THE SENSES. 705 



the rods and cones and separates them from the layer beneath. This 

 is called the external limiting membrane, (fig. 417A, le). 



2. External Umuular Layer. The cells of the external granular 

 layer are the bipolar or visual cells which contain the protoplasm not yet 

 transformed into rods and cones in the layer above. The cells whose 

 bodies are continued upward as cones are different in shape from those 

 which are connected with the rods. The cells of the cones are situ- 

 ated close to the external limiting membrane. They have a large ovoid 

 nucleus. From the inner side of the cell-body a process descends 

 toward the external molecular layer where it ends in a slight dilatation 

 (see fig. 417). On its outer side a process of the body ascends through 

 into the external limiting membrane and swells into a cone (fig. 417, c). 



The bipolar cells giving birth to the rods lie at deeper levels in the 



Fig. 418. The posterior half of the retina of the left eye, viewed from before; s, the cut 

 edge of the sclerotic coat ; ch, the choroid; r, the retina; in the interior at the middle the 

 macula lutea with the depression of the fovea centralis is represented by a slight oval shade; 

 toward the left side the light spot indicates the colliculus or eminence at the entrance of the 

 optic nerve, from the centre of which the arteria centralis is seen spreading its branches into 

 the retina, leaving the part occupied by the macula comparatively free. (After Henle.) 



granular layer. They contain an ovoid nucleus of a smaller volume than 

 those of the cone cells. The protoplasm of the cell-body gives off two 

 fibres, one ascending, and the other descending. The ascending fibre 

 runs up through the limiting membrane and is continued as a rod. The 

 descending fibre goes into the molecular layer and ends here in a small 

 nodule. According to Cajal, these cells of the visual layer have no di- 

 rect anatomical continuity with the cells of the bipolar layer below, 

 though Dogiel and others have denied this. 



3. The external molecular layer or external plexiform layer (fig. 417, C) 

 is composed of numerous protoplasmic processes (dendrites) which coine 

 from the cells of the internal granular layer below and from the visual 

 cells above. Some subdivisions of this layer are made, there being an 

 outer part in which the rod cells meet the branching fibres of the bi- 

 45 



