THE CEREBRAL NERVES. 425 



Physiology of the Third, Fourth, and Sixth Cerebral or Cranial 



Nerves. 



The physiology of these nerves may be in some degree com- 

 bined, because of their intimate connection with each other in 

 the actions of the muscles of the eyeball, which they supply. 

 They are probably all formed exclusively of motor fibres : 

 some pain is indicated when the trunk of the third nerve is ir- 

 ritated near its origin ; but this may be because of some fila- 

 ments of the fifth nerve running backwards to the brain in .the 

 trunk of the third, or because adjacent sensitive parts are in- 

 volved in the irritation. 



The third nerve, or motor oculi, supplies the levator palpe- 

 brse superioris muscle, and, of the muscles of the eyeball, all 

 but the superior oblique or trochlearis, to which the fourth 

 nerve is appropriated, and the rectus externus, which receives 

 the sixth nerve. Through the medium of the ophthalmic or 

 lenticular ganglion, of which it forms what is called the short 

 root, it also supplies the motor filaments to the iris. 



When the third nerve is irritated within the skull, all those 

 muscles to which it is distributed are convulsed. When it is 

 paralyzed or divided, the following effects ensue : first, the 

 upper eyelid can be no longer raised by the levator palpebrse, 

 but drops and remains gently closed over the eye, under the 

 unbalanced influence of the orbicularis palpebrarum, which is 

 supplied by the facial nerve : secondly, the eye is turned out- 

 wards by the unbalanced action of the rectus externus, to 

 which the sixth nerve is appropriated : and hence, from the 

 irregularity of the axes of the eyes, double-sight is often ex- 

 perienced when a single object is within view of both the eyes : 

 thirdly, the eye cannot be moved either upwards, downwards, 

 or inwards ; fourthly, the pupil is dilated. 



The relation of the third nerve to the iris is of peculiar in- 

 terest. In ordinary circumstances the contraction of the iris 

 is a reflex action, which may be explained as produced by the 

 stimulus of light on the retina being conveyed by the optic 

 nerve to the brain (probably to the corpora quadrigemina), 

 and thence reflected through the third nerve to the iris. 

 Hence the iris ceases to act when either the optic or the third 

 nerve is divided or destroyed, or when the corpora quadri- 

 gemina are destroyed or much compressed. But when the optic 

 nerve is divided, the contraction of the iris may be excited by 

 irritating that portion of the nerve which is connected with 

 the brain ; and when the third nerve is divided, the irritation 

 of its distal portion will still excite contraction of the iris, in 

 which its fibres are distributed. 



