THE CIRCULATION OF THE BLOOD 537 



explained also by variations in the irritability of 

 the several parts. In the latter case, the excita- 

 tion would pass as usual, but its action on any 

 part, for example the ventricle, would be in- 

 creased or diminished by changes in the irri- 

 tability of the cardiac muscle in that region. 

 Engelmann has found that ventricular systole 

 lowers the conductivity of the ventricle for a 

 time. 



Tonus. Pass the very fine copper wire through 

 the wall of the auricle of the tortoise and attach 

 the wire to the heart lever, so that the contrac- 

 tions of the auricle may be recorded. Let the 

 drum move so slowly that the individual contrac- 

 tions will be nearly but not quite fused. 



Two sorts of contractions can be distinguished, 

 (1) the usual frequent contraction or beat of the 

 auricle, (2) the tonus oscillations. The tonus 

 oscillations include from twenty to forty beats. 

 In the tortoise auricle, the beats usually become 

 less extensive during the rise of tonus. 



The Influence of " Load " on Ventricular Contrac- 

 tion. Kecord the contractions of the frog's 

 ventricle. Increase the intraventricular pressure 

 (i. e. the load against which the ventricular muscle 

 contracts) by clamping the aortae with forceps 



