706 



HANDBOOK OF PHYSIOLOGY. 



polar layer, arid a slightly deeper layer in which the cone cells come 

 in contact with the dendrites of the bipolar cells. 



4. The internal granular layer (fig. 417, E) is an inner subdivision of a 

 layer of bipolar cells, and is the most complicated of any of the layers of the 

 retina. It is made up, however, mainly of bipolar cells, which are fusi- 

 form in shape, vertical in arrangement, and have two processes, one as- 

 cending and the other descending. The descending fibre is always single 

 and ends at different levels in the internal molecular or plexiform layer, 

 where it forms flattened and brush-like expansions. The ascending 

 process is often multiple, and it ends in a large number of different 

 branches, which arrange themselves in something like a horizontal layer 

 in the lower part of the external molecular layer. 



Fig. 418A. Perpendicular section of the retina of a mammal. A, External grainsor bodies of 

 rods; .B, bodies of cones; a, horizontal external or small cell; ft, horizontal internal or large cell; 

 c, horizontal internal cell with descending protoplasmic appendages; e, flattened arborization of one 

 of the large cells; /. g, /t, j, I. spongioblasts ramifying in the various strata of the internal molecular 

 zone; m, ?i, diffuse spougioblasts; o, ganglionic cell; 1, external molecular zone; 2, internal mole- 

 cular zone. (Cajal.) 



Besides these vertical bipolar cells there are flattened star-shaped 

 cells lying just beneath the external molecular layer, sending out branches 

 parallel to the periphery and ending in numerous ramifying expansions 

 which come in contact with the different descending branches of the 

 cone cells. Their general arrangement is horizontal. These little cells 

 appear to have as their function the connecting of the visual cells with 

 each other (fig. 418A, r, />). There are other horizontal cells, larger than 

 these, but having practically the same shape and arrangement, and lying 

 somewhat more deeply in the layer; these connect the processes of the rod 

 cells with each other and have thus an associative function. There is, in 

 addition, in this layer, a series of larger cells, called by Cajal spongioblasts^ 

 which lie deep in the internal granular layer, and whose branches take 

 a horizontal direction and appear to have the function of associating the 

 cells of the ganglionic layer below (see fig. 418A). 



5. The internal molecular layer is composed of a plexus of fibres 



