CARDIAC MUSCLE 



S3 1 



2 cm. per second. Use care not to overfatigue the muscle, i.e., stimulate 

 it only 2 or 3 seconds at a time. Repeat this test, increasing the rate of stimu- 

 lation each time by 5, that is, stimulate at 10, 15, 20, etc., per second. In the 

 first stimulus there will be a series of simple contractions with almost com- 

 plete intervening relaxations. As the rate is increased these relaxations be- 

 come less and less until presently a rate is found which produces continuous, 

 apparently uninterrupted contraction. This is a tetanus, the others are in- 

 complete tetani. The frog's gastrocnemius at a temperature of 20 C. is 

 tetanized with a stimulation of from 25 to 35 per second. 



14. Cardiac Muscle. Cardiac muscle differs from voluntary in that 

 the contractions occur rhythmically and automatically. This is shown by 



FIG. 356. Arrangement of Apparatus for Studying the Contractions of the Strip of the 



Apex of the Ventricle. 



the isolated frog's heart, which continues to contract when bathed with 

 blood or salt solution, often for hours. This isolated heart, however, has a 

 complicated local nervous mechanism. The apex of the ventricle of the 

 terrapin's heart is practically free from nerve ganglia and may be used to 

 demonstrate the characteristics of pure cardiac muscle. Cut a strip off the 

 apex of the terrapin's ventricle, as shown in figure 214, and mount it by 

 means of light silk-thread ligatures tied around the two ends of a strip and 

 attached to the apparatus, shown in figure 356. When such ventricular 

 strips are immersed in ordinary o . 7 per cent, sodium chloride they will begin 

 contractions in a few minutes, twenty minutes or so. The contractions will 

 be regular in rate and will continue through two or three hours, gradually 

 becoming smaller and smaller until the standstill is reached. If the strip is 

 immersed in its own serum it will give only occasional contractions, but it re- 

 mains irritable and capable of contracting at any moment. If changed to 



