58 ELEMENTARY PHYSIOLOGY. X. 



the foot will be withdrawn. Dip the foot into the water, 

 in order to wash away the acid. Measure, with the aid 

 of a rapidly beating metronome, the time between the 

 v moment when the toe comes into contact with the acid 

 and the moment when it is withdrawn. Make, at in- 

 tervals of two minutes, three such observations (being 

 careful that the toe dips in the acid to exactly the same 

 extent) and take the mean of the three. 



11. Cut a small piece of blotting paper one or two 

 mms. square, moisten it with 1 to 5 p.c. acetic acid, and 

 place it on the flank of the animal. The legs of the same 

 side will be speedily drawn up and swept over the flank 

 as if to remove the piece of paper. Wash away the acid. 



12. Place similar pieces of acid-paper on different 

 parts of the body ; different movements will be witnessed 

 in consequence ; all however tending to remove the irri- 

 tating substance. 



13. Wash off all the acid from the frog, and when it 

 has become perfectly quiet, place it in a basin of water ; 

 it will sink to the bottom (unless the lungs be accident- 

 ally much distended with air), and no movements of 

 any kind will be witnessed. 



Observe that all the movements produced in the foregoing 

 observations, although complicated, co-ordinated, and pur- 

 poseful in character, are partial, and only by accident bring 

 about locomotion. However stimulated, the animal never 

 springs or leaps forward. 



14. Make a small cut through the skin of the back 

 and inject half-a-dozen drops of chloral hydrate, leave 

 for five minutes, and repeat 10; the reaction time will 

 be much prolonged. 



