EYES 215 



nected by the inner molecular layer with the rest of the ganglionic 

 layer which lines the inside of the retinal cup. 



In order to understand the latter layer and the relations of the 

 optic nerve, an account of the development is necessary. At first the 

 retinal layer is comparatively thin, but it increases in thickness, in part 

 by a multiplication of cells, in part by their increase in length and the 

 development of the dendrites of the molecular layers. Each cell of 

 the inner layer (the one turned toward the lens) also develops an axon 

 which runs over the free surface of the cells to the chorioid fissure, 

 passes through this and along the ventral groove of the optic stalk to 

 the diencephalon. 



As will readily be understood, it is these fibres and not the optic 

 stalk which form the optic nerve (p. 184). When the chorioid fis- 

 sure closes, the nerve appears to leave through the centre of the 

 retina, and as this part contains no sense cells, the point of exit con- 

 stitutes the 'blind spot' of physiological works. Besides the cells 

 already mentioned the retina contains supporting or radial cells, Uke 

 other sense organs or Uke the brain itself (neuroglia). These extend 

 through from the nerve fibres to the basal membrane. Either rods 

 or cones may be absent in isolated groups of vertebrates. Usually 

 there is a spot, the macula lutea (yellow spot) or fovea centralis at 

 the centre of the retina where vision is most distinct. Here the rod 

 and cone cells are shorter and more crowded than elsewhere. 



Here may be mentioned a point of morphological importance. It will be 

 recalled (p. 144) that the ependymal surface of the brain corresponds to the 

 external surface of the ectoderm of the rest of the body. Therefore, as a glance 

 at fig. 228 will show, the rods and cones are on the primitively outer and the 

 ganglion cells and nerve fibres are on the deeper surface of the ectoderm. Hence 

 rods and cones correspond to the percipient cuticular structures of other sensory 

 organs like the lateral line, taste buds and the like. Before it can affect the 

 sensory cells the light has to traverse the whole of the retina and then the nerv- 

 ous impulses have to pass back through the same layers to reach the optic nerve. 

 This constitutes an 'inverted eye' which, with the exception of a few molluscs, 

 is unknown, except in the vertebrates. A comparison with the parietal eye of 

 reptiles (fig. 162) is very instructive. 



The cavity between lens and retina is filled with a semisolid 

 vitreous body, the origin of which is in dispute. In mammals blood- 

 vessels and mesenchymatous cells enter the optic cup through the 

 chorioid fissure before its closure. Some suppose that the vitreous 

 body arises from a modification of these cells, some regard it as an 

 exudate from the blood-vessels, and others think it a retinal secretion. 



