SECTION VII 

 THE RETINAL CHANGES INVOLVED IN VISION 



IN nearly all sense-organs the essential constituent is a bipolar 

 nerve-cell having one process extending towards the surface and 

 ending between epithelium- cells covering that surface, and a central 

 process, which runs towards the central nervous system, where it forms 

 synapses with the processes of other nerve-cells (Fig. 282). In some 

 cases, such as the olfactory cells and the sense-cells embedded in the epi- 



B 



C 



FIG. 282. 



A, olfactory sense-cell ; B, auditory sense-cell ; c, connections of 

 gustatory fibres (taste-bud) ; D, nerve-ending in skin or corneal 

 epithelium (probably pain fibres). 



dermis of worms and other invertebrata, the peripheral process is quite 

 short. In other cases, as in the ordinary posterior root ganglion-cell, 

 the peripheral process may be several feet in length. The retina, 

 however, cannot be regarded as a simple sense-organ, but is homologous 

 with a complete lobe of the brain. It is formed, like the cerebral hemi- 

 spheres, as a hollowed outgrowth from the fore-brain or anterior cere- 

 bral vesicle. The stalk of this outgrowth narrows so as to produce an 

 optic vesicle connected with the rest of the fore-brrJn by the optic stalk. 

 As the vesicle grows towards the surface its anterior wall is invaginated 

 so that an ' optic cup ' is formed, at the mouth of which the lens and 

 other parts of the eye are developed at the expense of the over- lying 

 epiblast and the surrounding mesoblast. The posterior wall of the cup 

 develops into the retinal pigment, while the nervous elements which 



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