EXERCISE 86 



Some of the movements performed by human beings and other animals result from the contraction 

 of muscles over which there is no direct control (for example, heart movements, stomach movements). 

 Other movements result from the contraction of muscles which we can sometimes control, but which 

 ordinarily act without any attention from us (for example, the eyelid movements, diaphragm move- 

 ments). In still other cases voluntary, or striped, muscles contract as a result of some stimulation, without 

 our being able to control them at all. Such forced movements are called reflexes. 



Problem. Can we control any of our reflexes ? 



What to use. Live human beings with definite reflexes. (Students work in pairs.) 



What to do. 1. The knee-jerk reflex. Sit comfortably on a chair, in relaxed position, with one knet 

 crossed over the other. Strike the tips of the fingers just below the kneecap of the upper knee and 

 note movement of foot resulting. 



(It may require several trials to locate the exact spot and to learn the best way of striking. A good 

 way to proceed is to hold a ruler firmly with the edge right under the kneecap, and then to tap the 

 opposite edge.) 



When you have made sure that you can produce the knee jerk, repeat several times, the subject 

 trying to prevent the jerk while the operator produces the stimulation. 



2. The winking reflex. Bring finger quickly in front of subject's eye, without touching it or the 

 eyelashes. Note movements of the eyelids. Touch eyelashes gently from the side or from above with 

 a feather or light straw so that the subject cannot see the object approaching. Repeat, with the subject 

 trying to prevent the wink. 



3. Grasping reflex. If it is possible to have access to a very young infant, note the baby's reaction to 

 a solid object (stick, finger) placed across the palm of the hand. Find out whether the reaction is uniform ; 

 whether it depends upon the character of the stimulating object hard or soft, large or small, rough or 

 smooth, etc. With a spring scale fastened to the object that the baby clasps, measure the force of 

 the grip ; that is, find out how hard one must pull to force the object out of the baby's hand, 



Record. Describe the results of the stimulation in each case and of the attempt to prevent the 

 reactions from taking place. What is the answer to the Problem ? 



NOTE. A reflex is a form of movement that depends upon certain nerve connections which are inborn with other 

 structural characters of the organism. The reflex system consists of a receptor (that is, a receiving element), an effector 

 (that is, an effect-producing element), and a connecting element. In each of the three elements there is always a nerve part; 

 in the receptor there is also a special sensitive portion ; and in the effector there is a muscular (moving) portion or a 

 glandular portion. 



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