CHAPTER XX. 



CRANIAL NERVES. 



THE cranial nerves are twelve pairs of nerves which reach their 

 respective terminations after passage through foramina located in 

 the base of the cranium. They are designated numerically, begin- 

 ning from the anterior portion of the base of the brain backward, as 

 well as by names dependent upon their functions and distribution. 

 They are as follows: 



1. Olfactory. 5. Trifacial. 9. Glosso-pharyngeal. 



2. Optic. 6. Abducent. 10. Pneumogastric. 



3. Motor oculi. 7. Facial. 11. Spinal accessory. 



4. Pathetic. 8. Auditory. 12. Hypoglossal. 



Origin of the Cranial Nerves. Upon examination, each cranial 

 nerve is found to possess a point of superficial origin as well as a 

 nucleus of deep origin. 



The superficial origin is that point upon the brain's surface 

 where each nerve emerges. This is but the apparent origin of each 

 pair of nerves, since their individual fibers may be traced more 

 deeply. 



Each cranial nerve has a special nucleus of gray matter lying 

 deeply within the brain-substance. The nucleus consists of a col- 

 lection of cells from whose prolongations spring the axis-cylinders 

 which constitute the fibers of the nerves. 



The gray masses which represent the prolongations of the 

 anterior horns of the cord into the medulla oblongata form the 

 nuclei of origin of the cranial motor nerves. The base, separated 

 from the head of the horn by decussation of the pyramidal columns, 

 remains contiguous to the central canal. It is prolonged in its 

 entirety upon the floor of the fourth ventricle, lying upon each side 

 of the raphe. Beneath the trigonum hypoglossi lies the nucleus of 

 the hypoglossal; beneath the eminentia teres is found the common 

 nucleus of the facial and abducent ; the nuclei of the oculomotor and 

 pathetic lie upon each side of the aqueduct. 



The head of the anterior horn, cut into fragments by the motor 

 decussation, forms that which is known as the antero-lateral nucleus. 

 This is the motor nucleus of the mixed nerves. By its most internal 

 (820) 



