Page:Optics.djvu/123

 The choroid is continued, so as to form, by a sort of doubling or fold, the ciliary processes, and is again continued in front of these.

This continuation is called the uvea, and is like a circular basin thinned away towards the center, where there is an aperture $B$.

This aperture is denominated the pupil. It is of various forms in different animals, and is capable of being contracted, or enlarged. In man it is always circular, but in animals of the feline kind, its vertical diameter is invariable, so that its figure varies from a circle to a straight line (Fig. 148.). In ruminating animals on the contrary, it is transversely oblong, and when contracted to the utmost becomes a horizontal straight line (Fig. 149.).

The change in the size or form of the pupil, is effected by certain muscles, which in general perform their office spontaneously, being of the kind called involuntary. The cat however is said to have a great command over this mechanism.

The iris is a coloured membrane coating the exterior surface of the uvea. It is of different hues in men, varying through many shades of blue, gray, brown, and green.

The interior surface of the choroid is covered with a dark mucus, in which is imbedded a fine net-work, called the retina. This proceeds from the optic nerve, which enters obliquely at the back of the eye through a tube (part of which is shown in the figure) which connects the eye with the brain, the coats of which, called the dura mater and pia mater, are by some writers said to be identical with the sclerotica and choroid. M. Cuvier says on this subject: "Le nerf optique parcourt la portion postérieure de la sclérotique par un canal d'un pouce et demi de longueur, dont les parois sont formées par la dure-mère; et il est très visible que les fibres blanches, qui font la base de la sclérotique, se détachent successivement de la face extèrne de la dure-mère, dont elles semblent être un épanouissement. Cela pourroit décider, en faveur des anciens, la question de savoir si la sclérotique est ou non une continuation de la dure-mère; question assez difficile à résoudre dans les autres animaux où ces deux membranes ne se touchent que dans une espace très mince." As to the other point, he seems