68 EXPERIMENTAL PHYSIOLOGY 



held in the hand. Put the secondary coil far from the primary, 

 and break the primary circuit. Determine the excitability of the 

 preparation by ascertaining to what division of the scale the secondary 

 coil must approach the primary before a contraction is produced. 

 This represents the liminal stimulus. Now bring the secondary 

 nearer the primary and again stimulate. The contraction is not 

 appreciably larger. Repeat with a still stronger stimulus. In every 

 case the extent of contraction is the same 1 (compare with skeletal 

 muscle, p. 39). 



2. Refractory period. Put in a second stimulus (with the hand) 

 at varying intervals after the first. If put in very soon after so as to 

 reach the heart whilst it is still in process of contraction, no additional 

 effect is produced ; there is no superposition (compare with skeletal 

 muscle, p. 36). In other words, whilst the contraction produced by 

 the first stimulus is proceeding, cardiac muscle is refractory to a 

 second stimulus. This refractory phase is continued to a less extent 

 during the period of relaxation of the muscle. A consequence of the 

 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 reach the muscle always at the commencement 

 of the period of relaxation, there is an appearance of fusion of the 

 successive contractions ; but they never overtop one another as in 

 tetanus of skeletal muscle. To show this, put in a succession of 

 stimuli by rapidly making and breaking the primary circuit with the 

 hand (compare with tetanus of skeletal muscle, p. 36). 



3. Staircase phenomenon. This is not peculiar to cardiac muscle 

 (^ee p. 43), but is often well shown by it. Using the Stannius prepara- 

 tion, and after a period of rest, stimulate a number of times in suc- 

 cession at intervals of about two seconds, keeping the strength of 

 the stimulus the same throughout. 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. 



1 See, however, what is said as to the staircase phenomenon (paragraph 3). 



