500 



MUSCLE-NERVE PHYSIOLOGY 



second, record the contractions on the drum moving at a speed of about 

 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 incom- 

 plete 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 



FIG. 354 A. Arrangement of Apparatus for Studying the Contractions of the Strip of the Apex 



of the Ventricle. 



by 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 215. When such ventricular strips are 

 immersed in ordinary 0.7 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 im- 

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



