THE PHYSIOLOGY OF THE NERVOUS SYSTEM 207 



b. Reflex Response to Various Stimuli. Suspend a pithed frog by 

 a hook through its mandible. The body and legs should hang freely, 

 and should be several inches from the table. 



(8) MECHANICAL STIMULI. With forceps pinch one of the toes 

 (hot the longest) of the hind leg. Pinch the skin of the flank. Pinch 

 the folds of skin about the anus. Note response when stimulus 

 is varied in strength and applied to either side. 



(9) THERMAL STIMULI. With a hot wire touch the skin at several 

 points, noting response. 



(10) ELECTRIC STIMULI. With single-induction shocks stimulate 

 skin of legs, thighs, or flank, using fine platinum-wire electrodes, and 

 touching the moist skin. 



If single shocks elicit no response, use a rapid succession of shocks 

 produced by bringing the Neef hammer into the primary circuit. 



(11) CHEMICAL STIMULI. Cut some pieces of filter paper not over 

 2 mm square. Dip a piece into 50 per cent, acetic acid, taking care 

 that the paper is saturated and that there is no excess of the acid. 



Apply the acid paper in turn to the web of the foot; to the flank; 

 to the ventrum; median line; to the anus. Note the character of 

 the response, as to extent, single-sided or double-sided. After each 

 application of acid to the skin of the frog, the acid should be thor- 

 oughly rinsed or swabbed away. 



c. The Characteristics of Reflex Response. (12) PURPOSIVE CHAR- 

 ACTER OF RESPONSE. In the responses already studied the observer 

 could easily note that the movements possessed the manifest pur- 

 pose of removing the offending object. In many cases the move- 

 ments involved several sets of muscles, but in every case all the 

 muscles involved in the response acted with perfect co-ordination, 

 and the movement was well directed toward the removal of the 

 irritation. This is what is meant by the purposive character. 



In order further to illustrate this characteristic of response, repeat 

 Foster's instructive experiment: "Choosing a strong frog in which 

 reflex action has been found to be highly developed; suspend it; hold 

 the right leg firmly down, and apply a square of acid paper to the 

 right flank. Twitchings and convulsive movements of the right leg 

 are at first witnessed; then the left leg is brought up to rub the right 

 flank." (Handbook for the Physiological Laboratory, p. 409.) 



(13) THE LATENT PERIOD OF REFLEX RESPONSE. The observer 

 will remember that in most of the above experiments the responses 

 did not follow instantaneously upon the application of the stimulus; 

 this was especially noticeable in the case of one of the weaker stimuli. 



In order further to illustrate this characteristic, prepare five dilu- 

 tions of sulphuric acid in as many watch-glasses; hold a glass so that 

 the tip of the longest toe just dips into the acid. Note the time 

 required to elicit a response. After each response rinse off the part 

 stimulated and allow the animal to rest several minutes, testing 



