891. The apparatus of vision consists of the _Op´tic Nerve_, the _Globe_ and _Muscles_ of the eye, and its _Protecting Organs_. 892. The OPTIC NERVE arises by two roots from the central portion of the base of the brain. The two nerves approach each other, as they proceed forward, and some of the fibres of each cross to the nerve of the opposite side. They then diverge, and enter the globe of the eyes at their back part, where they expand, and form a soft, whitish membrane. 893. The GLOBE, or ball of the eye, is an optical instrument of the most perfect construction. The sides of the globes are composed of _Coats_, or membranes. The interior of the globe is filled with refracting _Humors_, or _me´di-ums_. -=-=-=-=-=-=-=-=-=-=-=-= 890. Which sense contributes most to the enjoyment of man? What do we perceive by this sense? What is said of the mechanism of the eye? 891-916. _Give the anatomy of the organs of vision._ 891. Of what does the apparatus of vision consist? 892. Describe the optic nerve. 893. Describe the globe of the eye. -=-=-=-=-=-=-=-=-=-=-=-= 894. The COATS are three in number: 1st. The _Scle-rot´ic_ and _Corn´e-a_. 2d. The _Cho´roid_, _Iris_, and _Cil´ia-ry processes_. 3d. The _Ret´i-na_. 895 The HUMORS are also three in number: 1st. The _A´que-ous_, or watery. 2d. The _Crys´tal-line_, (lens.) 3d. The _Vit´re-ous_, or glassy. [Illustration: Fig. 137. The second pair of nerves. 1, 1, Globe of the eye: the one on the left is perfect, but that on the right has the sclerotic and choroid coats removed, to show the retina. 2, The crossing of the optic nerve. 5, The pons varolii. 6, The medulla oblongata. 7, 8, 9, 10, 11, 12, 13, The origin of several pairs of cranial nerves.] 896. The SCLEROTIC COAT is a dense, fibrous membrane and invests about four fifths of the globe of the eye. It gives form to this organ, and serves for the attachment of the muscles that move the eye in various directions. This coat, from the brilliancy of its whiteness, is known by the name of "the white of the eye." Anteriorly, the sclerotic coat presents a bevelled edge, which receives the cornea in the same way that a watch-glass is received by the groove in its case. -=-=-=-=-=-=-=-=-=-=-=-= 894. Name the coats of the eye. 895. Name the humors of the eye. Explain fig. 137. 896. Describe the sclerotic coat. -=-=-=-=-=-=-=-=-=-=-=-= 897. The CORNEA is the transparent projecting layer, that forms the anterior fifth of the globe of the eye. In form, it is circular, convexo-concave, and resembles a watch-glass. It is received by its edge, which is sharp and thin, within the bevelled border of the sclerotic, to which it is firmly attached. The cornea is composed of several different layers; its blood-vessels are so small that they exclude the red particles altogether, and admit nothing but serum.