CHAP. IX.] NERVOUS SYSTEM AND ORGANS OF SENSE. 



293 



The rods stand in close apposition. The cones * are not so close 

 set and do not extend as far outwards as do the rods. Both structures 

 become shorter as they approach the ora serrata. 



Within the rods and cones is a thin layer of granulated substance, 

 connected with the fibres in which the rods and cones end in- 

 ternally. This layer is the external granular, or outer nuclear layer, 

 within which is the internal granular, or inner nuclear lager, formed 

 of nucleated cells and fibres, separated from the former by the inter- 

 granular, or internuclear lager, formed of plexiform tissue enclosing 

 a few nuclei and smooth cells, with 

 coarser fibres running parallel to the 

 surface of the retina. 



Within the internal granular layer, 

 again, is a thicker inner molecular 

 layer (or internal granulated layer), 

 which contains much connective 

 tissue, within which again is a layer 

 of ganglionic cells, and lastly and 

 most internally, is a layer of fibres of 

 the optic nerve, ramifying on the 

 inner surface of the retina, and con- 

 nected with the gaugHa placed ex- 

 ternal to it. This layer of nervous 

 fibres is bounded within by an ex- 

 tremely delicate, glossy membrane, 

 the memhrana limitans, which is 

 continued forwards as the imrs ciliaris retime, and becomes con- 

 tinuous with the suspensory ligament of the lens. Doubtless the 

 rods and cones are continuously though complexly connected with 

 the ganglia and fibres of the innermost layer. 



At the point of entrance of the optic nerve {i.e., at the blind spot) 

 rods and cones are wanting. In the eye, as in the nose and tongue, 

 the special sense is subserved by minute nervous rods proceeding 

 from cells. 



Filling up the great concavity bounded by the memhrana Uniitana 

 Kning the innermost layer of the retina is the vitreous humour 

 (or vitreous body), forming nearly four-fifths of the ball of the eye. 

 This is a transparent, jelly-like, almost quite fluid mass enclosed (as 

 just observed) in the memhrana Limitans of the retina. 



The vitreous humour is reducible to water with a few salts and a 

 little albumen. It appears to consist of concentric layers of slightly 

 different density, but not separated from one another by any 

 membrane. 



The CRYSTALLINE LENS of the cyc is a transparent, doubly convex 

 solid structure, interposed between the posterior surface of the 



Fig. 136.— Laminated Structure of the 

 Crystalline Lens, shown after 

 hardening in alcohol. 



1. Central portion. 



2, 2, 2. Concentric lamina, wliieh have 



become detached along the radiating 

 linos of the external siu-face of the 

 leus. 



* Tlie cones must be looked upou as 

 collections of nerve terminations ; they 

 appear to be longitudinally striated, and 

 they pass into a thick fibre (cone-fibre), 



which consists of a bundle of the finest 

 axis cylinders, which separate in the 

 granular layers of the retina. 



