EYE. 



173 



iiished with a circle of prominent folds called 

 ciliary processes, by means of which it is united 

 to the corresponding surface of the hyaloid 

 membrane of the vitreous humour. The an- 

 nexed figure represents a section of this hollow 

 sphere lodged within the sclerotic sphere. The 

 external circle, a a, between the two black 

 lines represents a section of the strong opaque 

 membrane called the sclerotic, which coasti- 

 tutes the case or resisting sides of the organ ; 

 4 is the transparent lenticular window called 

 cornea, which tills the aperture left in the ante- 

 rior part of the sclerotic for its reception ; dd is 

 the place of union between the sclerotic and 

 cornea, to which the ciliary ligament on the 

 outside of the anterior margin of the choroid 

 sphere corresponds ; e e the circle bounded 

 by the line marking the inner surface of the 

 sclerotic externally, and by the shaded part in- 

 ternally, represents a section of the hollow 

 sphere called choroid. At the point d d, cor- 

 responding to the place of union between the 

 sclerotic and cornea, this choroid projects exter- 

 nally, encroaching upon the sclerotic in a pecu- 

 liar manner, to be presently described as the 

 ciliary ligament; while at the same point it 

 projects internally in the shape of a series of 

 folds, to be described as the ciliary processes. 

 The white productions extending from the same 

 points in a vertical direction into the chamber 

 of the aqueous humour, between the cornea and 

 crystalline lens, represent a section of the 

 screen called the iris. / is a section of the 

 crystalline lens. 



Fig. 102. 



ed by the cornea being very small, and that 

 Ixihind bounded by the retina being very large. 

 Tins large posterior chamber is distended by a 

 spherical transparent mass, called the vitrn'>us 

 humour, which does not, however, fill this pos- 

 terior chamber completely, but is discontinued 

 or compressed at a short distance behind the 

 iris, leaving a narrow space between it and that 

 membrane, called the posterior chamber of the 

 aqueous humour. This spherical mass is of ex- 

 tremely soft consistence, and is composed of a 

 delicate transparent cellular membrane called 

 the hyaloid membrane, the cells of which are 

 distended with a transparent fluid. In the 

 small space between the anterior part of the 

 vitreous humour and the back of the iris, called 

 the posterior chamber of the aqueous humour, 

 and lodged in a depression formed for its re- 

 ception in the vitreous humour, is placed the 

 double convex lens called the crystalline lens. 

 The relation of these parts to each other may 

 be seen in the last figure, and the one below 

 represents the optic nerve expanded in the 

 form of a spherical membrane over the sphere 

 of vitreous humour, with the crystalline lens 

 lodged in a depression on the anterior part of 

 that sphere, and surrounded by a circle of 

 radiating lines, which are delicate folds corres- 

 ponding to the folds of the choroid, called the 

 ciliary processes. 



Fig. 103. 



Through a small aperture in the sclerotic 

 and choroid membranes in the bottom of the 

 eye, the optic nerve is transmitted, and imme- 

 diately expands into a texture of the most ex- 

 quisite delicacy, called the retina. This con- 

 stitutes a third spherically disposed membrane, 

 not however of the same extent as the sclerotic 

 or choroid, being discontinued at a distance of 

 about an eighth of an inch from the anterior 

 margins of these membranes. This is the ner- 

 vous expansion endowed with the peculiar 

 description of sensibility which renders the ani- 

 mal conscious of the presence of light. The 

 globe of the eye, as above described, is ob- 

 viously divided by the iris into two chambers of 

 very unequal dimensions ; that in front bound- 



The piece of animal optical mechanism thus 

 constructed is lodged in an open cavity of the 

 skull called the orbit, and is furnished with six 

 small muscles for its motions inserted into the 

 outside of the sclerotic coat. The transparent 

 cornea through which the light is transmitted is 

 necessarily exposed, and not being in its nature 

 suited to such exposure, is covered with a 

 membrane called conjunctiva, which alsoextends 

 over the sclerotic, where that membrane con- 

 stitutes the anterior part of the globe, and then 

 being reflected, lines the eyelids, and finally be- 

 comes continuous with the skin of the face. 



The human eye is, as has been stated above, 

 probably a sphere of about one inch in diameter. 

 Petit, however, who appears to have first made 

 the attempt to determine the proportions of the 

 organ accurately, describes the axis to be to the 

 diameter as 135 to 136, and the younger Som- 

 merring, apparently from his own observations, 

 as 10 to 9.5. This belief in a slight differ- 

 ence in dimension may, however, have been 



