Till-: OPTIC TRACTS 



829 



fibres from the nasal halves of each retina, and, in addition, by the fibres of Ciudden's 

 commissure which is ('untamed in it. 



Heyond the cliiasma the optic lihrcs continue as the optic tracts which course 

 posteriorly around the cerebral peduncles to attain their entrance into the t halamen- 

 ce|ihalon and mesencephalon. I ]>on reaching the pulvinar of the t halamus each 

 optic tract divides into two roots, a lateral and mesial. The lateral root contains 

 practically all of the true visual fibres fibres arisini: from the lateral half of the re- 

 tina of the same side and the nasal half of the retina of the opposite side. The-e 

 film's are distributee I to three localities: ( 1 ) perhaps the greater portion terminate in 

 the lateral geniculale body; (2) a portion pass over and around the lateral geniculate 

 body and enter the pulvinar; (15) a considerable portion enter the superior quadrigemi- 

 nal brachium and course in it to terminate in the nucleus of the superior quadrigemi- 

 nate body. The most evident function of this latter portion is to bear impulses 



FIG. 618. DIAGRAM OF THE PRINCIPAL COMPONENTS OF THE OPTIC APPARATUS. 



Cunningham.) 



(After 



which, by way of the neurones of the quadrigeminate body, are distributed to the 

 nuclei of the oculomotor, troehlear. and abducent nerves, and thus mediate eye- 

 moving reflexes. The cells of the lateral geniculaie body and the pulvinar. about 

 which the retinal fibres terminate, give off axones which terminate in the cortex of 

 the visual area, chiefly the cuneus. of the occipital lobe. In reaching this area they 

 curve upwards and backwards, coursing in a compact band of white substance known 

 as the optic radiation (radiatio oceipito-thalamica, fig. 643). Some of the fibres 

 of the optic radiation probably arise also in the superior quadrigeminate body. It 

 also is in large part composed of fibres arising from the cells of the occipital cortex, 

 which pass from the cortex to the pulvinar. superior quadrigeminate bodies, and 

 possibly some to the medulla ohloniiata and spinal cord. 



The mesial root of the optic tract contains few. if any, true visual fibres. It runs 

 into the medial geniculate body, and neither it nor this body are appreciably affected 

 after extirpation of both eyes. It may be considered as wholly representing 



