THE OPTIC TRACTS 



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encephalon and mesencephalon. The optic apparatus consists of the retinae 

 and optic nerves, the optic chiasma, the optic tracts, the superior quadrigeminate 

 bodies with their relations with the nuclei of the eye-moving nerves, the meta- 

 thalamus, the pulvinar of the thalamus, and the visual area of the cerebral cortex 

 of the occipital lobe. The fibres of the optic nerves arise from the cells of the 

 ganglion-cell layer of the retinse. The fibres which arise in the mesial or nasal 

 halves of each retina cross the mid-line to find their nuclei of termination in the 

 central grey substance of the opposite side, while those from the outer or lateral 

 halves terminate on the same side (fig. 670.) 



The optic chiasma (optic commissure) is functionally independent of the struc- 

 tures of the optic portion of the hypothalamus adjacent to it. It is formed by the 



Fig. 670.- 



-DlAGRAM OF THE PRINCIPAL COMPONENTS OF THE OpTIC APPARATUS. 



Cunningham.) 



(After 



CORP.GEN.MEO 



approach and fusion of the two optic nerves, and is knit together by the decussat- 

 ing fibres from the nasal halves of each retina, and, in addition, by the fibres of 

 Gudden's commissure which is contained in it. 



Beyond the chiasma the optic fibres continue as the optic tracts which course 

 posteriorly around the cerebral peduncles to attain their entrance into the thalam- 

 enchephalon and mesencephalon. Upon reaching the pulvinar of the thalamus 

 each optic tract divides into two roots, a lateral and mesial. 



The lateral root contains practically all of the true visual fibres—fibres arising from the latera 

 half of the retina of the same side and the nasal half of the retina of the opposite side. These 

 fibres are distributed to three localities: — (1) part of them terminate in the lateral geniculate 

 body; (2) the greater portion pass over and around the lateral geniculate body and enter the 

 pulvinar; (3) a considerable portion enter the superior quadrigeminal brachium and course in 

 it to terminate in the nucleus of the superior quadrigeminate body. The most evident function 

 of this latter portion is to bear impulses which, by way of the neurones of the quadrigeminate 

 body, are distributed to the nuclei of the oculomotor, trochlear, and abducent nerves, and thus 

 mediate eye-moving reflexes. The cells of the lateral geniculate body and the pulvinar, about 

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

 area, chiefly the gyri about the calcarine fissure of the occipital lobe. In reaching this area they 

 curve upward and backward, coursing in a compact band of white, substance known as the optic 



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