702 



DISSECTION OF THE EYE. 



and before ; 



ending in 

 front ; 



outer and 

 inner sur- 

 faces ; 



thickness ; 



circular 

 sjnus ; 



composed 

 of fibrous 

 tissue ; 

 vessels 

 and nerves. 



Cornea : 



extent and 

 size; 



form ; 



thickness ; 



surfaces ; 



curve ; 



condition 

 after death. 



It consists 

 of laminar 

 fibrous 

 tissue, with 

 conjunctiva 

 in front, 



and an 

 elastic 

 membrane 

 behind ; 



of an inch), the optic nerve (I) is transmitted through an aperture 

 in it ; this opening decreases in size from without inwards, and is 

 cribriform when the nerve is drawn out, the lattice-like condition 

 being due to the bundles of fibrous tissue between the funiculi of 

 the nerve. Small apertures for the passage of vessels and nerves 

 are situate around the optic nerve ; and there are others for vessels 

 at the front and the centre of the ball. Anteriorly the sclerotic is 

 continuous with the transparent cornea. 



On the outer surface this coat is smooth, except where the 

 muscles are attached : on the inner aspect it is of a dark colour, 

 with flocculi of fine areolar tissue (membrana f usca) uniting it to 

 the next coat, and with the ends of ruptured vessels and nerves. 



The sclerotic covering is thickest at the back of the eyeball, but 

 it becomes thinner and whiter about a quarter of an inch from the 

 cornea, where it is visible as the " white of the eye." Towards 

 the junction with the cornea it is again somewhat thickened. 

 In its substance, near the union with the cornea, is a small 

 flattened venous space, the canal of Schlemm (sinus circularis iridis ; 

 fig. 288, r). 



Structure. The sclerotic consists of bundles of fibrous tissue, 

 which interlace with one another, but run for the most part longi- 

 tudinally and transversely. Its vessels are very scanty. Minute 

 filaments of the ciliary nerves have been described, entering the 

 deep surface of the membrane. 



CORNEA (fig. 288, d). This firm transparent membrane forms 

 about one-sixth of the eye-ball, and measures about half an inch 

 across. Its shape is circular, though when viewed from the front 

 it appears larger in the transverse direction, in consequence of the 

 opaque sclerotic structure encroaching farther on it above and below 

 than on the sides. 



It is convex in front and concave behind ; and its thickness is 

 nearly uniform (from ^V^h ^ "sV^h ^ an. inch), except near the 

 circumference, where it is somewhat thicker at the junction with 

 the sclerotic. The anterior surface of the cornea is slightly less 

 extensive than the posterior, owing to its being overlapped by the 

 sclerotic. Supported by the aqueous humour, it is tense and 

 nearly spherical during life ; but its radius of curvature varies in 

 different individuals, and in the same person at different ages, 

 being shorter in the young. After death it becomes flaccid from 

 the trarisudation of the aqueous humour ; or if the eye is immersed 

 in water, it is rendered opaque by infiltration of the tissues by that 

 fluid. 



Structure. The substance of the cornea is composed of a special 

 kind of connective tissue, arranged in irregular layers. Over the 

 front the conjunctiva (which is here reduced to its epithelium) is 

 continued ; and covering the back of the cornea proper is a very 

 thin elastic stratum known as the membrane of Descemet. The 

 latter may be peeled off, after a cut has been made through it, in 

 shreds which curl up with the attached surface innermost. At the 

 circumference of the cornea the membrane of Descemet breaks up 



