Part 49

The common way of watching the movements of the iris is to regard it closely in a looking-glass while the amount of light entering the eyes is varied. Place yourself before a looking-glass and with your face to the window. Probably the iris will be expanded, and there will only be a very small opening or pupil in the center. Now shut one eye suddenly, while narrowly watching the other in the glass all the time. At the moment the light is cut off from one eye, the iris of the other contracts or is drawn up so as to enlarge the pupil. This shows that there is a remarkable interdependence between the curtains of the two eyes, as well as that they are affected by variations in the quantity of light falling on them.

Perhaps one of the most interesting ways of watching the movements of these sympathetic eye curtains is one which may be followed while you are out walking on the street some dark winter night. A gas lamp seen at a distance is, comparatively speaking, a point of light, with bars of light emanating from it in many directions. These bars, which give the peculiar spoked appearance to a star, are probably formed by optical defects of the lens within the eye, or by the tear fluid on the exterior surface of the eye, or by a combination of all these causes. Be that as it may, the lengths of the spokes of light are limited by the inner margin of the eye curtain; if the curtain be drawn up, then the spokes are long; if the curtain be let down, or, in other words, if the pupil be very small and contracted, then one can not see any spokes at all. Hence, as I look at a distant gaslight, with its radiating golden spokes, I am looking at something which will give me a sure indication of any movements of the eye curtains. I strike a match and allow its light to fall into the eyes; the spokes of the distant gas lamp have retreated into the point of flame as if by magic; as I take the burning match away from before my eyes, the spokes of the gas-lamp venture forth again. The experiment may be utilized to see how much light is required to move the window curtains of the eyes. Suppose you are walking toward two gas lamps, A and B; B about fifty yards behind A. If you steadfastly look at B and at the golden spokes apparently issuing from it, you may make these spokes a test of how soon the light of A will move your iris. As you gradually approach A, you come at last to a position where its light is strong enough to make the spokes of B begin to shorten; a little nearer still and they vanish altogether. I have found that about a third of the light which is competent to contract the pupil very markedly will serve to commence its movement.--WILLIAM ACKROYD.

PURKINJE'S FIGURES (p. 222).--Stand in a dark room with a lighted candle in hand. Shutting the left, hold the candle very near the right eye, within three or four inches, obliquely outward and forward, so that the light shall strongly illuminate the retina. Now move the light about gently, upward, downward, back and forth, while you gaze intently on the wall opposite. Presently the field of view becomes dark from the intense impression of the light, and then, as you move the light about, there appears projected on the wall and covering its whole surface, a shadowy, ghost-like image, like a branching, leafless tree, or like a great bodiless spider with many branching legs. What is it? It is an exact but enlarged image of the blood vessels of the retina. These come in at the entrance of the optic nerve, ramify in the middle layer, and therefore in the strong light cast their shadows on the bacillary layer of the retina. The impression of these shadows is projected outward into the field of view, and seen there as an enlarged shadowy image. These have been called Purkinje's Figures, from the discoverer.--PROF. JOSEPH LE CONTE, in Sight.

XI.

APPENDIX.

QUESTIONS FOR CLASS USE.

The questions include the Notes and the Selected Readings. The figures refer to the pages.

INTRODUCTION.

Illustrate the value of physiological knowledge. Why should physiology be studied in youth? When are our habits formed? How do habits help us? Why should children prize the lessons of experience? How does Nature punish a violation of her laws? Name some of Nature's laws. What is the penalty of their violation? Name some bad habits and their punishments. Some good habits and their rewards. How do the young ruin their health? Compare one's constitution with a deposit in the bank. Can one in youth lay up health as he can money for middle or old age? Is not the preservation of one's health a moral duty? What is suicide?

THE SKELETON.

3. How many bones are there in the body? Is the number fixed? Is the length of the different bones proportional? What is an organ? A function? Name the three uses of the bones. Why do the bones have such different shapes?

4. Why are certain bones hollow? Round? Illustrate. Compare the resisting property of bone with that of solid oak. What is the composition of bone? How does it vary? How can you remove the mineral matter? The animal matter? Why is a burned bone white and porous? What food do dogs find in bones?

5. What is the use of each of the constituents of a bone? What is "boneblack"? What is ossification? Why are not the bones of children as easily broken as those of aged persons? Why do they unite so much quicker? What are the fontanelles?

6. Describe the structure of a bone. What is the object of the filling? Why does the amount vary in different parts of a bone? What is the appearance of a bone seen through a microscope?

7. What is the periosteum? Is a bone once removed ever restored? What are the lacunæ? The Haversian canals? Why so called? Ans. From their discoverer, Havers. Define a bone. [Footnote: Bone structure may be summarized as follows: A bone is a collection of Haversian elements, or rods. An Haversian element consists of a tube surrounded by lamellæ, which contain lacunæ, connected by canaliculi.--DR. T. B. STOWELL.] What occupies the lacunæ? Ans. The bone cells (osteoblasts). How do bones grow?

8. Illustrate. How does a broken bone heal? How rapidly is bone produced? Illustrate. Objects of "splints"? Describe how a joint is packed. Lubricated.

9. How are the bones tied together? What is a tissue? Illustrate. Name the three general divisions of the bones. What is the object of the skull? Which bone is movable? How is the lower jaw hinged? Describe the construction of the skull. What is a suture?

10. Tell how the peculiar form and structure of the skull adapt it for its use. Illustrate the impenetrability of the skull.

11. Describe the experiment of the balls. What does it show? What two cavities are in the trunk? Name its principal bones. Describe the spine.

12. What is the object of the processes? Of the pads? Why is a man shorter at night than in the morning? Describe the perfection of the spine.

13. Describe the articulation of the skull with the spine. Why is the atlas so called?

14. Describe the ribs. What is the natural form of the chest? Why is it made in separate pieces? How does the oblique position of the ribs aid in respiration? (See note, p. 80.)

15. How do the hipbones give solidity? What two sets of limbs branch from the trunk? State their mutual resemblance. Name the bones of the shoulder. Describe the collar bone.

16. Describe the shoulder blade. Can you describe the indirect articulation of the shoulder blade with the trunk? Name the bones of the arm. Describe the shoulder joint. The elbow-joint.

17. Describe the wrist. Name the bones of the hand. How many bones in the fingers? The thumb? What gives the thumb its freedom of motion?

18, 19. Name and describe the fingers. In what lies the perfection of the hand? How do the gestures of the hand enforce our ideas and feelings? Describe the hip joint. What gives the upper limbs more freedom of motion than the lower? How does the pressure of the air aid us in walking? Illustrate.

20. Name the bones of the lower limbs. Describe the knee joint. The patella. What is the use of the fibula? Can you show how the lower extremity of the fibula, below its juncture with the tibia, is prolonged to form a part of the ankle joint? Name the bones of the foot. What is the use of the arch of the foot? What makes the step elastic? Describe the action of the foot as we step.

21. In graceful walking, should the toes or the heel touch the ground first? What are the causes of deformed feet? What is the natural position of the big toe? Did you ever see a big toe lying in a straight line with the foot, as shown in statuary and paintings? How should we have our boots and shoes made? What are the effects of high heels? Of narrow heels? Of narrow toes? Of tight-laced boots? Of thin soles? What are the rickets? Cause of this disease? Cure? Is there any provision for remedying defects in the body? Name one.