THE CRANIAL NERVES. 447 



OPTIC NERVES. We have already given some account of these 

 nerves and their decussations, in connection with the history of the 

 tubercula quadrigemina. They consist of rounded bundles of white 

 fibres, running between the tubercles and the retinae. As the reti- 

 nae themselves are membranous expansions consisting mostly of 

 vesicular or cellular nervous matter, the optic nerves, or " tracts,' 1 

 must be regarded as commissures connecting the retinae with the 

 tubercles. We have also seen that they serve, by some of their 

 fibres, to connect the two retinae with each other, as well as the two 

 tubercles with each other. 



The optic nerves convey only the special impression of light from 

 without inward, and give origin to the reflex action of the optic 

 tubercles, by which the pupil is made to contract. According to 

 Longet, the optic nerves are absolutely insensible to pain through- 

 out their entire length. When a galvanic current is passed through 

 the eyeball, or when the retina is touched in operations upon the 

 eye, the irritation has been found to produce the impression of lumi- 

 nous sparks and flashes, instead of an ordinary painful sensation. 

 The impression of colored rings or spots may be easily produced 

 by compressing the eye in particular directions; and a sudden 

 stroke upon the eyeball will often gi ve rise to an apparent discharge 

 of brilliant sparks. Division of the optic nerves produces complete 

 blindness, but does not destroy ordinary sensibility in any part of 

 the eye, nor occasion any muscular paralysis. 



AUDITORY NERVES. The nervous expansion in the cavity of 

 the internal ear contains, like the retina, vesicles or cells as well as 

 fibres; and the auditory nerves are therefore to be regarded, like 

 the optic and olfactory, as commissural in their character. They 

 are also, like the preceding, destitute of ordinary sensibility. Ac- 

 cording to Longet, they may be injured or destroyed without giving 

 rise to any sensation of pain. They serve to convey to the brain 

 the special sensation of sound, and seem incapable of transmitting 

 any other. Longet 1 relates an experiment performed by Yolta, in 

 which, by passing a galvanic current through the ears, the observer 

 experienced the sensation of an interrupted hissing noise, so long 

 as the connection of the wires was maintained. Inflammations 

 within the ear, or in its neighborhood, are often accompanied by 

 the perception of various noises, like the ringing of bells, the 



1 Traite de Physiologic, vol. ii. p 286. 



