THE SENSES. 707 



formed by the processes of the bipolar cells from above and of the gan- 

 glioriic cells below, and of fibres from the spongioblasts. 



6. The most internal of the nervous layers is a layer of ganglionic 

 cells, consisting of large multipolar nerve-cells, with large round nuclei. 

 In some parts of the retina, especially near the macula lutea, this layer 

 is ver} T thick and consists of several distinct strata of nerve-cells. These 

 cells lie in the spaces of the connective-tissue framework. They are ar- 

 ranged with their single neuraxon or axis-cylinder processes directed in- 

 ward. These pass into and are continuous with the layer of optic fibres. 

 Externally the cells send up numerous branching processes or dendrites 

 which interlace with the fibres of the bipolar cells and the horizontal 

 processes of the spongioblasts. 



All the elements of the retina are sustained and isolated by large 

 cells lying vertically which are known as the fibres of Muller, or epi- 

 thelial retinal cells. Like the corresponding cells of the olfactory 

 mucous membrane, these fibres have upon their sides an infinite number 

 of facets which serve as receptacles to the nerve-corpuscles and fibres 

 of the retina. The nucleus of the fibre of Miiller is found at the 

 level of the internal granular layer, and the two extremities of the proto- 

 plasm or cell-body are condensed in two homogeneous layers, known as 

 the external and internal limiting layer. The external limiting layer is 

 placed, as already described, just between the layer of rods and cones 

 and that of the visual cells. The other is situated upon the internal 

 surface of the retina. The fibres of Miiller are completely independent 

 of each other, having between themselves and the nerve elements only 

 the relation of contact. It is believed that their function is that of 

 supporting the nerve-tissues and also isolating them. 



It will be seen now that the retina is composed essentially of three 

 layers of vertical cells, whose processes have a vertical direction, and 

 which are connected with each other by contact of these processes; that 

 there are also two other sets of cells which form horizontal layers of 

 nerve-processes, these being in the inner and outer parts of the internal 

 granular layer. There are, therefore, strictly speaking, five layers of 

 nerve-cells, three vertical and two horizontal. Two other layers are 

 made up by the modification of the protoplasm of the fibres of Miiller 

 and are purely mechanical in function. They are the external and in- 

 ternal limiting layers. 



Pigment-cell layer , which was formerly considered part of the choroid, 

 consists of cells which cover and entirely surround the outer limbs of the 

 rods and cones. 



The further subdivisions of the retina are more for purposes of fine 

 anatomy than of functional importance. 



Differences in Structure of Different Parts. Toward the centre of the 

 macula lutea all the layers of the retina become greatly thinned out and 



