THE CIRCULATION OF THE BLOOD. 235 



of the ventricle, in which ganglia have not been found, does not, under 

 ordinary circumstances, possess the power of automatic movement. 



It has, however, been shown by Gaskell that the extreme apex of the 

 ventricle of the heart of the tortoise, which contains no ganglia, may 

 under appropriate stimuli be made to contract rhythmically. This 

 proves that the muscular tissue of the heart itself is capable of rhyth- 

 mical contraction independent of the ganglia. Thus it seems probable 

 that the rhythmic contractility of the heart is a power inherent in the 

 muscular tissue, which is quite independent as far as its commencement, 

 at any rate, is concerned of nerve influence. 



The heart-muscle exhibits another property which distinguishes it 

 , from ordinary skeletal muscle, viz., the way in which it reacts to stimuli. 

 ' The latter as will be described at greater length in its appropriate place, 

 , reacts slightly to a slight stimulus above the minimal, and with an in- 

 \ crease of the strength of the stimuli will give increasingly ample con- 

 tractions until the maximum contraction is reached; in the case of the 

 I heart-beats this is not so, since the minimum stimulus ivliich has any 

 effect is followed />// flic- miu-ininm contraction; in other words the weak- 

 ! est effectual stimulus brings out as great a beat as the strongest. There 

 f is another great difference between the contraction of the heart and of 

 skeletal muscle, viz., the inability of the former to enter into a state of 

 \ tetanus under the influence of stimuli repeated very rapidly. If the 

 heart be stimulated by a series of rapid interrupted induction shocks, 

 there is no summation of the contractions, as there would be,supposing 

 an ordinary skeletal muscle were stimulated in the same way. This 

 phenomenon is said to be due to the following fact, viz., that in order to 

 produce an extra contraction of the excised frog's heart, the stimulus 

 must be applied during diastole or period of rest or relaxation, and in 

 that case the next contraction happens at an earlier period than if the 

 stimulus were not applied. If applied during the systole, the stimulus 

 has scarcely any eifect; the period during which the muscle is refractory 

 to stimuli is much longer in the case of the heart than in the case of 

 other muscles. In order to produce a tetanus in skeletal muscle, the 

 second stimulus must be sent into the muscle before it has had time to 

 recover from the effect of the first stimulus and relax, and so on with 

 the third, fourth, and other stimuli. If, as we may suppose, the same 

 conditions for the production of tetanus are necessary in heart-muscle, 

 the reasons of the impossibility of producing tetanus, i.e. that a stim- 

 ulus applied during contraction is ineffectual, are sufficiently obvious. 

 It appears, however, that if the stimuli are sufficiently strong and rap- 

 idly repeated, the refractory period during which the muscle is prac- 

 tically insensible to stimuli diminishes, and a very rapid repetition of 

 the beats occurs. As a rule the beats are fewer with rapid stimulation, i 



