SENSE OF SIGHT. 441 



of the membrane of the retina. In the course of their short 

 passage, these fibres exhibit swellings, the signification of 

 which is unknown. Some of these form genuine nerve cells, 

 and terminate by dilating into a peculiar element, which is 

 either small and delicate (rods, or larger and more bulky 



S 



Fig. 119. Diagram of the retina and the optic nerve.* 



(cones) Fig. 120) ; we see, by this arrangement, that the rods 

 and cones form, by their juxtaposition, the external layer of 

 the retina (Fig. 119) ; this layer, which is easily separated, 

 was long known under the name of Jacob's membrane. 



Max Schultze and other German histologists who have 

 lately made investigations on this subject fix the number of 

 layers at ten, which, thus stratified, form the substance of the 

 retina. These are, beginning from within (proceeding from 

 the vitreous humor to the choroid tunic) an internal limiting 

 membrane (Fig. 120, /) ; the layer of filaments of the optic 

 nerve (Fig. 120, /) ; the layer of the nerve cells (g] ; the 

 granular layer (n) ; the internal nuclear division of the gran- 

 ular layer (k) ; the external nuclear division of the granular 

 layer ; external granular layer (Jsf) ; external limiting mem- 

 brane of Schultze; the layer of cones and rods (Fig. 120, s); 

 and, finally, a layer of pigment, which is diffused between 

 the extremities of the cones and rods, and which every thing 



* S, S, Sclerotic. CA, Choroid. Nqo, Optic nerve. P, Its papilla, whence 

 the fibres radiate, and form the retina (K,R). M, Central fosselte of the .retina 

 (orfovea centralis ntinat). 



