290 TEXT-BOOK OF PHYSIOLOGY. 



through the cavities of the excised heart will so increase the tone, 

 or the contractile power, that complete relaxation is prevented, 

 until finally the heart comes to a standstill in the condition of 

 systole. The passage of dilute solutions of lactic acid, mus- 

 carine, etc., through the heart has the opposite effect a gradual 

 diminution in contractile power, the heart finally coming to a 

 standstill in diastole. In the first instance the tonicity is said to 

 be increased; in the second instance, decreased. 

 The Response of the Heart to the Action of a Stimulus. 

 The heart in many animals may be brought to a standstill by ligation 

 of the tissues between the sinus venosus and the auricles the first 

 Stannius ligature. Under such circumstances the heart may be made 

 to contract by stimulating it with the induced electric current. With 

 each passage of the current the muscle contracts. Contrary to what is 

 observed in other muscles, the heart-muscle, if it contracts at all, at 

 once reaches its maximal value. Increase or decrease in the strength 

 of the stimulus has no effect on the extent or force of the contraction. 

 If a series of successive stimuli be thrown into a muscle, the effect 

 will vary in accordance with their time 

 intervals. Should this be less than about 

 three seconds, there will be a gradual 

 increase in the height for some half- 



1 dozen contractions, a result to which the 



FIG. 127. THE EXTRA CON- term "staircase" contractions has been 

 TRACTION AND THE COM- given. If a second stimulus is thrown 



^T^rixoSS 'f the Scle d "g ^ P* of 



line indicates the moment relaxation, a second contraction is de- 



the electric current passes veloped, which Superposes itself On the 



first, but the height of the contraction is 

 not any greater than the height of the' 



first (Fig. 127). There is here no summation of effects. With the 

 relaxation of the heart after the second contraction, a considerable 

 pause in the heart's action is noticed, to which the term "compensa- 

 tory" is given. During this pause the muscle accumulates a new 

 supply of contractile material sufficient for the next contraction. If 

 the stimulus be thrown into the heart during the period of contraction, 

 it will have no effect. For this reason the heart is said to be refrac- 

 tory to the second stimulus during the entire contraction period. 

 The explanation at hand is that the irritability is very much lowered 

 from the decomposition of energy-holding compounds. In the case 

 of the skeletal muscle the refractory period is confined to the latent 

 period. Since there is no summation of the contractions of the heart- 

 muscle, tetanization of the muscle can not be produced. 



Though many of the experiments relating to the properties of the 

 heart-muscle have been made on the hearts of frogs, turtles, and 



