ed by the cornea being very small, and that behind bounded by the retina being very large. This large posterior chamber is distended by a spherical transparent mass, called the vitreous 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.
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, whi eh also extends 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 adopted from not making allowance for the projection of the cornea, which is a portion of a smaller sphere than the globe itself, and con sequently projects beyond its circumference. From the flaccid state of the eye even shortly after death, it must be very difficult to measure it accurately. The question is, however, for tunately of little practical importance. The eyeball of the male is generally a little larger than that of the female ; and if a close inquiry be made into the matter, much difference in this respect might probably be detected in different individuals. I have seen the eyeball in an adult of full size not larger than that of a child of five years old ; and there is much apparent difference in consequence of the difference in the depth of the orbit, and in the gape of the eyelids. Although the human eyeball is nearly a perfect sphere, that precise form is obviously not an essential requisite in the construction of a perfect organ of vision. In all the vertebral animals the bottom of the eye, where the retina is expanded, is probably a portion of a correct sphere, but in many the anterior part is com pressed, or in other words the sphere is trun cated, to adapt it to the form and dimensions of the head, or to bring the cornea and lens nearer to the retina. In the mysticete whale the axis is to the diameter as 20 to 29; in the swan as 7 to 10; in the turtle as about 8 to 10; and in the cod as 14 to 17. This deviation from the spherical form demands a corresponding provi sion in the construction of the sclerotic, to be noticed when describing that membrane. For a fuller account of the comparative proportional measurements of the eye, the student is referred to the works of Cuvier .and D. W. Sommer ring, as quoted at the end of this article ; the limits of which do not admit of a greater detail of facts derived from comparative anatomy than the illustration of the description of the human organ absolutely demands. - Having attempted to give a general notion of the mechanism of the eye in the preceding paragraphs, it remains to consider each com ponent part separately, and to determine its organization, properties, and application, as well as the changes to which it is liable from age, disease, or other circumstances.