540 THE NERVOUS SYSTEM. 



Nerves of motion . . . Third, fourth, lesser division of the fifth, 



sixth, facial, and hypoglossal. 

 Mixed nerves .... Pneumogastric, and accessory. 



The physiology of the several nerves of the special senses 

 will be considered with the organs of those senses. 



Physiology of the Third, Fourth, and Sixth Cerebral or 

 Cranial Nerves. 



The physiology of these nerves may be in some degree 

 combined, 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 irritated near its origin ; but this may be 

 because of some filaments of the fifth nerve running back- 

 wards to the brain in the trunk of the third, or because 

 adjacent sensitive parts are involved in the irritation. 



The third nerve, or motor oculi, supplies the levator 

 palpebrse superioris muscle, and, of the muscles of the eye- 

 ball, 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 opthalmie 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 orbi- 

 cularis palpebrarum, which is supplied by the facial nerve : 

 secondly, the eye is turned outwards by the unbalanced 

 action of the rectus externus, to which the sixth nerve is 

 appropriated : and hence, from the irregularity of the axes 



