196 HUMAN PHYSIOLOGY. 



First nerve, or olfactory. Seventh nerve, or facial, portio dura. 



Second nerve, or optic. Eighth nerve, or acoustic. 



Third nerve, or motor oculi communis. Ninth nerve, or glosso-pharyngeal. 



Fourth nerve, or trochlearis. Tenth nerve, or pneumogastric. 



Fifth nerve, or trigeminal. Eleventh nerve, or spinal accessory. 



Sixth nerve, or abducent. Twelfth nerve, of hypoglossal. 



The cranial nerves may also be classified physiologically, according to 

 their function, into three groups: 



1. Nerves of special sense e. g., olfactory, optic, acoustic, gustatory (glosso- 

 pharyngeal and chorda tympani). 



2. Nerves of motion e. g., motor oculi, pathetic, small root of the trigeminal, 

 abducent, facial, spinal accessory and hypoglossal. 



3. Nerves of general sensibility e. g., large root of the trigeminal, the glosso- 

 pharyngeal and the pneumogastric. 



ORIGINS OF THE CRANIAL NERVES. 



The nerves of special sense have their origin in neuro-epithelial cells in the 

 sense organs with which they are connected. 



The nerves of motion have their origin in nerve cells situated in the gray 

 matter beneath the floor of the aqueduct of Sylvius and the floor of the fourth 

 ventricle. 



The nerves of general sensibility have their origin in the ganglia situated on 

 their trunks. 



First Nerve. Olfactory. 



The olfactory nerve is situated in the upper third of the nasal fossa. It 

 consists of from 20 to 30 branches. 



Origin. From neuro-epithelial cells situated among the epithelial cells 

 covering the mucous membrane. From these cells the nerve-fibers pass 

 upward through foramina in the cribriform plate of the ethmoid bone and 

 arborize around nerve-cells, in the olfactory bulb. 



The Olfactory Tract. The olfactory tract consists of both gray and white 

 fibers which pass from their origin in the bulb, to the base of the cerebrum 

 where it divides into three branches, viz., an external white root, which passes 

 across the fissure of Sylvius to the middle lobe of the cerebrum; an internal 

 white root, which passes also into the middle lobe; a gray root, which is in 

 relation with the anterior lobe. The white fibers at least terminate around 



