PHYSIOLOGY OF THB CKANIAL NEKVES. 531 



gata in 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: 



A. Nerves of special sense, . . Olfactory, Optic, Auditory, 



part of the Glosso-pharyn- 

 geal, and part of the Fifth. 



B. Nerves of common sensation, The greater portion of the 



Fifth. 

 0. Nerves of motion, Third, Fourth, lesser division 



of the Fifth, Sixth, Facial, 



and Hypoglossal. 

 D. Mixed nerves, ...... Glosso-pharyngeal, Vagus, and 



Spinal accessory. 



The physiology of the First, Second, and Eighth will be considered 

 with the organs of Special sense. 



The Third Nerve, or Motor Oculi. 



Functions. The Third nerve, or motor oculi, which arises in three 

 distinct bands of fibres from the gray matter beneath the aqueduct of 



3 



FIQ. 366. Diagram of a longitudinal section through the pons, showing the relation of the 

 nuclei for the ocular muscles. CQ, corpora quadrigemina; 3, third nerve; in., its nucleus; 4, fourth 

 nerve; iv., its nucleus, the posterior part of the third; 6, sixth nerve. The probable position of the 

 centre and nerve fibres for accommodation is shown at a and a' ; for the reflex action of iris, at 6, 

 and &'; for the external rectus muscles, at c, c'. The lines beneath the floor of the fourth ven- 

 tricle indicate fibres, which connect the nuclei. (Gowers.) 



Sylvius near the middle line in conjunction with the fourth nerve. It 

 supplies the levator palpebrae superioris muscle, and all of trie muscles 

 of the eye-ball, but the superior oblique, 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 motor filaments 

 to the iris and ciliary muscle. The fibres which subserve the three 

 functions, accommodation, contraction of the pupil, and nerve-supply 

 to the external ocular muscles, arise from three distinct groups of cells. 



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

 to which it is distributed are convulsed. When it is paralyzed or di- 

 vided the following effects ensue: (1) the upper eyelid can be no longer 



