THE VISUAL APPARATUS 205 



ondary optic vesicle or optic cup) is formed whose walls are two-layered, 

 being composed of both the original lateral and medial parts of the primary 

 optic vesicle (Fig. 97, on the right side). This secondary cavity contains 

 the vitreous humor in the adult eye; the layer of the secondary optic vesicle 

 which borders the vitreous humor forms the retina; the outer layer of the 

 vesicle forms the pigment layer of the retina; and the stalk forms the optic 

 nerve by the ingrowth of fibers throughout its length from the retina and 

 brain (Fig. 100). 



The retina, then, is as truly a part of the brain as is the cerebral 

 hemisphere and its structure is, in general, similar to that of 

 other parts of the brain. There are supporting cells, the fibers 

 of Miiller (Fig. 98, M), and neuroglia elements (Fig. 98, d.s. and 

 s.s.), and lying among these are the neurons. The latter can 

 be classified in general in four groups: (1) the rods and cones 

 (Fig. 98, A); (2) the bipolar cells (Fig. 98, D); (3) the so-called 

 ganglion cells which give rise to fibers of the optic nerve (Fig. 

 98, F); (4) horizontally disposed correlation neurons (Fig. 98, 

 h) . All of these types except the third are intrinsic to the retina, 

 i. e., they send none of their fibrous processes beyond the limits 

 of the retina itself. The axons of the neurons of the third type 

 pass out of the retina and form the so-called optic nerve, termi- 

 nating in the thalamus or midbrain. 



Immediately external to the nervous layer of the retina is the 

 pigment layer (Figs. 99, 100), which is formed from the outer 

 epithelial layer of the secondary optic vesicle (Fig. 97). Figure 

 99 illustrates the ten layers of the retina as figured by the older 

 histologists, and Fig. 98 illustrates the relations of some of the 

 nervous elements as revealed by the Golgi method. It is evi- 

 dent that the "nuclear" or "granular" layers are characterized 

 chiefly by the presence of the cell bodies of the neurons and their 

 nuclei, while the "molecular" layers are composed chiefly of the 

 fibrillar nerve-endings which form the synapses between the 

 various groups of neurons. 



The rods and cones of the retina are the receptors and also 

 the neurons of the first order in the optic path. Their free ends 

 project through the external limiting membrane into the pigment 

 layer. Rays of light which pass through the dioptric apparatus 

 (lens, humors, etc.) of the eyeball must penetrate also the entire 

 thickness of the retina (which is very transparent) before they 

 reach these receptors (Fig. 100). 



