The Inner Coat, or Retina.—This is a delicate membrane containing the expanded termination of the optic nerve. It rests upon the choroid coat and spreads over about two thirds of the back surface of the eyeball. Although not more than one fiftieth of an inch in thickness, it presents a very complex structure, essentially nervous, and is[pg 377] made up of several distinct layers. Of chief importance in the outer layer are the cells which are acted upon directly by the light and are named, from their shape, the rods and cones. In contact with these, but occupying a separate layer, are the ends of small afferent nerve cells. These in turn communicate with nerve cells in a third layer, known as the ganglion cells, that send their fibers into the optic nerve (Fig. 160).
In the center of the retina is a slight oval depression having a faint yellowish color, and called, on that account, the yellow spot. This is the part of the retina which is most sensitive to light. Directly over the place of entrance of the optic nerve is a small area from which the rods and cones are absent and which, therefore, is not sensitive to light. This is called the blind spot. (See Practical Work.)
The Crystalline Lens.—Immediately back of the iris and touching it is a transparent, rounded body, called the crystalline lens. This is about one fourth of an inch thick and one third of an inch through its long diameter, and is more curved on the back than on the front surface. It is inclosed in a thin sheath, called the membranous capsule, which connects with a divided sheath from the sides of the eyeball, called the suspensory ligament (Fig. 159). Both the lens and the capsule are highly elastic.
Chambers and "Humors" of the Eyeball.—The crystalline lens together with the suspensory ligament and the ciliary processes form a partition across the eyeball. This divides the eye space into two separate compartments, which are filled with the so-called "humors" of the eye. The front cavity of the eyeball, which is again divided in part by the iris, is filled with the aqueous humor. This is a clear, lymph-like liquid which contains an occasional[pg 378] white corpuscle. It has a feeble motion and is slowly added to and withdrawn from the eye. It is supplied mainly by the blood vessels in the ciliary processes and finds a place of exit through a small lymph duct at the edge of the cornea (Fig. 159).
The back portion of the eyeball is filled with a soft, transparent, jelly-like substance, called the vitreous humor. It is in contact with the surface of the retina at the back and with the attachments of the lens in front, being surrounded by a thin covering of its own, called the hyaloid membrane. The aqueous and vitreous humors aid in keeping the eyeball in shape and also in focusing.
How we see Objects.—To see an object at least four things must happen:
1. Light must pass from the object into the eye. Objects cannot be seen where there is no light or where, for some reason, it is kept from entering the eye.
2. The light from the object must be focused (made to form an image) on the retina. In forming the image, an area of the retina is stimulated which corresponds to the form of the object.
3. Impulses must pass from the retina to the brain, stimulating it to produce the sensations.
4. The sensations must be so interpreted by the mind as to give an impression of the object.