268 SPECIAL ANATOMY. 



Its internal surface is applied loosely to the vitreous body; its anterior 

 extremity reaches as far as the posterior border of corpus ciliare (according to 

 the older view as far as the lens, before or behind the zonula Zinnii). At the 

 posterior extremity, where the rather projecting optic nerve enters (medullary 

 elevation), it presents in the axis of the eye, that is to say, on the external cir- 

 cumference of the optic nerve, a transparent spot (probably not perforated), 

 the so-called 



Foramen centrale retina, hidden behind a yellow transverse fold, macula 

 lutea. This last is found in man (and apes) only, and even here but for a time 

 after birth. Its object is unknown. 



Structure : 



1. The external layer of the Retina, which is connected with Choroid, and 

 is described as Tunica Jacobi, consists of the so-called rod-shaped corpuscles, 

 that is to say, closed cylinders, which are soft, flexible, easily torn, filled with 

 an oily material, and only loosely joined with one another. 



2. The proper nervous layer arises from the manner in which the optic 

 nerve sends out nerve tubules in bundles from its place of entrance in all 

 directions, and forms long meshed plexus. Whether the nerve fibres termi- 

 nate freely, or are reflected in loops (Valentin), is yet uncertain. Likewise, 

 whether a proper (ganglion) globular layer exists on the internal nervous 

 layer. 



3. This lies immediately within a dark granular layer, in which the vessels 

 pass, and which towards the vitreous body is bounded by a kind of large- 

 celled epithelium (vascular layer of the Retina). This last passes over the 

 processus ciliares, and extends over the zonula Zinnii as far as the capsule of 

 the lens (whilst the nervous and cylindrical layers do not reach the corp. cili- 

 are), and forms the so-called ciliary portion of the Retina (Henle). According 

 to Bidder, the layer of cylindrical cells does not reach to the zonula ciliaris. 

 His layer of ganglion globules (continuations of the gray substance) and the 

 stratum nerveum (continuation of the white substance) reaches as far as the 

 capsule of the lens. According to Langenbeck, jun., and Pappenheim, a 

 sinus passes on the internal surface in the anterior border of the retina, sinus 

 8. circulus venosus retinee. 



[Art. : See Angiology, Art. ophthal.] 



Function of the Retina : it presents a susceptible nervous surface 

 towards the light, the sensation of which is transmitted by the optic 

 nerve to the cerebrum. 



d. The transparent nucleus of the eye. 

 502. 1. Humor aqueus, the aqueous humour, 



a transparent fluid, as clear as water, of spec. grav. 1*005, con- 

 tains most water and only a little common salt and albumen, fills 



