72 EXPERIMENTAL PHYSIOLOGY 



preparation, and a very slow drum. After a period of rest, put in 

 single maximal stimuli at intervals of about two seconds, and record 

 each contraction. Notice that there is a slight increase in the extent of 

 the first few successive contractions, the second ordinate being a little 

 higher than the first, the third than the second, and so on, but they 

 soon become of exactly the same height. The experiment shows that 

 the effect of a first contraction is to increase the contractile power of 

 the tissue for the contraction immediately succeeding it, and similarly for 

 the next contraction, but only up to a certain point. 



2. Refractory period. For this experiment the drum must move 

 fast enough to spread out the curve of the ventricular contraction to 



FIG. 62. Frog cardiograph. /, Frog; h, heart; I, lever. 



about 1 centimetre long. A first stimulus is put in, and during the 

 record of the contraction caused by it a second stimulus is allowed to 

 follow it at varying intervals after the first. Mark the moment of put- 

 ting in the second stimulus by a dot on the curve. If the second comes 

 soon after the first, so as to reach the heart whilst it is still in process 

 of contraction, no additional effect is produced ; there is no super- 

 position (compare with skeletal muscle, p. 38). In other words, whilst 

 the contraction produced by the first stimulus is actually proceeding, 

 cardiac muscle is entirely refractory to a second stimulus. This re- 

 fractory condition is continued in a modified degree during the period of 

 relaxation of the muscle. A consequence of the long refractory phase 

 and of the lack of superposition is that cardiac muscle never shows a 

 true tetanus, although by sending in successive stimuli so that they 



