216 



EXPERIMENTAL PH YSIOLOG Y 



1. The latent period or reflex time. 



Arrange a time marker recording \ seconds and a signal to write 

 vertically over one another on a blackened surface set to rotate at about 

 1 cm. per 2^ seconds. Take some of each of the four strengths of sulphuric 

 acid, 0*1 per cent., 0*2 per cent., 0*3 per cent., and 0"4 per cent., in small 

 beakers, labelling each. Dip the foot up to the ankle in the weakest solution, 

 recording the instant of immersion by closing the key of the signal. Wait 

 until the foot is withdrawn and then open the key of the signal. Count the 

 number of oscillations of the time marker which have occurred dming the 

 closure of the current through the signal. This is the reflex time for that 

 strength of acid. Wash the skin thoroughly, allow the frog to rest for a 

 time, and then repeat the experiment, using the 0*2 per cent, solution. In the 

 same way repeat for the other strengths of solution. 



Arrange the results thus obtained in tabular form as in the follow- 

 ing instance : — 



2. Purposive character of the reflex. — In all the reflex actions 

 studied it will be noticed that the muscular response is a very complex 

 one. It is in no way an irregular series of twitches of the limb 

 muscles, but is a movement similar in nature to those carried out by 

 the frog during its life. It involves several muscles, each of which 

 contracts at the right instant, to the proper extent, and at the proper 

 rate, and another set of muscles which relax to the right degree, and 

 at the right time ; i.e. it is a co-ordinated movement. In addition to 

 this the response obtained is different according to the part stimulated, 

 and when examined is seen to tend either to remove the irritating 

 body, to move that part of the body from the irritant, or to remove the 

 whole body. This purposive character of the response is well seen in 

 the following experiment : — 



Take some squares of filter paper, about 4 mm. each side, and dip them 

 into some 20 or 40 per cent, acetic acid. Remove the excess of acid from one 

 of these and place it upon the flank of the frog. After a latent period the 

 limb on the same side is drawn up, and the flank rubbed with it as if to wipe 

 away the irritating body. In this the movement frequently succeeds. Wash 

 the skin, and after a period of rest apply another square and hold the leg of 

 the same side. The leg of the opposite side will probably be moved so as to 

 remove the irritant. Wash the skin again and study the effects of altering 

 the position of the irritant. In all cases characteristic but different movements 

 are produced. 



3. Irradiation of reflex movements. — In many of the experiments 

 we have so far tried it has been noted that with a given stimulus 



