THE KERVOUS SYSTEM. 



597 



The roots of the first or olfactory and of the second or optic nerves 

 will be mentioned elsewhere. The third and fourth nerves arise from 

 gray matter beneath the corpora quadrigemina; and the roots of origin 

 of the remainder of the cranial nerves can be traced to gray matter 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 



Nerves of common sensation 



c. Nerves of motion 



d. Mixed nerves 



Olfactory, Optic, Auditory, part of the 



Glosso-pharyngeal, and part of the 



Fifth. 



The greater portion of the Fifth. 

 Third, Fourth, lesser division of the 



Fifth, Sixth, Facial, and Hypoglossal. 

 Glosso-pharyngeal, Vagus, and Spinal 



accessory. 



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

 with the organs of Special sense. 



The Illrd Nerve (Motor Oculi). 



Origin. The third nerve arises in three distinct bands of fibres from 

 the gray matter surrounding the aqueduct of Sylvius near the middle 

 line ventral to the canal. The nucleus of origin consists of large multi- 



Fig. 363. Section through anterior corpus quadrigeminum and part of optic thalamus. s., 

 Aqueduct of Sylvius; gr., gray matter of the aqueduct; c.q.s., quadrigeminal eminence; *., 

 stratum lemnisci ; o. , stratum opticum ; c. , stratum cinereum ; Th. , pulvinate of optic thala- 

 mus; c.g.e., c.g.i., lateral and median corpora geniculata; br.s., br.i., superior and inferior 

 bracnia;/., fillet; p. L, posterior longitudinal bundle; r., raphe; ///., third nerve, and n.IIL, 

 its nucleus; I. p. p., posterior perforated space; s.n., substantia nigra, above this is the tegmen- 

 tum with the circular area of the red nucleus; cr., crusta; //., optic tract; M., medullary centre 

 of hemisphere; w.c., nucleus caudatus; st., stria terminalis. (After Quain, from Meynert.) 



polar ganglion-cells, and extends to the back part of the third ventricle 

 as far as the level of the anterior corpus quadrigeminum. The fibres 

 pass from their origin partly through the red nucleus to their superficial 



