NEURO-MUSCULAR MECHANISM 



121 



Between the hyaloid membrane and the choroid is the 

 retina (Ret.}. This is an expansion of the optic nerve, which 

 enters the eye at 3 to 4 mm. to the inner side of the posterior 

 optic axis (fig. 53). The white nerve fibres pass through the 

 sclerotic, through the choroid, and through the retina, to form 

 the white optic disc, and then losing their white sheath, they 

 spread out in all directions over the front of the retina, to 

 form its first layer the layer of nerve 

 fibres (1). These nerve fibres take 

 origin from a layer of nerve cells (2) 

 behind them, forming the second 

 layer. The dendrites of these cells 

 arborise with the dendrites for the 

 next set of neurons in the third 

 layer, the internal molecular layer (3). 

 The cells of these neurons are placed 

 in the next or fourth layer, the inner 

 mwlear layer (4), and from these cells, 

 processes pass backwards to form syn- 

 apses in the fifth, or outer molecular 

 layer (5), with the dendrites of the 

 terminal neurons. These terminal 

 neurons have their cells in the sixth 

 or outer nuclear layer (6) of the retina, 

 and they pass backwards and end in 

 two special kinds of terminations in 

 the seventh layer of the retina the 

 rods and cones (7). These structures 

 are composed of two segments a 

 somewhat barrel-shaped basal piece, 

 and a transparent terminal part which 

 in the rods is cylindrical and in the cones is pointed. Over 

 the central spot of the eye in man or apes there are no rods, 

 but the cones lie side by side, and the other layers of the retina 

 are thinned out. The rods and cones are imbedded in the last 

 or eighth layer of the retina the layer of pigment cells, or 

 tapet-um niyrum. The retina in front thins out, but the tapetum 

 nigrum, along with another layer of epithelial cells representing 

 the rest of the retinal structures, is continued forwards over 

 the ciliary processes and over the back of the iris. 



FIG. 53. Diagram of a Sec- 

 tion through the Retina 

 stained by Golgi's method. 

 For description, see text. 

 (From VAN GEHUCHTEN.) 



