tina.

[Fig. 160]

Fig. 160--*Diagram showing main nervous elements in the retina.* Light
waves stimulate the rods and cones at back surface of the retina, starting
impulses which excite the ganglion cells at the front surface. Fibers from
the ganglion cells pass into the optic nerve.

*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 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, ly