450 THE NERVOUS SYSTEM. 



Here some of its fibres are connected with the gray substance of this 

 ganglion ; while others pass onward to their termination in the poste- 

 rior part of the optic thalamus. This represents the central connec- 

 tion of the optic tracts, as described by Wagner, Henle, and Huguenin, 

 and as generally accepted by modern anatomists. The optic tracts have 

 accordingly their origin in three separate nuclei or deposits of gray 

 substance, namely, 1st, the anterior tubercula quadrigemina ; 2d, the 

 corpus geniculatum externum ; and 3d, the optic thalamus. 



But there are, beyond question, further indirect connections between 

 these nuclei and the cortex of the hemispheres. They consist of diverg- 

 ing fibres from both the optic thalamus and the corpora geniculata, 

 which, according to Gratiolet, Meynert, and Huguenin,* take part in 

 the formation of the corona radiata and pursue their course toward the 

 posterior part of the hemispheres. 



Physiological Properties of the Optic Nerves. The optic nerves 

 are nerves of special sense, and may be regarded as tracts of fibres 

 connecting the gray matter of the cerebrum with the retinal expansion 

 in the eyeball. They are destitute of tactile sensibility and convey 

 inward only the impression caused by luminous rays. In the central 

 parts of the brain this impression produces the sensation of light ; and 

 the optic nerves are therefore the channels for the sense of vision. 

 Magendie found in quadrupeds both the retina and the optic nerves 

 throughout their length insensible to mechanical irritation ; and, in 

 man, touching the retina with a cataract needle excited no perceptible 

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

 eyeball, that section of the optic nerve is not painful ; and, according 

 to Longet, these nerves in the lower animals may be pinched, pricked, 

 cauterized, divided, or injured in various ways without causing signs 

 of pain. 



On the other hand, their division at once produces blindness. The 

 impressions received by the retina are no longer transmitted to the 

 central organ, and the animal becomes insensible to light, without any 

 loss of tactile sensibility or the power of motion. 



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

 nerves are the channels for a special reflex action ; namely, that of the 

 contractile movements of the iris. 



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

 is regulated by the size of the pupil, are involuntary in character, but 

 are due to impressions conveyed inward by the optic nerve. The im- 

 pression, first received upon the retina, passes through 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 nucleus of origin of the oculomotorius 

 nerves. Thus both the optic nerves and the tubercula quadrigemina 

 are essential to the movements of the pupil under the influence of light. 



Anatomie des Centres Nerveux. Paris, 1879, pp. Ill, 135. 



