256 EXPERIMENTAL PHYSIOLOGY. 



drawn up again towards the body. Pinch the flank with a 

 pair of forceps; the leg of the same side is rapidly extended, 

 then drawn up towards the spot stimulated. Pinch sharply 

 the skin round the anus with forceps. Immediately, both 

 legs are pushed out and pulled up towards the body, as if to 

 dislodge the offending body. 



2. Bend a long (6 cm.) straight pin into the form of a hook, 

 and push it through the tips of both jaws, and by means of the 

 hook hang up the frog on a suitable support. It hangs vertically 

 with the legs pendant. At first the legs may make a few move- 

 ments, but they soon cease to do so, and hang quite motionless. 



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

 leg is flexed and drawn up. If a toe of the left leg be 

 pinched, either with the nail or forceps, the left leg is 

 drawn up. 



3. The Latent Period (TUrck's Method). 



(a.) Prepare dilutions of sulphuric acid containing 1, 2, 

 3, and 4 cc. per litre i.e., (H, 0-2, O3, and O4 per cent, of 

 sulphuric acid, and place some of each in four i 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. 



(6.) 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'l 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 withdrawn from the acid. After the leg is withdrawn, 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 hundredths of a minute i.e., the latent period. Allow 

 the frog to rest at least five minutes, and repeat the experi- 

 ment. 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 

 experiment with acid of 0*2, 0*3, and 04 per cent., noting 

 that as the strength of the acid increases, the latent period 

 becomes shorter, but not in the ratio in which the acid is 

 stronger. 



