30 INNERVATION. [CHAP, xvi I. 



wall is a diaphanous membrane with nuclei projecting at intervals, 

 and the meshes average -j-J^- of an inch diameter. 



Behind the vesicular gray layer is the granular layer, a term we 

 shall apply to it, because it seems to consist of a close aggregation 

 of small granules, which refract the light more powerfully than 

 the neighbouring parts, and have scarcely any appearance of in- 

 tervening matrix ; they might be regarded perhaps as analogous 

 to the nuclei of cells, and much resemble a layer of granules in 

 the substance of some of the cerebral convolutions, and of the 

 laminae of the cerebellum. They are made more evident by acetic 

 acid. This layer is divided into two, of which the inner is much 

 the narrower, by a pale stratum, which can only be seen by very 

 careful manipulation. 



On the outside of the granular layer is that remarkable lamina, 

 known by the name of its discoverer, the membrana Jacoli. It 

 Fig. \i9. consists of club-shaped rods, placed 



uprightly, the thin end inwards, the 

 thick outwards ; and it is very easily 

 detached from the rest of the retina, 

 when the choroid is removed, so as 

 to float as delicate shreds, visible to 

 the naked eye, in the water in which 

 the eye is immersed. The rods have 

 a tendency to separate from one 

 another when placed in water, and 

 the club-shaped extremities are then 

 a often seen to be formed by a sud- 

 den bending back of the stem like 



a crook, which may be more or less opened out. Interspersed 

 among the rods are seen on the outer surface a number of clear 

 spaces, as though transparent cells were disseminated among them. 

 This layer forms the connecting medium between the retina and 

 the choroidal epithelium. 



It has been before stated that the optic nerve pierces the sclerotic 

 and joins the retina about an eighth of an inch on the inner side 

 of the axis of the eye. Precisely in this axis, the retina is of a 

 decidedly yellow colour in a roundish spot of about -J 4 - of an inch 

 diameter, called, after its discoverer, the yellow spot of Sasmmerring. 

 This spot exists only in man and the monkey among mammalia, 

 but an analogous part has been found by Dr. Knox in reptiles. It has 

 been described by some as a fold, by others as a foramen in the re- 

 tina, and after several examinations we should speak of it as a small 



