1 66 



THE PINEAL ORGAN 



proliferate and give rise to many layers of small round cells, the outer of 

 which pass through the basement membrane and form the intermediate 

 and ganglionic layers of the retina. The inner ends of the receptive cells, 

 however, retain their primary position inside the basement membrane and 

 form the layer of visual-rods. Between the visual cells are a certain 

 number of cells which develop pigment. These form a conspicuous band 

 external to the bacillary layer (Fig. 121) ; outside the pigment layer is a 

 narrow, clear zone of nerve-fibres — inner plexiform layer ; this is succeeded 

 by a wide intermediate band of small, round nuclei — middle nuclear layer — 

 another layer of fine nerve-fibres — outer plexiform layer — and finally a 

 layer of ganglion cells, appearing in the greater part of its extent as a single 



OUTER GRAN. LAYER 

 INNER GRAN. LAYER 



LOB CER. POST 



Uti. 



'com fs 



LOB. CER ANT 

 s OPTIC GANGLION 



OPTIC LOBE 



Fig. 122. — Scheme of the Visual Paths and their Central Connections 

 in a Cephalopod, combined from von Lenhossek, Kopsch, and Cajal. 

 According to Cajal most Bipolar Cells (b) are located in the outer 



GRANULAR LAYER. (FROM C.U. ArIENS KAPPERS, SLIGHTLY MODIFIED.) 



Axons coming from the retinal cells, decussate in the optic nerve, where they form 

 a chiasma. The inverted image on the retina which results from rays passing 

 through the narrow pupil and lens is thus corrected in the cortex of the optic 

 lobe, a, b, c, d, d 1 , d 2 , d 3 , d*, e, e 1 , e 2 , denote relays of neurons. 



stratum of large cells, lying in an outer nerve-fibre layer which is limited 

 by an external basement membrane. The nerve-fibres converge towards, 

 the proximal or posterior pole of the eyeball, where, forming a short, 

 thick optic nerve, they pierce the sclera and enter the optic lobe (Fig. 36, F, 

 Chap. 3, p. 50). 



The course of the nerve-fibres, according to Cajal, is indicated in 

 Fig. 122, which shows a decussation of axons coming from the retina in the 

 optic nerve ; this corrects the reversal of the image formed on the retina. 

 In the optic lobe there are two layers of granule cells placed superficially 

 and separated by a plexiform layer of nerve-fibres ; while in the centre is 

 an area consisting of nerve-cells and nerve-fibres, the nerve-cells being 

 chiefly aggregated in a central nucleus and in a peduncular nucleus . From 



