518 THE CRANIAL NERVES. 



only the impression produced upon the retina by luminous rays. In 

 the central parts of the brain with which they are connected, this im- 

 pression becomes the sensation of light ; and the optic nerves are there- 

 fore the channels for the sense of vision. Magendie found that in 

 quadrupeds both the retina and the optic nerves throughout their 

 length were insensible to mechanical irritation; and, in man, that touch- 

 ing the retina with the point of a cataract needle excited no perceptible 

 sensation. It has also been remarked, in cases of extirpation of the 

 eyeball, that the section of the optic nerve is not a painful part of the 

 operation; and, according to the observations of Longet upon animals, 

 these nerves may be pinched, pricked, cauterized, divided, or injured in 

 various ways without producing any signs of pain. 



On the other hand, division of these nerves at once produces a state 

 of blindness. The impressions received by the retina are no longer 

 transmitted to the central organ, and the animal becomes insensible to 

 light, without losing any of his ordinary tactile sensibility or power of 

 voluntary motion. 



Beside their immediate function in the perception of light, the optic 

 nerves are also channels for a special reflex action, connected with the 

 mechanism of vision ; namely, that of the contractile movements of the 

 iris. By these movements the orifice of the pupil enlarges or diminishes 

 according to the intensity of the light to which the eye is exposed. On 

 first entering a dark room everything is nearly invisible; but gradually, 

 as the pupil dilates and as more light is admitted, objects show them- 

 selves with greater distinctness, and at last we can see tolerably well 

 where it was at first almost impossible to perceive a single object. On 

 the other hand, when the eye is exposed to a brilliant light, the pupil 

 contracts and shuts out so much of it as would be injurious to the retina. 



These movements, by which the quantity of light admitted to the eye 

 is regulated to suit the sensibility of the retina, are involuntary in cha- 

 racter, but are due to impressions conveyed inward by the optic nerve. 

 On the division of these nerves, or the destruction of the tubercula 

 quadrigemina, not only is the perception of light abolished, but the pupil 

 remains immovable, whatever may be the intensity of the light to which 

 it is exposed. In the production of this reflex act, the impression, which 

 is first received upon the retina, passes inward through the fibres of the 

 optic nerve to the tubercula quadrigemina. Its transformation into a 

 motor impulse is either accomplished in these bodies, or is commenced 

 in them and completed by transmission to the gray matter at the origin 

 of the oculomotorius nerves. Thus both the optic nerves and the tuber- 

 cula quadrigemina are essential to the movements of the pupil under the 

 influence of light. The proof that this action is of a reflex nature is 

 afforded by the results of dividing and irritating the optic nerves. After 

 section of the nerve, according to the experiments of Herbert Mayo and 

 Longet, upon pigeons, dogs, and rabbits, irritation of its peripheral end, 

 that is, the portion still connected with the eyeball, produces no effect 

 upon the pupil ; but irritation of its central portion, which is connected 



