ble tissues and
turned in different directions by a special set of muscles. A cup-shaped
receptacle is provided within the orbit, by layers of fat, and a smooth
surface is supplied by a double membrane that lies between the fat and the
eyeball. In front the eyeballs are provided with movable coverings, called
the _eyelids_. These are composed of dense layers of connective tissue,
covered on the outside by the skin and lined within by a sensitive
membrane, called the _conjunctiva_. At the base of the lids the
conjunctiva passes to the eyeball and forms a firmly attached covering
over its front surface. This membrane prevents the passage of foreign
materials back of the eyeball, and by its sensitiveness stimulates effort
for the removal of irritating substances from beneath the lids. The
eyelashes and the eyebrows are also a means of protecting the eyeballs.

*The Eyeball*, or globe of the eye, is a device for _focusing_ light upon
a sensitized nervous surface which it incloses and protects. In shape it
is nearly spherical, being about an inch in diameter from right to left
and nine tenths of an inch both in its vertical diameter and from front to
back. It has the appearance of having been formed by the union of two
spherical segments of different size. The smaller segment, which forms
about one sixth of the whole, is set upon the larger and forms the
projecting transparent portion in front. The walls of the eyeballs are
made up of three separate layers, or coats--an _outer coat_, a _middle
coat_, and an _inner coat_ (Fig. 159).

[Fig. 159]

Fig. 159--*Diagram of the eyeball in position.* 1. Yellow spot. 2. Blind
spot. 3. Retina. 4. Choroid coat. 5. Sclerotic coat. 6. Crystalline lens.
7. Suspensory ligament. 8. Ciliary processes and ciliary muscle. 9. Iris
containing the pupil. 10. Cornea. 11. Lymph duct. 12. Conjunctiva. 13.
Inferior and superior recti muscles. 14. Optic nerve. 15. Elevator muscle
of eyelid. 16. Bone. _A._ Posterior chamber containing the vitreous humor.
_B._ Anterior chamber containing the aqueous humor.

*The Outer Coat* surrounds the entire globe of the eye and consists of two
parts--the sclerotic coat and the cornea. The _sclerotic coat_ covers the
greater portion of the larger spherical segment and is recognized in front
as "the white of the eye." It is composed mainly of fibrous connective
tissue and is dense, opaque, and tough. It preserves the