ORGANS OF SENSE. 169 



of tlie vitreous humor by a series of folds, called ciliary processes, arranged 

 in a circular form around the margin of the crystalline lens. The third, or 

 innermost tunic, is the retina, the delicate expansion of the optic nerve; 

 this extends from the entrance of the nerve, at the back of the eye, all 

 around the interior of the choroid membrane, and terminates in front about 

 two lines behind the anterior border of the latter; this is the essential part 

 of the organ, being endowed with sensibility to light ; all the other parts may 

 be considered as subordinate to it. The humors of the eye are the aqueous, 

 the crystalline lens, and the vitreous ; these, though of different densities, 

 and inclosed in membranous capsules, are all transparent, and exactly fill 

 the globe. The aqueous fluid occupies the space between the cornea and 

 the lens ; the iris floats and moves in this fluid, and divides the space into 

 two chambers, the anterior and posterior. The anterior is between the 

 cornea and iris, and the posterior between the iris and the lens and vitreous 

 humor ; this space is very small, and both communicate freely through the 

 pupil, or the circular aperture in the centre of the iris. All the posterior 

 region of the eye is filled by the vitreous humor, a soft, gelatinous, transpa- 

 rent mass, composed of the most delicate cellular membrane, the cells filled 

 with fluid. In a depression on the fore part of this substance, and behind 

 the pupil and iris, is placed the crystalline fluid or humor, of greater density 

 than either of the other two, and of the form of a double convex lens. We 

 shall next proceed to the examination of these parts of the eye, in greater 

 detail. 



1. Tunica Sclerotica. This, together with the cornea in front, forms 

 the external coat or case of the eye, and extends from the optic nerve to 

 the circumference of the cornea ; it is a dense, opake, fibrous membrane, 

 very strong and inelastic, preserving the figure of the organ and protecting 

 from injury and pressure the delicate structure within. Near the entrance 

 of the optic nerve it is pierced by numerous small foramina, for the short 

 ciliary nerves and vessels ; more anteriorly, by small oblique canals for the 

 long ciliary; and a little h-liind the transverse axis there are four or six 

 larger canals for the exit of veins. The internal surface of the sclerotic 

 is in contact with the choroid membi-ane, and is stained by the brown 

 pigment. When washed clean it presents a smooth glossy appearance, 

 owing partly to the presence of a very fine serous-like lamina. The thick- 

 ness of the sclerotic is greatest posteriorly, about the optic nerve ; thence it 

 thins out towards the centre. The optic nerve perforates the sclerotic 

 about a line and a half internal to the antero-posterior axis. As the nerve 

 approaches this point, it is suddenly constricted ; its fibrous sheath of dura 

 mater is intimately united to the sclerotic, and the contracted nerve passes 

 through a small aperture in the membrane which appears traversed by 

 fibres, and has received the name of lamina crihrosa. When the nerve 

 is squeezed, a central dark point will be observed in a section, called 

 the porus opticus, and showing the position of the cerebral artery of the 

 retina. 



2. The Cornea forms the anterior fifth of the eyeball, and completes the 

 external case, by joining on to the sclerotic ; it projects beyond the level 



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