THE ORGANS OF SPFXIAL SENSE 



567 



of the cone cells. Their axones pass only into the inner molecular 

 layer where they end in arborizations around the dendrites of neu- 

 rones whose cell bodies are in the layer of nerve cells (neurone system 

 No. II.). (c) Horizontal neurones (Fig. 373, A, 11 and 12). These 

 serve as association neurones between the visual cells and may be 

 divided into rod association neu- 

 rones and cone association neu- 

 rones. The cone association 

 neurones are the smaller and 

 more superficial, and both den- 

 drites and axones end in the 

 outer molecular layer around the 

 terminal expansions of the cone 

 visual cells (Fig. 373, A, 11). 

 The rod association neurones are 

 larger, more deeply seated, and 

 behave in a similar manner to- 

 ward the rod visual cells (Fig. 

 373, A, 12). Some of these cells 

 send processes to the inner mole- 

 cular layer. 



Xeuroxe System No. II. — 

 This has been already partly de- 

 scribed in connection with the 

 axone terminations of neurone 

 system No. I. The cell bodies 

 of the second neurone system 

 (Fig. 373, A, 8, 9) are in the 

 layer of nerve cells and are, as 

 above noted, associated either 

 directly or by means of their 

 dendrites \^dth the axones of the 

 first neurone system. Their 



axones pass into the layer of nerve fibres and ultimately become 

 fibres of the optic nerve (Fig. 373, A, 10). 



The optic nerves (Fig. 376, No) unite at the base of the brain to 

 form the optic decussation or chiasma (Fig. 376, CM). Here the 

 axones from the mesial part of the retina cross to the optic tract of 

 the opposite side, while those from the lateral part of the retina remain 

 in the optic tract of the same side. The axones of the optic tract 



Fig. 376. — Diagram showing Main 

 Relations of Optic Tract. (Testut.) 

 R, Retina; No, optic nerve; CM, optic 

 decussation or chiasma; Tro, optic tract; 

 Tlio, thalamus; Cgl, lateral geniculate 

 body; Qa, anterior corpus quadrigemi- 

 num; Rd, fibre of optic tract passing 

 directly to cortex; Sm, third neurone sys- 

 tem of optic tract (excepting Rd) connect- 

 ing thalamus, lateral geniculate body, 

 and anterior corpus quadrigemnium with 

 the cortex, Co. 



