THE CRANIAT, NERVES. 447 



which they originate, and which is called their "nucleus." For all 

 except the olfactory and optic nerves, these nuclei of origin are situ- 

 ated along the floor of the fourth ventricle or about the aqueduct of 

 Sylvius. 



The peculiarities of peripheral distribution, in the cranial nerves, are 

 more apparent than real in importance. The oculomotorius, patheti- 

 cus, and abducens emerge from the brain at very different points, and, 

 running forward through the cranial cavity in the form of separate 

 cords, are enumerated as three nerves. But they all originate from 

 the same layer of gray substance, two of them, the oculomotorius and 

 the patheticus, in close proximity to each other ; they all pass from 

 the cranial into the orbital cavity by the sphenoidal fissure ; and they 

 are all distributed to muscles moving the eyeball. In a physiological 

 point of view, therefore, they are branches of a single nerve. Even 

 when two or more nerves emerge from the cranium by different foram- 

 ina, like the three divisions of the trigeminus, they are nevertheless, 

 properly speaking, parts of the same nerve, if they have similar physi- 

 ological properties and are distributed to the same region. It is the 

 character and ultimate destination of a nerve, and not its course 

 through the bones of the skull, which determine its physiological 

 position. In the bull-frog, as shown by Wyman,* both the facial 

 nerve and the abducens are given off as branches from the fifth pair ; 

 and in most quadrupeds, the frontal branches of the ophthalmic divi- 

 sion of the trigeminus are nearly wanting, in accordance with the 

 imperfect sensibility of the forehead and vertex. 



The cranial nerves may, therefore, be conveniently arranged in pairs 

 according to their distribution and functions, notwithstanding the inci- 

 dental peculiarities of their course or subdivision. The olfactory, optic, 

 and auditory nerves form a separate specific group ; while the remain- 

 der consist of motor and sensitive nerves, supplying the muscles and 

 integument of different regions. 



CRANIAL XERYES. 



Nerves of Special Sense. 



1. Olfactory. 2. Optic. 3. Auditory. 



Motor nerves. Sensitive nerves. Distributed to the 



Oculomotorius 

 Patheticus 



1st PAIR. J Abducens . Trigeminus. Upper, middle, and 



Facial lower facial regions. 



Small root of 5th pair 

 2d PAIR. Hypoglossal Glosso-pharyngeal. Tongue and pharynx. 



3d PAIR. Spinal accessory Pneumogastric. Passages of respira- 



tion and deglutition. 



This division of the cranial nerves, according to their physiological 

 character, is not perfect in all particulars. For while the hypoglossal 



* Nervous Systems of Rana pipiens. Smithsonian Institution ; Washington, 1853. 



