A Practical Physiology: A Text-Book for Higher Schools · Chapter 17Additional Experiments
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Experiment 162. Use a small pair of wooden compasses, or an ordinary pair of dividers with their points guarded by a small piece of cork. Apply the points of the compasses lightly and simultaneously to different parts of the body, and ascertain at what distance apart the points are felt as two. The following is the order of sensibility: tip of tongue, tip of the middle finger, palm, forehead, and back of hand.
Experiment 163. Test as in preceding experiment the skin of the arm, beginning at the shoulder and passing downwards. Observe that the sensibility is greater as one tests towards the fingers, and also in the transverse than in the long axis of the limb. In all cases compare the results obtained on both sides of the body.
Experiment 164. By means of a spray-producer, spray the back of the hand with ether, and observe how the sensibility is abolished.
Experiment 165. Touch your forehead with your forefinger; the finger appears to feel the contact, but on rubbing the forefinger rapidly over the forehead, it is the latter which is interpreted as “feeling” the finger.
Experiment 166. Generally speaking, the sensation of touch is referred to the cutaneous surfaces. In certain cases, however, it is referred even beyond this. Holding firmly in one hand a cane or a pencil, touch an object therewith; the sensation is referred to the extremity of the cane or pencil. If, however, the cane or pencil be held loosely in one’s hand, one experiences two sensations: one corresponding to the object touched, and the other due to the contact of the rod with the skin. The process of mastication affords a good example of the reference of sensations to and beyond the periphery of the body.
Experiment 167. Prepare a strong solution of sulphate of quinine with the aid of a little sulphuric acid to dissolve it (bitter), a five-per-cent solution of sugar (sweet), a ten-per-cent solution of common salt (saline), and a one-per-cent solution of acetic acid (acid). Wipe the tongue dry, and lay on its tip a crystal of sugar. It is not tasted until it is dissolved.
Experiment 168. Apply a crystal of sugar to the tip, and another to the back of the tongue. The sweet taste is more pronounced at the tip.
Experiment 169. Repeat the process with sulphate of quinine in solution. It is scarcely tasted on the tip, but is tasted immediately on the back part of the tongue. Test where salines and acids are tasted most acutely.
Experiment 170. To illustrate the muscular sense. Take two equal iron or lead weights; heat one and leave the other cold. The cold weight will feel the heavier.
Experiment 171. Place a thin disk of cold lead, the size of a silver dollar, on the forehead of a person whose eyes are closed; remove the disk, and on the same spot place two warm disks of equal size. The person will judge the latter to be about the same weight, or lighter, than the single cold disk.
Experiment 172. Compare two similar wooden disks, and let the diameter of one be slightly greater than that of the other. Heat the smaller one to over 120° F., and it will be judged heavier than the larger cold one.
Experiment 173. To illustrate the influence of excitation of one sense organ on the other sense organs. Small colored patches the shape and color of which are not distinctly visible may become so when a tuning-fork is kept vibrating near the ears. In other individuals the visual impressions are diminished by the same process. On listening to the ticking of a watch, the ticking sounds feebler or louder on looking at a source of light through glasses of different colors. If the finger be placed in cold or warm water the temperature appears to rise when a red glass is held in front of the eyes.
Experiment 174. Formation of an inverted image on the retina. Take a freshly removed ox-eye; dissect the sclerotic from that part of its posterior segment near the optic nerve. Roll up a piece of blackened paper in the form of a tube, black surface innermost, and place the eye in it with the cornea directed forward. Look at an object—e.g., a candle-flame—and observe the inverted image of the flame shining through the retina and choroid, and notice how the image moves when the candle is moved.
Experiment 175. Focus a candle-flame or other object on the ground-glass plate of an ordinary photographic camera, and observe the small inverted image.
Experiment 176. To illustrate spherical aberration. Make a pin-hole in a blackened piece of cardboard; look at a light placed at a greater distance than the normal distance of accommodation. One will see a radiate figure with four to eight radii. The figures obtained from opposite eyes will probably differ in shape.
Experiment 177. Hold a thin wooden rod or pencil about a foot from the eyes and look at a distant object. Note that the object appears double. Close the right eye; the left image disappears, and vice versa.
Experiment 178. To show the movements of the iris. It is an extremely beautiful experiment, and one that can easily be made. Look through a pin-hole in a card at a uniform white surface as the white shade of an ordinary reading-lamp. With the right eye look through the pin-hole, the left eye being closed. Note the size of the (slightly dull) circular visual field. Open the left eye, the field becomes brighter and smaller (contraction of pupil); close the left eye, after an appreciable time, the field (now slightly dull) is seen gradually to expand. One can thus see and observe the rate of movements of his own iris.
Experiment 179. To show the blind spot. The left eye being shut, let the right eye be fixed upon the cross as in Fig. 145. When the book is held at arm’s length, both cross and round spot will be visible; but if the book be brought to about 8 inches from the eye, the gaze being kept steadily upon the cross, the round spot will at first disappear, but as the book, is brought still nearer both cross and round spot will again be seen.
Experiment 180. To illustrate the duration of retinal impressions. On a circular white disk, about halfway between the center and circumference, fix a small, black, oblong disk, and rapidly rotate it by means of a rotating wheel. There appears a ring of gray on the black, showing that the impression on the retina lasts a certain time.
Experiment 181. Mark off a round piece of cardboard into black and white sectors as in A (Fig. 146). Attach it so as to rotate it rapidly, as on a sewing machine. An even gray tint will be produced as in B.
Experiment 182.To illustrate imperfect visual judgments. Make three round black dots, A, B, C, of the same size, in the same line, and let A and C be equidistant from B. Between A and B make several more dots of the same size. A and B will then appear to be farther apart than B and C.
For the same reason, of two squares absolutely identical in size, one marked with alternately clear and dark cross-bands, and the other with alternately clear and dark upright markings, the former will appear broader and the latter higher than the other.
Experiment 183. Make on a white card two squares of equal size. Across the one draw horizontal lines at equal distances, and in the other make similar vertical lines. Hold them at some distance. The one with horizontal lines appears higher than it really is, while the one with vertical lines appears broader, i.e., both appear oblong.
Experiment 184. Look at the row of letters (S) and figures (8). To some the upper halves of the letters and figures may appear to be of the same size as the lower halves, to others the lower halves may appear larger. Hold the figure upside down, and observe that there is a considerable difference between the two, the lower halves being considerably larger.
S S S S S S S S 8 8 8 8 8 8 8 8
Experiment 185. To illustrate imperfect visual judgment. The length of a line appears to vary according to the angle and direction of certain other lines in relation to it (Fig. 147). The length of the two vertical lines is the same, yet B appears much longer than A.
Experiment 186. In indirect vision the appreciation of direction is still more imperfect. While leaning on a large table, fix a point on the table, and then try to arrange three small pieces of colored paper in a straight line. Invariably, the papers, being at a distance from the fixation-point, and being seen by indirect vision, are arranged, not in a straight line, but in the arc of a circle with a long radius.