LXVIII.] REFLEX ACTION, ETC. 319 



the legs may make a few movements, but they soon cease to do 

 so, and hang motionless. 



(a.) Pinch the tip of any toe of the right leg ; the right leg is 

 drawn up. If a toe of the left leg be pinched, the left leg is drawn 

 up. These are unilateral reflex movements. 



(b.) Mechanical Stimuli. Pinch the tip of one toe very fee my, 

 perhaps only the foot will be flexed at the ankle-joint. Pinch 

 more strongly, and a greater reflex movement will be obtained. 

 It is evident, therefore, that the reflex movement varies not only 

 with the part of the skin stimulated (1, d.\ but also with the 

 intensity of the stimulus. Very violent stimulation may cause 

 reflex movements in all the other limbs. This is due to irradiation 

 of the reflex movement in the cord. 



3. The Latent Period (Tiirck's Method). Summation of 

 Stimuli. 



(a.) Prepare and label dilutions of sulphuric acid containing 

 i, 2, 3, and 4 cc. per litre i.e., o.i, 0.2, 0.3, and 0.4 per cent 

 of sulphuric acid (by volume) and place some of each in lour 

 shallow glasses. Arrange also a large beaker of water to wash the 

 frog. Adjust a metronome to beat one hundred times per 

 minute. Cause it to beat. 



(b.) Hold the frog in the left hand by means of the hook, and 

 in the right take a glass rod to hold one leg aside. Dip the other 

 leg up to the ankle into the o.i per cent, acid, and on doing so 

 count the number of beats before it is withdrawn from the acid. 

 After the leg is withdrawn, wash the leg in water to remove the 

 acid. Note the time in hundred ths of a minute, i.e., the latent 

 period. Allow the frog to rest at least five minutes, and repeat 

 the experiment. Take the mean of the two observations or, if 

 you prefer it, of three or more observations and this will give the 

 " latent period." 



(c.) Repeat with suitable intervals of repose the same experi 

 ment with acid of 0.2, 0.3, and 0.4 per cent., noting that, as tb* 

 strength of the acid increases, the latent period becomes shorter, 

 but not in the ratio in which the acid is stronger. 



(<?.) If only the longest toe is dipped into the acid, then the 

 summation of stimuli takes place more slowly. 



4. Chemical Stimulation. (Purposive Characters of Reflex.) 



(a.) In a small glass place some strong acetic acid and a few 

 pieces of filter-paper 3 mm. square. Either when the frog is 

 lying on its back or while it is suspended, apply with a pair of 

 forceps one of the pieces of paper moistened with acid the surplus 



