ACTION OF HEAT UPON SINGLE BEATS 



117 



also be studied a little more closely. 

 This can readily be done by the aid of a 

 heart brought to a standstill by the Stan- 

 nius ligature. 



Experiment 2. — Prepare the apparatus 

 for recording single contractions as in ex- 

 periment 5, p. 106. The drum should rotate 

 2 cm. per second. Pith a frog, expose its 

 heart, and apply the Stannius ligature. 

 Hook a bent pin through the tip of the 

 ventricle apex. The heart may now be 

 excised and attached as in fig. 93. Fix 

 the electrodes for stimulating the ventricle. 

 Take a small beaker full of the diluted 

 blood (or Ringer's solution), which has pre- 

 viously been cooled in a freezing mixture, 

 and bring it up around the cork base, so as 

 to immerse the heart. Take the tempera- 

 ture of the dilute blood. After about a 

 minute's immersion lower the beaker and 

 record a contraction. Raise the temperature 

 of the dilute blood five degrees by placing 

 the beaker for a few seconds in hot water. 

 Again immerse the heart for one to two 

 minutes. Then lower the beaker, see that 

 the writing point is again at the same level, 

 and record a second contraction over the first. 

 In this way record a series of contractions, 

 increasing the temperature each time until 

 a range of from 5° C. to 30° C. has been 

 reached. Mark the point of stimulation, 

 draw an abscissa line, and take a time- 

 tracing of 30 per second beneath the zero 

 abscissa. 



Pig. 95 represents such a series of 

 curves. Measurements of each should 

 be taken and arranged in a tabular form, 

 as has been done for this experiment in 

 the table on next page. 



Prom this figure and its accompany- 

 ing table the following points are clear : 



1. As the temperature rises the 

 latent period becomes shorter. 



2. As the temperature rises the 

 period of contraction becomes shorter, 

 at first very rapidly and then more 

 slowly. 



3. The period of relaxation shows 

 a sudden shortening from 7 to 10° C. It 



