144 HUMAN BIOLOGY 



Experiment 18. Astigmatism (effect of unequal curvature of cornea 

 or lens along certain lines). With end of crayon draw about twelve 

 straight, even lines crossing at one point on the blackboard. Have 

 the lines of equal distinctness. How many pupils report that the lines 

 in certain directions are blurred? Inquire whether these pupils have 

 frequent headaches from eye strain. 



Experiment 19. Can Sound reach the Ear through the Bones? 

 Hold a watch between the lips and notice its ticking. Close the teeth 

 down upon it and notice any change in the sound. Cover one, then 

 both ears, and note the result. 



Experiment 20. Test keenness of hearing by having pupils walk 

 away from a ticking watch until it becomes inaudible. Test each 

 ear. A " stop " watch is preferable. 



Experiment 21 . Advantage of Two Ears over One. Have the class 

 stand in a circle. Blindfold some one and place him in the middle of 

 the circle. Let various pupils clap the hands as the teacher points to 

 each. Can the blindfolded one point in the direction whence the sound 

 comes ? Stop one ear with a handkerchief and repeat. Result? Con- 

 clusion? From what two points in the circle does the sound fall upon 

 both ears alike ? 



Experiment 22. The Cause of Nasal Tones. Let a pupil go to the 

 back of the room and read a paragraph, and hold his nose until partly 

 through the reading. Or the teacher may read with his face and hand 

 hidden by a large book. Let the other pupils raise their hands when 

 they notice a change in the quality of the reader's voice. Does the 

 experiment show that a "nasal" tone comes partly through 'the nose 

 or through the mouth only ? Does stoppage of the nostrils by catarrh 

 cause a nasal tone ? 



Five Differences between Special and General Sensation. First, the 

 nerves of special sense all end in special organs at the surface ; for 

 instance, the touch corpuscles are for touch, the eye is for sight, etc. 

 There are many nerves in the body that do not end in special organs ; 

 these nerves give what is called general sensation. A second difference 

 is that general sensation tells of the condition of the interior of the body, 

 while special sensations tell us of the condition of the surface of the 

 body and of the outside world. Third, general sensations are not so 

 exact as the reports of the special senses. One can locate a point on 

 the skin that has been touched much more accurately than he can locate 

 an internal pain. A fourth difference is that the meaning of each special 

 sensation must be learned (usually in infancy) ; but the meaning of gen- 

 eral sensations is inherited. This inherited knowledge of what general 

 sensations mean is also called instinct. Fifth, the sympathetic nerves 



