THE NERVOUS SYSTEM. 141 



voluntarily exercise them. It is, probably, for conferring this sensibility 

 on the muscles, that the branches of the fifth nerve communicate so fre- 

 quently with those of the facial and hypoglossal, and the nerves of the 

 muscles of the eye; and it is because of the loss of this sensibility that 

 when the fifth nerve is divided, animals are always slow and awkward in 

 the movement of the muscles of the face and head, or hold them still, or 

 guide their movements by the sight of the objects toward which they 

 wish to move. 



Again, the fifth nerve has an indirect influence on the muscular move- 

 ments by (b) conveying sensations of the state and position of the skin 

 and other parts: which the mind perceiving, is enabled to determine 

 appropriate acts. Thus, when the fifth nerve or its infra-orbital branch 

 is divided, the movements of the lips in feeding may cease, or be imper- 

 fect. Bell supposed that the motion of the upper lip in grasping food 

 depended directly on the infra-orbital nerve; for he found that, after he 

 had divided that nerve on both sides in an ass, it no longer seized the 

 food with its lips, but merely pressed them against the ground, and used 

 the tongue for the prehension of the food. Mayo corrected this error. 

 He found, indeed, that after the infra-orbital nerve had been divided, the 

 animal did not seize its food with the lip, and could not use it well during 

 mastication, but that it could open the lips. He, therefore, justly attrib- 

 uted the phenomena in Bell's experiments to the loss of sensation in the 

 lips; the animal not being able to feel the food, and, therefore, although 

 it had the power to seize it, not knowing how or where to use that power. 



The fifth nerve has also (e), an intimate connection with muscular 

 movements through the many reflex acts of muscles of which it is the 

 necessary excitant. Hence, when it is divided and can no longer convey 

 impressions to the nervous centres to be thence reflected, the irritation of 

 the conjunctiva produces no closure of the eye, the mechanical irritation 

 of the nose excites no sneezing. 



Through its ciliary branches and the branch which *forms the long 

 root of ths ciliary or ophthalmic ganglion, it exercises also (d), some in- 

 fluence on the movements of the iris. 



When the trunk of the ophthalmic portion is divided, the pupil be- 

 comes, according to Valentin, contracted in men and rabbits, and dilated 

 in cats and dogs; but in all cases, becomes immovable even under all the 

 varieties of the stimulus of light. How the fifth nerve thus affects the 

 iris is unexplained; the same effects are produced by destruction of the 

 superior cervical ganglion of the sympathetic, so that, possibly, they are 

 due to the injury of those filaments of the sympathetic which, after join- 

 ing the trunk of the fifth, at and beyond the Gasserian ganglion, proceed 

 with the branches of its ophthalmic division to the iris; or, as R. Hall 

 ingeniously suggests, the influence of the fifth nerve on the movements 

 of the iris may be ascribed to the affection of vision in consequence of 



