640 THE SENSE OF SIGHT. 



only point of the surface of the retina from which the 

 power of vision is absent. 



On making a vertical section of the retina, it is seen, 

 under the microscope, to be composed of several layers 

 which differ from each other in structure and arrangement, 

 while besides these there are fibres, the so-called fibres of 

 Midler, which extend through the different layers, and 

 perforate them, so to speak. Fig. 182 represents a verti- 

 cal section of a small piece of the retina. On examina- 

 tion it will be seen that there are three principal layers, 

 bounded on the inner aspect by a membrana limitans, 

 and on the outer by the choroid coat. I . The outermost 

 is the membrane of Jacob, or the columnar layer. 2. In 

 the middle is the granular layer. 3. The innermost 

 is the nervous layer. Each of these layers, again, is com- 

 posed of different strata, after the fashion shown in the 

 figure. 



The columnar layer (Jacob's membrane) is composed of 

 cylindrical or staff-shaped, transparent and highly refrac- 

 tive bodies, arranged perpendicularly to the surface of the 

 retina, with their outer extremities imbedded, to a greater 

 or less depth, in a layer of black pigment of the choroid 

 coat. Recent researches seem to have determined that 

 this membrane, instead of being, as was formerly con- 

 sidered, an independent covering, is intimately associated, 

 both in structure and function, with the sensitive part of 

 the retina ; for the conical and staff-shaped bodies, of 

 which it is composed, appear to be connected, by means of 

 delicate fibres issuing from them, with the nerve-vesicles 

 of the retina, and even to become continuous with the 

 radiating processes which some of these vesicles present. 

 Concerning the use of these bodies, the discovery of their 

 connection with the sensitive part of the retina supports 

 the opinion entertained by Kolliker and H. Miiller, that 

 their special office is to receive and transmit impressions 

 of light. 



