The Organs of Sight.
THE organs of vision are the optic nerve, the globe of the eye, the
muscles of the eye, and the organs of protection. The Optic Nerve begins
by two roots at the base of the brain, the fibers from which meet, as
they come forward, and some of them cross
each other. The two nerves then separate, and enter the back part of the
globe of the eyes, and then spread out into a kind of membrane. In Fig.
53: 1, 1, show the globe of the eye ; 2, the crossing of the optic
nerve; 8, the origin of two pairs of cranial nerves. The Globe of the
Eye is a better constructed optical instrument than man ever made. Its
interior is filled with what are called refracting humors or mediums,
which are surrounded and
held in their place by membranes, called coats. The Coats are the
sclerotic and cornea ; the choroid, iris, and ciliary processes ; and
the retina. The Sclerotic Coat is a fibrous membrane, covering the
largest portion of the globe. To this the muscles are attached. It is
the part which is called the white of the eye. It has a beveled edge in
front, into which the cornea is fitted. The Cornea is a transparent
layer which projects in front, and forms about onefifth of the globe. It
is shaped like a watchglass. Its bloodvessels are too small to receive
the red particles of blood. The Choroid Coat is a vascular membrane. Its
color is brown externally, and black within. It is connected with the
sclerotic coat externally, and internally with the retina. It is
composed of three layers. The Iris is named from its having a variety of
colors in different persons. It is the partition between the anterior
and posterior chambers of the eye, and has a circular opening in the
centre called the pupil. Of its two layers, the fibers of the anterior
one are radiating, and dilate the pupil, while those of the other are
circular, and cause its contraction.
The Ciliary Processes are a number of folds formed from the internal layer of the choroid coat.
The Retina has three layers. The external is extremely thin; the middle
is nervous, being an expansion of the optic nerve; the internal is
vascular, and consists of a ramification of minute blood vessels.
The divided edge of their coats may be seen in Fig. 54, namely, the
sclerotic, the choroid, and the retina: 2, is the pupil; 3, the iris; 4,
the ciliary process; 5, the scofloped border of the retina.
The Humors of the Eye are the aqueous, the crystalline, and the
The Aqueous or watery humor is situated in the chambers of the eye. It is an albuminous fluid, with an alkaline reaction, and a specific gravity a little greater than distilled water. The Crystalline Humor is immediately behind the pupil. It is a lens, and is convex both on the posterior and the anterior surface. The Vitreous Humor is also an albuminous fluid something like the aqueous humor, but more dense. In Fig. 65 we have in E a good view of the cornea fitted into the sclerotic coat; A, is the choroid; B, the pigmentum nigrum, C, the retina; K, the vitreous humor; D, the optic nerve; 1, the lens; C, the Iris, painted on the back side with pigment; F, the aqueous humor. The muscles of the eye, six in number, are attached to the bones of the orbit behind, and to the cornea in front, by their tendons. These tendons give the eye its pearly appearance. In Fig. 66, five of the muscles are indicated by a, 6, c, d, e; f, is the optic nerve. If the internal muscle be too short, the eye is drawn in towards the nose, and the squinting called 11 crosseye " is produced. The Orbits are bony sockets which enclose the eye. The optic nerve passes through a large hole at the bottom. The Eyebrows are the projecting arches above, covered with short hair. They prevent the sweat from running down into the eyes, and also shade them from strong light. The Eyelids are the curtains which rise and fall in front. The smooth membrane which lines them is called the conjunctiva. It secretes a fluid which makes the eyelids open and shut easily.
The Lachrymal Gland is at the upper and outer angle of the orbit.
Several small ducts open from it upon the upper eyelid, through which
the tears run down upon the conjunctiva.
The Lachrymal Canals begin near the internal angle of the eye, by two smalltear points, which communicate with the sac at the upper part of the nasal duct.
The Nasal Duct is a canal about threequarters of an inch long, which
runs down to the inferior channel of the nose. Fig. 5 7 shows these
organ: 1, being the lachrymal gland; 2, the ducts leading to the upper
eyelid; 3, 3, the tearpoints (puncta lachr~ymalis); 4, the nasal sac; 5,
the termination of the nasal duct.
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copyright 2005, J. Crow Company, New Ipswich NH 03071