THE RETINA. 271 



in primary vision becomes more consistent. The divergent 

 pencil of light which proceeds from any visible point to the 

 eye, becoming convergent after having entered the refractive 

 media, passing through the perfectly transparent retina, is 

 probably brouglit to a point at the surface of tlie choroid or 

 outer part of the bacillaiy layer of the retina, and is not 

 entirely absorbed there, but is reflected as a divergent pencil. 

 In passing towards the point of reflection, the rays of the 

 pencil cannot impress any part of the retina, because they 

 cannot impinge on any of its nervous elements in the only 

 manner in wliich these can be affected — viz., against their free 

 extremities at a right angle. Certain of the rays, however, 

 of the reflected pencil — viz. those which pass along the axis 

 of a rod or cone in the bacOlarj' layer — will impinge in the 

 proper direction on the contained nerve, and produce the 

 luminiferous impression. No confusion, therefore, can result 

 from the multitude of convergent and divergent rays which 

 is passing through the chamber of the eye, and through the 

 retina, for those only are capable of impressing which are 

 reflected along the axes of the cones and rods. The human 

 sensorium receives from the retina the impression of a picture, 

 which is not continuous, but made up of detached points ; as 

 in the vision of the insect, wliich only sees an object by as 

 many points as can transmit rays along the axes of its eye- 

 tubules. 



The bacillary layer of the retina belongs morphologically 

 to the transparent humours of the eye. The original bulb of 

 the eye, whether it be a mere process of the brain, or partly a 

 pulp developed on the tegimientaiy membrane, forms over its 

 entire free surface transparent (cuticular) structures. That 

 portion of the free surface of the pidp which is directed 

 towards the orifice of the eye-follicle, developes the lens, the 

 cornea, and the %itreous humour. That portion again, in 

 contact witli the inner surface of the follicle, and which 



