CHAPTER III. 

 Tab. II.^ — the eye. 



Fig. 1. The crystalline lens of a fish; it is proportionably larger 

 than in other animals, and perfectly spherical. 



Fig. 2. A section of the human eye. It is formed of various coats, 

 or membranes, containing pellucid humours of different degrees of 

 density, and calculated for collecting the rays of light into a focus, up- 

 on the nerve situated at the bottom of the eye-ball. 



The external membrane, called sclerotic, is strong and firm, and is 

 the support of the spherical figure of the eye : it is deficient in the 

 centre, but that part is supplied by the cornea, which istranparent and 

 projects like the segment of a small globe from one of larger size. 

 The interior of the sclerotic is lined by the choroid, which is covered 

 by a dark mucous secretion, termed pigmentum nigrum, intended to 

 absorb the superfluous rays of light. The choroid is represented in 

 the plate by the black line. The third and inner membrane, which is 

 marked by the white line, is the retina, the expanded optic nerve. 



Within these coats of the eye, are the humours, a, the aqueous 

 humour, a thin fluid like water ; h, the crystalline lens, of a dense 

 texture ; c, the vitreous humour, a very delicate gelatinous substance, 

 named from its resemblance to melted glass. Thus the crystalline is 

 more dense than the vitreous, and the vitreous more dense than the 

 aqueous humour : they are all perfectly transparent, and together 

 make a compound lens, which refracts the rays of light issuing from 

 an object, d, and delineates its figure e, in the focus upon the retina, 

 inverted. 



Fig. 8. The lens of the telescope. 



Fig. 4. The crystalline lens, or, as it has been called, the crystalline 

 humour, of the eye. 



Fig. 5, 6. A plan of the circular and radiated fibres which the iris 

 is supposed to possess ; the former contracts, the latter dilates the pupil, 

 or aperture formed by the inner margin of the iris. *•■ 



Fig. 7. a, a, a, a, the four straight muscles, arising from the bot- 

 tom of the orbit, where they surround, c, the optic nerv^e ; and are 

 inserted by broad, thin tendons at the fore part of the globe of the eye 

 into the tunica sclerotica. 



