HISTOLOGY OF THE RETINA 



543 





between the limbs of the rods. The bases of these cells are firmly attached to the 

 choroid and thus give support to the rest of the retina. 



The object of these cell processes and the pigment granules which they 

 contain would appear to be either the preven- 

 tion of an image formed on one part of the 

 retina from spreading to the sensitive elements 

 of surrounding portions, or else the protection of 

 these elements from excessive light action. But 

 it has been definitely proved that the cells 

 themselves have another and important func- 

 tion to perform, namely the secretion of the 

 pigment called visual purple (rhodopsin). The 

 important functions of this pigment will be 

 described later. 



It should be noted that beside the struc- 

 tures described above, which have the func- 

 tional activities of the retina to perform, there 

 are a number of connective tissue elements 

 which form the retina into one coherent struc- 

 ture. Since the retina is developed from an 

 outgrowth of the brain, these structures are 

 similar in type to those met there ; we there- 

 fore find neuroglia and also long cells which 

 extend through the first seven layers and hold 

 them together, namely the fibres of Miiller. 



THE DEVELOPMENT OF THE RETINA. 

 The complex series of layers of which the 

 retina consists are developed from the two walls 

 of the primitive optic cup, which grows as a 

 hollow bud from the anterior cerebral vesicle of 

 the embryo. At first the two layers are of the 

 same thickness, but the outer becomes reduced 

 to a single layer of flattened cells, which become 

 pigmented, forming the stratum pigmenti. The 

 inner layer consists at first of a single layer of 

 elongated nucleated cells, which become differ- 

 entiated into spongioblasts, germinal cells and 

 neuroblasts, similar to those found in the de- 

 velopment of the spinal cord. The spongioblasts 

 form the inner and outer limiting membranes, 

 and a groundwork within which the functional 

 elements develop. The germinal colls give rise 

 to three series of neuroblasts in all. The first 



d - 



U. 



FIG. 273. 



I, a rod; II, a cone of mammalian 

 retina ; h, external limiting membrane. 

 (R. GREEFF.) 



fibr 



: 



set are much larger than the others, and become 

 the ganglion cells (these appear to be formed by mitotic division). The next two sets 

 are much smaller, and become the first and second nuclear layer (these seem to be 

 formed by amitotic division). Lastly the germinal cells themselves become trans- 

 formed into the rods and cones. The molecular layers are formed of the arborisations 

 of the processes of the cells between which they lie. The innermost layer of nerve 

 fibres is formed by the growth of long processes from the ganglion cells, which make 



tir way from the retina into the brain. 

 THE DIFFERENT PARTS OF THE RETINA show marked variation in detail, 

 the fovea cones alone are found ; each of these connects to one axon only. 

 Other structural differences are found beside (1) absence of rods, namely 

 (2) the cones are longer, more highly developed, and some say more rod-like than 



