542 



PHYSIOLOGY 



and a liunch of dendrites \\liii-li ramify with others in the inner molecular layer. Only 

 at the macula is more lhaii urn- layer of ganglion cells present ; this is due to their almost 

 complete absence at the fovea. The macula therefore not only has its own relays but 

 those; of the fovea as well. 



THE INNER MOLECULAR LAYER consists of a felt-work made up by the 

 interheing dendrites of the ganglion cells with those of the inner nuclear cells or second 

 order neurons. There are also the dendrites of horizontal cells or spongioblasts. These 

 possibly serve to associate the impulses from different parts of the retina, such as is 

 supposed to occur in the brain. It should be noted that they appear to be absent in 

 the. fovea and macula- 



THE INNER NUCLEAR LAYER largely consists of bipolar second order neuron 

 cells. There are however also present the nuclei of the horizontal cells, and also the 

 nuclei of similar cells, whose dendrites travel in the outer molecular layer. The bipolar 

 cells, which are fusiform in shape and nucleated, are of three f kinds : (a) those which 

 connect with rods, (6) those which connect with cones, and (c) giant bipolars which 

 connect with either. 



n.M 



am b. 



KM;. -21'2. Diagram to show the different cellular structures found in the retina. 



THE OUTER MOLECULAR LAYER is much like the inner; it consists of the 

 dendrites of the second order neurons and the first. 



THE OUTER NUCLEAR LAYER consists of the cells of the first order neurons 

 or the granules of the rods and cones. The cells are nucleated, somewhat smaller than 

 the bipolar cells, and their nuclei are striated. They give off two processes, one of \\ hich 

 toi !!!> dendrites in the lift h layer, the other connects with either a rod or cone as the case 

 may be. 



THE BACILLARY LAYER of rods and cones is separated from the previous layer 

 1>\ t he external limiting membrane. Both rods and cones consist of an outer and inner 

 limb, the forms of which are well shown in Fig. 273. It will be seen that the outer 

 limbs are striated, the cones coarsely, the rods finely. Like some types of striated 

 muscle they tend after hardening to break up into discs. The inner limbs of both rods 

 and cones have a strong alHnity for <l\ 



THE STRATUM PIGMENTI is the only one that is developed from the external 

 layer of the embryonic opt ic cup. The epithelium consists of a single layer of hexagonal 

 nucleated cells containing numerous pigment granules. The cells send fine processes 



