334 



PHYSIOLOGY 



CHAP. 



seventh layer ; these accordingly conduct in a centripetal direc- 

 tion, and end by ramifying in the grey matter of the superior 

 corpora quadrigemina and the lateral geniculate bodies. But, 

 according to Cajal, a small number of them are protoplasmic pro- 

 cesses of ganglion cells situated in the brain, and are therefore 

 fibres with centrifugal conduction. 



The two limiting membranes, outer and inner, are probably 

 formed by Miiller's sustentacular fibres, the most important of the 

 elements that bind the structures of the retina together. They 

 resemble the ependymal cells of the embryonic spinal cord. They 



FIG. 159. Diagram of histological structure of retina, reconstructed by Kallius from Ramon y 

 Cajal. A, layer of rods and cones; B, membrana limitans externa ; C, outer nuclear layer; 

 D, fibrous layer of Henle ; E. outer molecular layer ; F, inner nuclear layer ; G, inner mole- 

 cular layer;. H, ganglion -cell layer; J, nerve -fibre layer; K, membrana limitans interna ; 

 a, Miiller's supporting fibres ; b, rods ; c, cones ; d, bipolar cell arborising with rods ; e, f, g, h, i, 

 bipolar cells arborising with cones ; k, I, m, horizontal cells ; n, centrifugal nerve-fibre ; 

 Pi (?> r > s > *> ganglion cells of optic nerve ; a, /3, y, 8, 8', t, stratified spongioblasts (amacrine 

 cells) ; , S-, diffuse amacrine cells ; 77, neuro-spongioblasts. 



are epithelial elements of ectodermal origin, small and elongated 

 in form, and scattered through the entire depth of the retina. 

 Their characteristic form is shown at a of Fig. 159, and more 

 clearly in Fig. 160. 



In addition to Miiller's fibres neuroglia cells (spider cells), 

 which occur plentifully along the course of the optic nerve, form 

 part of the supporting tissue of the retina. 



This brief sketch of the complicated structure of the retina 

 shows that an excitation produced at the ends of the rods and 

 cones by light must pass through three different ganglion cells 

 (or chain of neurones) before it can reach the brain. The associa- 



