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 pro- 

 cesses of other nerve-cells (Fig. 284). In some cases, such as the olfactory 

 cells .and the sense-cells embedded in the epidermis of worms and other 



A B C 



FIG. 284. 



. 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). 



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 hemispheres, as a hollowed outgrowth from the fore-brain 

 or anterior cerebral vesicle. The stalk of this outgrowth narrows so as to 

 produce an optic vesicle connected with the rest of the fore-brain 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 make 

 up the retina are formed by division and differentiation from the anterior 



558 



