218 HUMAN PHYSIOLOGY. 



nerves, derived from the cerebro-spinal system and the medulla 

 oblongata, leave the spinal cord by the rami communicant es, enter 

 the sympathetic ganglia, and finally terminate in the muscular walls 

 of the blood-vessels. 



THE CRANIAL NERVES. 



The cranial nerves come off from the base of the brain, pass 

 through foramina in the walls of the cranium, and are distributed 

 to the structures of the head, the face and in part to the organs of 

 the thorax and abdomen. 



According to the classification of Soemmering, there are twelve 

 pairs of nerves, enumerating them from before backward, as fol- 

 lows viz. : 



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



Second pair or optic. Eighth pair, or auditory, portio 



Third pair, or motor oculi com- mollis. 



munis. Ninth pair, or glosso-pharyngeal. 

 Fourth pair, or patheticus, troch- Tenth pair, or pneumogastric. 



learis. Eleventh pair, or spinal accessory. 



Fifth pair, or trigeminal. Twelfth pair, or hypoglossal. 

 Sixth pair, or abducens. 



The cranial nerves may also be classified physiologically, according 

 to their function, into three groups : 



1. Nerves of special sense e. g., olfactory, optic, auditory, gustatory 

 (glosso-pharyngeal and chorda tympani). 



2. Nerves of motion e. g., motor oculi, patheticus, small root of 

 the trigeminal, 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. 



