448 



THE CRANIAL NERVES. 



washing of the waves, the humming of insects ; sounds which have 

 no external existence, but which are simulated by the morbid irri- 

 tation of the auditory nerve. 



It is evident, from the facts detailed above, that the nerves of 

 special sense are neither motor or sensitive, properly speaking; 

 and that they are distinct in their nature from the ordinary spinal 

 nerves. 



The remainder of the cranial nerves, however, present the 

 ordinary qualities belonging to the spinal nerves. Some of them 

 are exclusively motor in character, others exclusively sensitive; 

 while most of them exhibit the two properties, to a certain extent, 

 as mixed nerves. They may be conveniently arranged in three 

 pairs, according to the regions in which they are distributed, cor- 

 responding very closely with the motor and sensitive roots of the 

 spinal nerves. According to such a plan, the arrangement of the 

 cranial nerves would be as follows : 



CRANIAL NERVES. 



Nerves of Special Sense. 



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



1st PAIR. 



Motor nerves. 

 ' Motor oculi com. 

 Patheticus 

 Motor oc. externus 



Sensitive Nerves. 



Distributed to 



Small root of 5th pair 

 _ Facial 

 2d PAIR. Hypoglossal 

 3d PAIR. Spinal accessory 



Large root of 5th pair. Face. 

 Glosso-pharyngeal. Tongue. 



Pneumogastric. 



Neck, &c. 



The above arrangement of the cranial nerves is not absolutely 

 perfect in all its details. Thus, while the hypoglossal supplies the 

 muscles of the tongue alone, the glosso-pharyngeal sends part of 

 its sensitive fibres to the tongue and part to the pharynx ; and 

 while the large root of the 5th pair is mostly distributed in the 

 face, one of its branches, viz., the gustatory, is distributed to the 

 tongue. Notwithstanding these irregularities, however, the above 

 division of the cranial nerves is in the main correct, and will be 

 found extremely useful as an assistant in the study of their func- 

 tions. 



There is no impropriety, moreover, in regarding all the motor 

 branches distributed upon the face as one nerve ; since even the 

 anterior roots of the spinal nerves originate from the spinal cord, 

 each by several distinct filaments, which are associated into a single 



