Subject: Physiology • Group: Science • Class III • Time: 35 minutes
By J. Vera Good
The Parents’ Review, 1909, pp. 710-711
I. To complete and summarize children’s knowledge of the physiology of the eye.
II. To interest them in its structure, so wonderfully adapted to the conditions of its use.
III. To make the lesson clear by the use of diagrams and experiments.
Step I.—Ask the children to narrate all they have learned about the eye.
Step II.—Supplement their knowledge with the following:—
I. Protection and Movements of the Eyeball.
The eyeball is protected by,—
(a) Walls of orbit.
(b) Two lids, which are simply folds of skin.
(c) The conjunctiva, a membrane liningeyelids and covering the eye.
(d) The eyelashes which give warning of anything approaching the eye; the nerves at the base of the eyelashes, by a reflex action causing the eyelids to close.
(e) Tear gland, secreting tears which wash the eye.
The movements of the eyeball are controlled by six muscles. (Diagram, Foster & Shaw.) The fat padded orbit, allows eyeball to turn without friction.
II. Structure of Eyeball.
The eyeball is a hollow globe divided into two chambers by the lens. Small anterior chamber is filled by a thin watery fluid, large posterior by a gelatinous substance.
Eyeball has three coats:—
(1) Sclerotic, the strong fibrous outer coat, of which the cornea, the window before the lens, is a part.
(2) Chloroid, thickly supplied with bloodvessels; its inner surface is black. It absorbs the rays of light which pass through the retina, lying in front of it and so prevent a reflexion from back of eye. (Illustrate this by showing pupils black inside of a camera.) Part of this coat forms iris.
(3) Retina, a thin nerve film on which objects are focussed, made by spreading out of optic nerve. It consists of a dozen different layers. (Diagram, Schofield). The rod and cone layer is the most sensitive, as shown by the yellow spot, which it alone lines (Diagram from Hill’s Physiology). Where optic nerve enters retina there is a blind spot. Prove this by illustration in Schofield.
The iris is a curtain hung before the cornea, lined with a layer of cells containing black pigment which gives it its colour. Its function is to confine rays of light to middle of lens to prevent blurred images. Illustrate by showing diaphragm of camera and prove by experiment with candle.
Accommodation is the power which the eye possesses of altering the power of the lens. Remind children of manner of focusing in camera; the same mode is impossible with the eye, because both the retina and the lens are fixed, so the power of the lens must be changed. (Experiment with candle and lenses.) This is done by the muscle round the lens, which contracts it, thus making it more convex.
Step III.—Ask the children to read from Hill’s Physiology passage relating to how the brain learns to use the eye.
Step IV.—Let the girls narrate what they know about the eye.
Let them make Diagrams by which the main points of the structure of eyeball are illustrated.