493 



structure, consisting of a so-called rod or cone (which I distinguish 

 as the crystalline column), and its appended Miillerian filament, with 

 its nuclear enlargements. This structure constitutes a specific kind 

 of peripheral nervous organ, which, from its function, I term a 

 photcesthetic hody. 



A photsesthetic body consists of a distal segment, or dioptric por- 

 tion, elongated, cylindrical, or club-shaped, homogeneous, transparent, 

 and highly refractive, usually termed the rod or cone ; and a proxi- 

 mal segment or peduncle, with its nuclear enlargements, into which 

 the ultimate nervous filament passes, and within which it apparently 

 terminates, probably at its outer end. 



The entire aspect and arrangement of these photsesthetic bodies, 

 their predominance over the other parts of the retina at the axial 

 spot of the eye, and the direct continuity of their stems with the 

 nerve filaments at that spot, appear to me to indicate not only the 

 nature of their functions, but also the general features of the mode 

 in which it is effected. It appears to me that the rays which act 

 upon the nervous filaments, must be such rays as the arrangement 

 permits to pass from behind, forwards in the axis of the photsesthe- 

 tic bodies. It has now been ascertained, that the quantity of light 

 reflected, and consequently irregularly dispersed within the eye- 

 ball from the choroid, and bacillary layer, &c. is very considerable ; 

 and it consequently becomes a very important question, to determine 

 in what manner this reflected and irregularly dispersed light is pre- 

 vented from affecting the retina. The view which I have already 

 given of the structure and probable mode of action, of the photaes- 

 thetic bodies, affords the basis of a hypothesis which meets all the 

 conditions of the question, and is in full accordance with the com- 

 parative anatomy and development of the organ of vision, I can- 

 not interpret the functions of the structure of the retina as now de- 

 termined, except by assuming that the photsesthetic columns are im- 

 pressed not by the light as it enters the eye, or as it is more or less 

 irregularly reflected and dispersed in its interior, but only by those 

 rays which, in their passage backwards to the pupil pass along, or 

 nearly in the axes of the crystalline rods or columns of the photses- 

 thetic bodies, so as to reach the photsesthetic spots under the re- 

 quired conditions. No confusion, therefore, can result from the mul- 

 titude of convergent and divergent rays which pass through the 

 chamber of the eye, and through the retina. By this means, the 



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