The Eye : Part 2

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 23 of 28)

The Eyeball, or globe of the eye, is a device for focusing light upon a sensitized nervous surface which it encloses 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.

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 form of the eyeball and protects the portions within. It is pierced at the back by a small opening which admits the optic nerve, and in front it becomes changed into the peculiar tissue that makes up the cornea.

The cornea forms the transparent covering over the lesser spherical segment of the eyeball, shading into the sclerotic coat at its edges. It has a complex structure, consisting in the main of a transparent form of connective tissue. It serves the purpose of admitting light into the eyeball.

The Middle Coat consists of three connected portions - the choroids coat, the ciliate processes, and the iris. These surround the larger spherical segment. All three parts are rich in blood vessels, containing the blood supply to the greater portion of the eyeball.

The choroids coat lies immediately beneath the sclerotic coat at all places except a small margin toward the front of the eyeball. It is composed chiefly of blood vessels and a delicate form of connective tissue that holds them in place. It contains numerous pigment cells which give it a dark appearance and serve to absorb surplus light. Near where the sclerotic coat joins the cornea, the choroids coat separates from the outer wall and, by folding, forms many slight projections into the interior space. These are known as the ciliate processes. The effect of these folds is to collect a large number of capillaries into a small space and to give this part of the eyeball an extra supply of blood. Between the ciliate processes and the sclerotic coat is a small muscle, containing both circular and longitudinal fibers, called the ciliate muscle.

The iris is a continuation of the choroids coat across the front of the eyeball. It forms a dividing curtain between the two spherical segments and gives the color to the eye. At its center is a circular opening, called the pupil, which admits light to the back of the eyeball. By varying the size of the pupil, the iris is able to regulate the amount of light which passes through and it employs for this purpose two sets of muscular fibers. One set of fibers forms a thin band which encircles the pupil and serves as a sphincter to diminish the opening. Opposing this are radiating fibers which are attached between the inner and outer margins of the iris. By their contraction the size of the opening is increased. Both sets of fibers act reflexively and are stimulated by variations in the light falling upon the retina.

The Inner Coat, or Retina. - This is a delicate membrane containing the expanded termination of the optic nerve. It rests upon the choroids 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.

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.

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 enclosed in a thin sheath, called the membranous capsule, which connects with a divided sheath from the sides of the eyeball, called the suspensor ligament. Both the lens and the capsule are highly elastic.

Chambers and "Humors" of the Eyeball. - The crystalline lens together with the suspensor ligament and the ciliate 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 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 ciliate processes and finds a place of exit through a small lymph duct at the edge of the cornea.

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 hyaloids membrane. The aqueous and vitreous humors aid in keeping the eyeball in shape and also in focusing.

D.C. Heath and Co. - Publishers
Original copyright 1909