658 



SPECIAL SENSES 



external surface a regular mosaic appearance ; and between these, is a 

 greater or less number of flask-shaped bodies, the cones. This layer is 

 about -g^-Q of an inch (76 /x) in thickness at the middle of the retina ; 

 ^Q- of an inch (62 yu), about midway between the centre and the periph- 

 ery ; and near the periphery, about |Q- of an inch (55 yu). At the 

 macula lutea the rods are wanting. Over the rest of the membrane, the 

 rods predominate and the cones become less and less frequent toward 

 the periphery. 



The rods are regular cylinders, their length corresponding to the 

 thickness of the layer, terminating above in truncated extremities, and 



Fig. 168. Diagram of the retina (Kallius). 



I, inner limitary membrane ; II, expansion of fibres of the optic nerve; III, layer of ganglion- 

 cells; IV, inner molecular layer ; V, inner nuclear layer ; VI, outer molecular layer; VII, outer 

 nuclear layer ; VIII, outer limitary membrane ; IX, layer of rods and cones ; X, layer of pigment- 

 cells. 



NOTE. The small letters in this figure refer to minute details of structure that have not been con- 

 sidered in the text. The lower border of the cut may be taken as representing the inner limitary 

 membrane. The numbering of the layers is from below upward, reversing the order in the original 

 diagram, and a number for the inner limitary membrane has been added. 



below in points which probably are continuous with the filaments of con- 

 nection with the nerve-cells. Their diameter is about T-J^Q-O ^ an ^ nc ^ 

 (2 fji). They are clear, of rather a fatty lustre, soft and pliable, but 

 somewhat brittle, and so alterable that they are with difficulty seen in a 

 natural state. They should be examined in perfectly fresh preparations, 

 moistened with liquid from the vitreous humor or with serum. When 



