THE OLFACTORY NERVES. 



513 



its physiological character and position. The details of branching and 

 division of the cranial nerves vary in different species of animals, or 

 even to some extent in the same individual on the two opposite sides 

 of the body, but their physiological characters remain the same. Thus 

 in the bull-frog, as shown by Wyman, 1 both the facial nerve and the 

 abducens, instead of existing as distinct trunks, are given off as branches 

 from the fifth pair ; and in most of the quadrupeds, the terminal frontal 

 branches of the ophthalmic division of the fcrigeminus are wanting, or 

 reduced to trifling dimensions, in accordance with the absence of sensi- 

 bility in the skin of the forehead and vertex. 



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

 according to their distribution and functions, rather than the incidental 

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

 auditory nerves thus form a group by themselves of a specific character ; 

 while the remainder consist of the motor and sensitive nerves, supply- 

 ing the muscular apparatus and the integument or mucous membrane 

 of different regions. 



CRANIAL NERVES. 

 Nerves of Special Sense. 



Distributed to the 



Upper, middle, and 

 lower facial regions. 



Tongue and pharynx. 

 Passages of respira- 

 tion and deglutition. 



This division of the nerves, though based on their physiological 

 characters and distribution, is not absolutely perfect in all particulars. 

 For while the hypoglossal nerve supplies the muscles of the tongue 

 alone, its associate, the gloss-pharyngeal, sends a part of its sensitive 

 fibres to the tongue and a part to the pharynx ; and while the trigeminal 

 nerve is mainly distributed to the external parts of the face, one of its 

 deeper branches, the lingual, is distributed to the tongue. Notwith- 

 standing, however, these irregularities, such an arrangement of the 

 cranial nerves is substantially correct, and may serve as a useful guide 

 in the study of their functions. 



First Pair. The Olfactory Nerves. 



What is called in man the " olfactory nerve," is a three-cornered pris- 

 matic tract, composed of both gray and white substance, running forward 

 in a longitudinal groove upon the inferior surface of the anterior cerebral 



1 Nervous System of Rana pipiens ; published by the Smithsonian Institution. 

 Washington, 1853. 



