96 PRACTICAL PHYSIOLOGY 



diastole proceeds. The energy of any cardiac contraction depends on 

 the previous activity of the heart, on the pressure of the diastolic 

 filling, on the resistance to systolic outflow, temperature, nutrition, 

 etc. It is independent of the strength of the stimulus so long as the 

 latter is efficient. Owing to the refractory period, the slow rate of 

 contraction, and the independence of the amplitude of contraction on 

 the strength of stimulus, the heart cannot be tetanised. 



By the study with the aid of the capillary electrometer or string 

 galvanometer of the electrical current of action which accompanies 

 the systole, it has been shown that the contraction of the heart is a 

 simple twitch, and not a tetanus. The current of action is triphasic in 

 the mammal (1) base negative, (2) apex negative, (3) base negative. 

 The excitatory wave travels from base to apex and from apex to base, 

 following the course of the muscle-bands, which start from the base, 

 run to the apex, and, turning in there, ascend on the inner wall 

 of the ventricle. The current of action travels at the same rate as the 

 excitatory state. 



The power of slow, sustained contraction seems to depend on the 

 richness of the heart-muscle in sarcoplasm. The heart-muscle possesses 

 tone, and this varies with the temperature and nutrition. Muscarine, 

 acids and chloroform weaken, while digitaline, caffeine, and alkalies 

 increase the tone of the heart. The auricular muscle of the toad 

 exhibits rhythmic alterations in tone. 



Antiperistalsis is difficult to produce because the excitatory process 

 in the ventricle is slow, and does not easily affect the more rapidly 

 contracting auricle. The refractory period which persists during 

 systole also prevents antiperistalsis. The excitatory wave is delayed in 

 passing through the more embryonic type of muscle in the sino-auricular 

 and auriculo-ventricular junctions, and therefore the auricle beats in 

 sequence to the sinus and the ventricle in sequence to the auricle. By 

 cooling the sinus and warming the ventricle the sequence of the heart 

 can be reversed, for the excitability of the ventrical is by these means 

 raised, while that of the sinus is lowered. 



By gently clamping a strip of tortoise auricle muscle between two 

 little bits of cork an artificial block can be created, and the piece of 

 auricle below the clamp then beats in sequence to the piece above the 

 clamp. The natural delay in conductivity at the auriculo-ventricular 

 junction is thus imitated (Gaskell). The conductivity is decreased by 

 the clamp. 



The nerve cells of the heart are placed in the least differentiated 

 parts : in the sinus (Remak's ganglion), in the inter-auricular septum 



