30 PRACTICAL PHYSIOLOGY 



B. THE INFLUENCE OF THE SPINAL CORD ON THE 

 SKELETAL MUSCLES 



I. Reflex Action in the Frog 



1. Phenomena of Reflex Action. 



Now hang the frog to the edge of the frogboard, fixing it by a pin 

 through the jaw. Pinch the toes of one foot and observe and 

 record what happens. This is a reflex action. 



1. Are reflex movements co-ordinated ? Apply a very small scrap 

 of blotting paper dipped in acetic acid to the flank of the animal. 

 Study and describe the movements. 



Having washed off the acetic acid, study 



2. The relationship of the reflex response to the stimulus. Pinch 

 the foot with forceps and study the result as regards 



(a) Movements which result. (Describe.) 



(6) Relation of these movements to the strength of the stimulus. 

 Vary the strength of the pinch, or, stimulating with the induced 

 current (Neef 's hammer, see Appendix, p. 83), vary its strength. 



(c) Duration of the movements. How long are the movements 

 maintained with different strengths of stimulus ? 



(d) Spread of the movements. Study the order of this. 



3. What is the effect of a series of subminimal stimuli in liberating 

 a reflex action? 



(A stimulus which is too weak to cause a reaction is called a 

 subminimal stimulus.) 



Stick the pin electrodes lightly into the skin of the foot. With- 

 draw the secondary coil of the inductorium till a single stimulus just 

 fails to elicit a response, then stimulate the foot with 



1. A single subminimal stimulus. 



2. A series of subminimal stimuli. 

 What conclusion do you draw ? 



2. Is Time taken in Reflex Action ? 



METHOD. Using the same frog, dip the foot first into the weak 

 acid supplied, and, after washing it by immersing in a vessel of 

 water, into the stronger acid. Record the difference in the time 

 of onset of the reflex action. 



3. Fatigue. 



Arrange as in 3 above, with the secondary coil pushed nearer the 

 primary. 



Continue a series of fairly strong stimuli till the leg no longer 

 responds till fatigue is produced. Now dissect out the sciatic 

 nerve. To do so snip through the skin on the dorsal surface of the 

 thigh, then, holding the thigh between the thumb and forefinger 

 of the left hand, separate the flexors and extensors and with a glass 

 hook draw forward the thin biceps muscle and the white nerve 

 will be clearly seen and may be separated from the surrounding 



