ELEMENTARY EXPERIMENTAL PHYSIOLOGY 57 



the mammalian heart it has been shown that muscle fibres of an 

 embryonic type connect the auricles with the ventricles. 



The above experiments prove that rhythmic contractility is the 

 inherent function of the cardiac muscle. The muscle of the sinus and 

 auriculo- ventricular junction is more embryonic in structure and possesses 

 greater power of initiating rhythm. It is less excitable, and conducts 

 a stimulus less rapidly than the muscle of the auricles and ventricle. 

 The auricular and ventricular muscle is more differentiated in structure. 

 The cross striae are more marked. It does not so easily initiate 

 rhythm. Owing to its greater excitability and conductivity it follows 

 the lead of the sinus. 



During the period of systole the heart is refractory to artificial 

 excitation. The excitability returns with diastole, increasing as 

 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 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 



