368 



LESLIE B. AREY 



from the apex of the optic sac. At this thickest region, the 

 distance from the optic capsule to the central zone, that is, the 

 length of the pigment cells, is approximately 35 /x. 



The unpigmented or sensory cells (fig. C, nl. cl. sns.) are much 

 more robust and have larger nuclei than either kind of pigment 

 cell just described. These cells are of an elongated spindle 

 shape, thickest in the region of the nucleus. The cell body 



fbrlo. 



fbrl. 



)rtn.ex^ 



pre. n't'. 



Fig. C A portion of an axial section of a Planorbis eye, showing the com- 

 ponent retinal elements (X 1000). ax. bac, axis of rod; cps. opt., optic capsule; 

 fbrl'., fibrillae of rod-axis; /6rZ"., fibrillae of rod-mantle; ivlr. bac, mantle (in- 

 volucrum) of rod; nl. cl. pig., nucleus of pigment cell; 7il. cl. sns., nucleus of sen- 

 sory cell; pig., pigment; pre. n't., neurite-process of sensory cell; rtn. ex., periph- 

 eral zone of retina; rtn. i., central zone of retina; rtn. m., middle or pigmented 

 zone of retina. 



continues through the pigment zone and ends, in the so-called 

 central zone, in a structure known as the 'rod' — this being the 

 photoreceptive portion of the cell. Each rod consists of two 

 parts, centrally a core or 'axis' (ax. bac), and peripherally a 

 radially striate "mantle' (ivlr. bac). Proximal to the nucleus, 

 each sensory cell gives off a neurite {pre. nH.), which courses 

 along the inner face of the capsule and is ultimately gathered 

 up with similar neurites to form the optic nerve (figs. A and B, 



