THE EYE AND ITS APPENDAGES. 35! 



lies a flattened annular channel, the canal of Schlemm ; the 

 inner wall of this canal is formed by intersecting delicate lamellae 

 whose loose disposition suggests an incomplete isolation of the 

 channel from the adjacent spaces of Fontana. The nature of the 

 canal of Schlemm, whether a venous or a lymphatic channel, has long 

 been a subject of discussion ; the weight of evidence warrants re- 

 garding it as a venous canal, between which and the lymph-clefts 

 represented by the spaces of Fontana free communication un- 

 doubtedly exists. 



THE RETINA. 



The inner nervous tunic of the eyeball includes the retina alone, 

 which extends from the optic entrance throughout the posterior seg- 

 ment of the ball and as far forward as the pupillary margin of the 

 iris. This extensive tract, corresponding in its morphological limits 

 to the secondary optic vesicle, falls into three divisions : (i) the 

 pars optica retinae, including the entire posterior segment and end- 

 ing at the ora serrata; (2) the pars ciliaris retinae, covering the 

 posterior surface of the ciliary zone and processes and extending from 

 the ora serrata to the base of the iris ; and (3) the pars iridica re- 

 tinae, passing over the posterior surface of the iris from the base to 

 the anterior edge of the pupil, where it terminates as a slightly-thick- 

 ened margin, which corresponds to the free lip of the double- layered 

 optic cup. 



The retina proper, or pars optica retinae, consists of an inner 

 and an outer lamina, which correspond to the inner and outer layers 

 of the optic vesicle ; the outer lamina includes the pigment-layer 

 alone, while the inner lamina embraces the remaining layers of the 

 retina. The inner lamina permits further subdivision of its structures 

 into the neuro-epithelial and the cerebral layer. The relations 

 of these divisions to the individual retinal layers may be expressed : 



/. Outer layer of optic ) Pigment . layer . A , Pigment-layer, 



vesicle. 



Layer of rods and cones ; \ B Neuro . epithelial 



Limiting membrane ; layer 



Outer nuclear layer ; 



Outer reticular layer ; ^ 



Inner nuclear layer ; 



Inner reticular layer ; J. c. Cerebral layer. 



Ganglion-cell-layer ; 



Nerve-fibre-layer. 



The retinal structures consist of two parts, the nervous ele- 

 ments and the supporting neuroglia. The supporting tissues 

 contribute a considerable part of the entire retina, but differ in their 

 amount in the several layers. The most conspicuous constituents 

 of the supporting framework are long neuroglia-fibres, the radial 



// Inner layer of optic 

 vesicle. 



