308 OF THE CIRCULATION OF THE BLOOD. 



frog, it appears that the heart only beats within certain limits of tempera- 

 ture, though these are not very well defined ; in regard to the lower limit of 

 temperature, the hearts of some animals cease to beat at 32 F., others not 

 till exposed to a temperature of 25 F. ; in regard to the higher limit of 

 temperature the variation is still greater, some hearts being arrested at 86 F., 

 and others not till 104. In ascending sloicly from the lower to the higher 

 limits the frequency of the heart's action steadily increases till it reaches a 

 maximum, and then as the higher limit is approached it suddenly but 

 somewhat irregularly falls, as is represented in the adjoining cut (from 

 Cyou's Memoir), where the abscissa may be considered as divided into de- 

 grees of temperature, whilst the ordinates would represent the relative fre- 

 quency of the heart's action. Near the higher limit, at which the heart's 



FIG. 125. FIG. 126. 



FIG. 125. Curve showing the increased rapidity of the heart's action as the temperature to which it 

 is exposed rises. 

 FIG. 126. Curves showing force of the contraction of the heart with increase of temperature. 



movements are arrested, the heart beats irregularly, so that the duration of 

 the successive periods of diastole vary considerably, and just before stopping 

 the contraction of the heart is no longer sudden and complete, but progres- 

 sive or peristaltic in character. In regard to the variation in the force of the 

 beats of the heart which occurs with rise of temperature, Cyou has shown 

 that the curve representing it attains its maximum very near the lower 

 limits of temperature, and remains at the same height till a temperature of 

 between 59 F. and 66 F. is reached, from which it progressively falls to 

 null-point. At the higher temperature little or no fluid is propelled from 

 the heart, owing to the peristaltic character of its contractions. The con- 

 clusion that may be drawn from the above facts is, that there is a certain 

 temperature at which the combined frequency and force of the heart's action 

 produces its greatest effect in driving the blood through the vessels. For the 

 frog, this lies, according to Cyon, between 64 and 72 F. The phenomena 

 in Cyon's experiments were somewhat different when the changes of tempe- 

 rature in the air surrounding the excised heart, and the serum flowing 

 through it were sudden. Cyon found that if a heart that had been beating 

 regularly at about 70 F. were suddenly cooled to 32 F., the force of the 

 contractions underwent great diminution, the movements assuming a peri- 

 staltic character, and the heart becoming considerably dilated (heat diastole). 

 After a time, however, it began to beat more strongly, and behaved as if it 

 had been gradually cooled. If after being kept at 32 F. for some time, it 

 was suddenly exposed to air and serum at a temperature of 104, the beats 

 succeeded one another more and more rapidly till the whole organ passed 

 into a state of tetanus (heat tetanus), lasting from 15 30 seconds. It then 

 began to beat again, and passed through the same stages in the course of 

 H 2 minutes that it presented in being gradually warmed. When a heart 

 was suddenly heated, not from 32 F., but from about 55 to 104 F., the 

 beats instead of becoming quicker became slower and fuller, and the traces 

 resembled those of a heart in which the vagus is irritated. This state lasted 



