THE NERVOUS SYSTEM. 137 



The roots of the olfactory tracts are connected deeply with the cortex 

 of the anterior cerebral hemisphere, and probably with the corpora striata 

 also. The optic nerves can be traced into the optic thalami, corpora 

 quadrigemina, and corpora geniculata. The third and fourth nerves arise 

 from grey matter beneath the corpora quadrigemina; and the roots of 

 origin of the remainder of the cranial nerves can be traced to grey matter 

 in the medulla oblongata beneath the floor of the fourth ventricle, and 

 in the more central part of the medulla, around its central canal, as low 

 down as the decussation of the pyramids. 



According to their several functions, the cranial nerves may be thus 

 arranged : 



Nerves of special sense . . Olfactory, optic, auditory, part of the 



glosso-pharyngeal, and of the lingual 

 branch of the fiftit. 



" of common sensation . The greater portion of the fifth. 

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



sixth, facial, and hypoglossal. 



Mixed nerves Glossopharyngeal, vagus, and spinal ac- 

 cessory. 



The physiology of the several nerves of the special senses will be con- 

 sidered with the organs of those senses. 



THIRD NERVE. 



Functions. The third nerve, or motor oculi, supplies the levator 

 palpebrae superioris muscle, and, of the muscles of the eyeball, all but 

 the superior oblique or trochlearis, to which the fourth nerve is appropri- 

 ated, 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 motor filaments to the iris 

 and ciliary muscle. 



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: (1), the upper eyelid can be no longer raised 

 by the elevator palpebrae, but droops (ptosis) and remains gently closed 

 over the eye, under the unbalanced influence of the orbicularis palpe- 

 brarum, which is supplied by the facial nerve: (2), the eye is turned out- 

 ward (external strabismus) by the unbalanced action of the rectus ex- 

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

 irregularity of the axes of the eyes, double-sight is often experienced when 

 a single object is within view of both the eyes: (3), the eye cannot be 

 moved either upward, downward, or inward: (4), the pupil becomes 

 dilated (mydriasis), and insensible to light: (5), the eye cannot "accom- 

 modate" itself for vision at short distances. 



