ANATOMY OF THE EYE 



107 



give 



cohesion and 

 different layers. 



on one or both edges of which are a few small black bodies called corpora 

 nigra (fig. 242, i>). The pupil is the opening through which light enters 

 the eye, and by which images of objects in the outer world are permitted 

 to be formed on the retina within. The iris, except in albinos, is perfectly 

 opaque, and acts as a diaphragm, regulating the amount of light that enters 

 the eye. In dim lights the pupil dilates (fig. 243), in bright lights it con- 

 tracts (fig. 244). 



The Retina. This, which is the 

 most internal of the tunics of the 

 eye, is a sheet of nerve tissues situated 

 between the choroid coat and the 

 vitreous humour, specially organized 

 to receive and transmit impressions 

 of light. Its structure (fig. 245) is 

 very complex, but it consists essen- 

 tially of an outer layer of rods and 

 cones, followed by several layers of 

 cells and nuclei, with interwoven 

 fibres, which 

 strength to the 



The fibres of the optic nerve ter- 

 minate in the retina, and conduct 

 the impressions made upon the re- 

 tina through the optic nerve to the 

 brain. The accompanying woodcut 

 will sufficiently demonstrate the 

 structure of the retina as now re- 

 ceived by the best observers. 



Fig. 245. Microscopic Section of the Ketma, 



The HumOUrS Of the Eye. Choroid, and Part of the Sclerotic 



The acpaeous humour (fig. 240, K) 

 is a limpid fluid which occupies the 

 space between the cornea and the 

 lens. The quantity is estimated at 

 about 1 fluid drachm. It appears 

 to be secreted by the ciliary pro- 

 cesses, and undergoes constant renewal. That which is freshly secreted 

 is poured forth into the posterior chamber of the eye, and therefore 

 occupies the space between the back of the iris, in front, and the tips 

 of the ciliary processes, the suspensory ligament of the lens, and the lens 

 itself, behind. The fluid passes through the aperture of the pupil into 

 the anterior chamber of the eye, and escapes by a natural channel situated 



1, Sclerotic. 2, External Vascular Portion of the 

 Choroid. 3, Internal Vascular Portion of the Choroid. 

 4, Pigment Cell Layer. 5, Layer of Rods and Cones. 

 (>, Membrana Limitans Externa. 7, External Nuclear 

 Layer. 8, External Molecular Layer. 9, Internal Nu- 

 clear Layer. 10, Internal Molecular Layer. 11, Gan- 

 glion Layer. 12, Nerve-fibre Layer traversed by Muller's 

 Sustentacular Fibres. 13, Membrana Limitans Interna. 



