of the nose), may be at the same time lost or gravely impaired; 

 so may the hearing, and commonly, whenever the fifth nerve is para- 

 lyzed, the tongue loses the sense of taste in its anterior and lateral parts, 

 and according to Growers in the posterior part as well. 



In relation to Taste, The loss of tactile sensibility as well as the 

 sense of taste, is no doubt due (a) to the lingual branch of the fifth 

 nerve being a nerve of tactile sense, and also because with it runs the 

 chorda tympani, which is one of the nerves of taste; partly, also, it is 

 due (#), to the fact that this branch supplies, in the anterior and lateral 

 parts of the tongue, a necessary condition for the proper nutrition of 

 that part; while (c), it forms also one chief link in the nervous circle for 

 reflex action, in the secretion of saliva. But, deferring this question un- 

 til 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 lost; 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 

 their several peculiar impressions for the reception and conduction of 

 which they are purposely constructed and supplied with special nerves 

 besides the fifth. The facts observed in these cases can, perhaps, be 

 only explained by the influence which the fifth nerve exercises on the 

 nutritive processes in the organs of the special senses. It is not unrea- 

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

 be the seats of such changes as are seen in the laxit}% the vascular con- 

 gestion, oedema, and other affections of the skin of the face and other 

 tegumentary 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 or- 

 gans of the special senses are adapted to their functions. 



The Sixth Nerve, or Abducens. 



Functions. The Sixth nerve, Nervus abducens or ocularis externus, 

 is also, like the fourth, exclusively motor, and supplies only the rectus 

 externus muscle. It arises from the floor of the fourth ventricle from 

 the anterior region in the deeper part. It is connected (Fig. 367) with 

 the nuclei of the third, fourth, and seventh nerves. It is nearer the 

 middle line than the nuclei of the fifth. 



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

 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 inwards, 

 and cannot be moved outwards. 



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- 



