206 INELUEXCE OF TEMPERATURE OX THE MAMMALIAN HEART. 



change in the number of the heart-beats which was produced 

 by an alteration of temperature was of course due to its action 

 on the nerves or muscular substance of the heart itself, as it 

 had been severed from all connection with other structures. 

 In this way he found that it is only within a certain limited 

 range of temperature that the frog's heart pulsates at all, the 

 lower limit being from 32° to 28-8° F., and the upper one 

 between 86° and 104° ¥., the limits varying somewhat with 

 each individual heart. When it is cooled down, its beats become 

 slower and slower till it reaches the lower limit, and then they 

 stop altogether. When it is slowly warmed, they become 

 quicker and quicker till they leach their maximum rapidity, 

 within a few degrees of the upper limit. They then become 

 slower and slower, and finally stop when the limit itself is 

 reached. During the time the heart is being warmed, the 

 number of its beats does not steadily increase throughout in 

 the same proportion to the rise of temperature. .From the 

 lower limit; the number of beats increases at first very slowly, 

 and then more and more quickly for each increment of tempera- 

 ture, till the maximum rapidity is attained, and then, with each 

 degree of rise in the temperature, the pulsations diminish in 

 number, rapidly though irregularly, and soon cease. In the 

 two or three degrees which precede complete stillstand the 

 beats become not only slow but irregular, so that almost no two 

 intervals between them are of the same length. 



At the under limit of temperature the heart contracts only 

 to a small extent, but a few degrees above it the maximum 

 amount of contraction is reached, and this amount continues 

 the same up to from 57-2° to 66-2° F., when the contractions 

 a^ain begin to get smaller and smaller. 



Just before it is stopped altogether by the heat, the ventricle 

 no longer contracts as a whole, but does so in a peristaltic 

 manner, so that it sometimes seems to be contracting vigor- 

 ously, and yet not a drop of the fluid it contains is expelled, 

 but it is merely moved about from place to place in the 

 ventricular cavity, one part expanding as another contracts, 

 and vice versa. 



When the heart is beating regularly, at moderate tempera- 



