560 



PHYSIOLOGY 



the brain along the fibres of the optic nerve, each of which is the axon of one 

 of the ganglion- cells. These axons, which form the inner layer of the retina, 

 the so-called ' nerve-fibre layer,' are non-medullated as they pass over the 

 surface of the retina, but acquire a medullary sheath as they pass out of the 

 eyeball through the cribriform plate of the sclerotic and join to form the 

 optic nerve. 



Most of the bipolar cells are connected with several rods or cones ; only 

 in the fovea centralis do we find a special bipolar cell provided for every cone. 



am b 



FIG. 286. Schema of retina. (From BOHM and DAVIDOFF after CAJAL.) 

 1, nerve- fibre layer ; 2, ganglion-cell layer ; 3, inner molecular layer ; 4, inner 

 nuclear layer ; 5, outer molecular layer ; 6, outer nuclear layer ; c, cone ; r, rod ; 

 b, bipolar cells ; S, spongioblast ; am, amacrine cell ; c.n, centrifugal nerve fibre ; 

 M , fibre of M tiller ; n. M , nucleus of fibre of Miiller ; n, neuroglia ; o, outer limiting 

 membrane. 



Every ganglion-cell comes into connection with and receives the impulses 

 from a considerable number of bipolar cells, so that the number of fibres in 

 the optic nerve running centrally is not so great as the number of sense 

 elements in the rod and cone layer of the retina. Besides these cells situated 

 on the direct path of the visual impulse, other cells of a nervous nature are 

 found in the inner nuclear layer (the outer and inner horizontal cells), and also 

 in the inner molecular layer, the so-called * amacrine ' cells. These cells have 

 been imagined to serve as a means of connection or association between 

 different parts of the retina, and may be taken as analogous to the association- 

 cells found in the cerebral cortex. The analogy of the retina with a lobe of 

 the brain is illustrated by the fact that, in addition to the fibres originating in 

 the retina and passing towards the brain, a considerable number of fibres pass 

 from the central nervous system into the retina, where they end chiefly in the 

 two molecular layers. These may have as one of their functions the correla- 

 tion of processes occurring in the retinae of the two eyes and may be asso- 

 ciated with phenomena such as those of binocular contrast, which we shall 

 have to study later on. 



