203 KECENT PROGRESS OF THE THEOEY OF VISION. 



with structures not elsewhere found, called rods and cones. 

 The rods are slender cylinders ; the cones, or bulbs, some- 

 what thicker, flask- shaped structures. All are ranged 

 perpendicular to the surface of the retina, closely packed 

 together, so as to form a regular mosaic layer behind it. 

 Each rod is connected with one of the minutest nerve 

 fibres, each cone with one somewhat thicker. This layer 

 of rods and bulbs (also known as membrana Jacobi) has 

 been proved by direct experiments to be the really sensi- 

 tive layer of the retina, the structure in which alone 

 the action of light is capable of producing a nervous 

 excitation. 



There is in the retina a remarkable spot which is placed 

 near its centre, a little to the outer (temporal) side, and 

 which from its colour is called the yellow spot. The 

 retina is here somewhat thickened, but in the middle of 

 the yellow spot is found a depression, the fovea centralis, 

 where the retina is reduced to those elements alone which 

 are absolutely necessary for exact vision. Fig. 29, from 

 Henle, shows a thin transverse section of this central de- 

 pression made on a retina which had been hardened in 

 alcohol. Lh (Lamina hyalina, membrana limitans) is 

 an elastic membrane which divides the retina from the 

 vitreous. The bulbs (seen at b) are here smaller than 

 elsewhere, measuring only the 400th part of a millimeter 

 in diameter, and form a close and regular mosaic. The 

 other, more or less opaque, elements of the retina are 

 seen to be wanting, except the corpuscles (^-), which 

 belong to the cones. At / are seen the fibres which unite 

 these with the rest of the retina. This consists of a layer 

 3f fibres of the optic nerve (ri) in front, and two layers of 

 nerve cells (gli and gle), known as the internal and exter- 

 nal ganglion layers, with a stratum of fine granules (gri) 

 between them. All these parts of the retina are absent 

 at the bottom of the fovea centralis, and their gradual 



