THE HEART 



81 



heart. Note the reduction in the rate and slowing of each individual 

 beat. After the heart has again resumed its normal rate and ampli- 

 tude of contraction bathe it in the same manner with warmed saline 

 solution (20 to 25 C.). Note the increase in its rate, due to a greater 

 rapidity of the individual contractions. 



3. Refractory Period. Extrasystole. Leave the heart in position, 

 but place its ventricular portion in the cup of a heart-holder. Adjust 

 the writing lever upon its surface, and connect the binding-posts with 



FIG. 5L THE HEART-HOLDER. (Porter.) 



the secondary coil of an inductorium. Arrange the electric apparatus 

 for stimulation with single shocks, and insert a signal in the primary 

 circuit. Place the writing point of the latter in the same ordinate with 

 that of the heart lever. Allow the ventricle to register its cycles upon 

 a drum revolving at a moderate speed. Stimulate at intervals first 

 during the systolic and then during the diastolic period of the heart. 

 Note that the former stimuli remain without effect, whereas the latter 

 produce an extra contraction (extrasystole). While the musculature 

 is in contraction it remains impervious to stimuli (refractory period). 



C7> 



FIG. 52. STIMULATION OF FROG'S HEART DURING DIASTOLE. 

 S, Moment of stimulation; E, extra contraction; CP, compensatory pause. 



4. The Transmission of the Wave of Excitation. The wave of exci- 

 tation, ordinarily started at the venous entrance of the heart, is trans- 

 mitted in the turtle and frog over muscular connections and activates its 

 different segments consecutively. The transmission of this impulse may 

 be interfered with by compressing these muscular bridges by means of 

 a screw-clamp applied to the auriculo ventricular junction. By grad- 

 ually tightening this screw a degree of compression may be established 

 which will allow only some of these waves of excitation to reach the 



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