Experiment 210. To illustrate the blind spot. Marriott’s experiment. On a white card make a cross and a large dot, either black or colored. Hold the card vertically about ten inches from the right eye, the left being closed. Look steadily at the cross with the right eye, when both the cross and the circle will be seen. Gradually approach the card toward the eye, keeping the axis of vision fixed on the cross. At a certain distance the circle will disappear, i.e., when its image falls on the entrance of the optic nerve. On bringing the card nearer, the circle reappears, the cross, of course, being visible all the time (see Experiment 180, p. 355).
Experiment 211. To map out the field of vision. A crude method is to place the person with his back to a window, ask him to close one eye, stand in front of him about two feet distant, hold up the forefingers of both hands in front of and in the plane of your own face. Ask the person to look steadily at your nose, and as he does so observe to what extent the fingers can be separated horizontally, vertically, and in oblique directions before they disappear from his field of vision.
Experiment 212. To illustrate imperfect judgment of distance. Close one eye and hold the left forefinger vertically in front of the other eye, at arm’s length, and try to strike it with the right forefinger.
On the first trial one will probably fall short of the mark, and fail to touch it. Close one eye, and rapidly try to dip a pen into an inkstand, or put a finger into the mouth of a bottle placed at a convenient distance. In both cases one will not succeed at first.
In these cases one loses the impressions produced by the convergence of the optic axes, which are important factors in judging of distance.
Experiment 213. Hold a pencil vertically about twelve inches from the nose, fix it with both eyes, close the left eye, and then hold the right index finger vertically, so as to cover the lower part of the pencil. With a sudden move, try to strike the pencil with the finger. In every case one misses the pencil and sweeps to the right of it.
Experiment 214. To illustrate imperfect judgment of direction. As the retina is spherical, a line beyond a certain length when looked at always shows an appreciable curvature.
Hold a straight edge just below the level of the eyes. Its upper margin shows a slight concavity.
Surface Anatomy and Landmarks.
In all of our leading medical colleges the students are carefully and thoroughly drilled on a study of certain persons selected as models. The object is to master by observation and manipulation the details of what is known as surface anatomy and landmarks. Now while detailed work of this kind is not necessary in secondary schools, yet a limited amount of study along these lines is deeply interesting and profitable. The habit of looking at the living body with anatomical eyes and with eyes at our fingers’ ends, during the course in physiology, cannot be too highly estimated.
In elementary work it is only fair to state that many points of surface anatomy and many of the landmarks cannot always be defined or located with precision. A great deal in this direction can, however, be done in higher schools with ingenuity, patience, and a due regard for the feelings of all concerned. Students should be taught to examine their own bodies for this purpose. Two friends may thus work together, each serving as a “model” to the other.
To the following syllabus may be added such other similar exercises as ingenuity may suggest or time permit.