456 EFFECT OF HEAT AND POISONS ON THE HEART OF THE FllOG. 



sention of one is then fastened by sealing-wax to one end of the 

 lever, and to the other end of the straw a round piece of white 

 paper cut to the size of a shilling or half-crown, according to 

 convenience, is also fixed by a drop of sealing-wax. The pin, 

 which acts as a pivot, should be just sufficiently beyond the 

 edge of tlie plate to allow the lever to move freely, and the lever 

 itself should lie fiat upon the plate. Its weight too, increased 

 as it is by the straw and paper flag, would now be tco great for 

 the heart to lift, and so it must be counterpoised. This is 

 readily done by clasping a pair of bull-dog forceps on the other 

 end. By altering the position of the forceps the weight of the 

 lever can be regulated with great nicety. If the forceps are 

 drawn back as at c. Fig. 158, the flag is more than counter- 

 balanced, and does not rest on the heart at all, wliile the 

 position a brings the centre of gravity of the forceps in front of 

 the pivot, and increases the pressure of the lever on the heart. 

 The isolated frog's heart is laid under the lever near the pivot 



Fig. 158. — Instrument for showing the Action of Heat and Cold and of Poisons 

 on the Frog's Heart. 



and as it beats the lever oscillates upwards and downwards. If 

 the tin plate be now laid on some pounded ice the pulsations 

 will become slower and slower, and if the room be not too 

 warm the heart may stand completely still in diastole. On 

 removing the plate from the ice the pulsations of the heart 

 become quicker. If a spirit-lamp be now held at some distance 

 below it the heart beats quicker and quicker as the heat 

 increases, until at last it stands still in heat tetanus. On again 

 cooling it by the ice its pulsations recommence. At first they 

 are quick, but they gradually become slower and slower. On 

 again applying the spirit-lamp they become quicker, and by 



