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 to set it at rest, by saying of the optic nerve that "arrivé à la choroide, il la perce par un trou rond, fermé d'une petite membrane cribleé d'une multitude de petis pores, au travers desquels la substance medullaire qui a traversé les longs canaux dont ce nerf est composé, semble s'écouler pour se mêler intimement et former cette expansion nerveuse qui double toute la concavité de la choroide, et que l'on nomme rétine."

Within the eye, near the uvea, is suspended in a transparent membranous capsule, a jelly-like substance $C$ in form of a double-convex lens of unequal radii, being less convex in front, than on the back; this is called the chrystalline lens. It is composed of a great number of laminæ, which are divisible into fibres, converging from the circumference towards the middle of the surface, and varying in hardness from the surface to the center, where the consistence is the greatest.

The space $D$ behind the chrystalline, which is the largest in the eye, is occupied by the vitreous humour, a viscous fluid contained in a cellular sponge-like substance enclosed in a very fine transparent membrane.

The cavity between the chrystalline and cornea, which is partly divided by the uvea, is filled with a fluid called the aqueous humour, which consists merely of water, with small quantities of albumer and salt, and is quite limpid, and devoid of smell. It is said by some authors to be lighter than distilled water, in the ratio of 975 to 1000.

136. The proportions of the spaces occupied by the three humours of the eye vary in different animals, as may be seen from the following Table, taken from M. Cuvier's Anatomie Comparée, which shows the parts of the axis lying in the several humours: