119 



the streng-th of the stimulus but do not stimulate more than 

 once in ten seconds, otherwise "staircase" contractions may 

 result. The force of the ventricular contraction will remain 

 the same in spite of the strong-er stimulus. If the heart is 

 capable of responding- at all it will, in each case, g-ive a maxi- 

 mum contraction. Stimulate either auricle in the same way 

 and note the result. 



371. Staircase Contractions of the Heart. Apply 

 the first Stannius ligature over the sino-auricular groove. 

 Connect the apex of the heart with the' heart lever. Record 

 on a slowly moving drum. Stimulate the quiescent heart 

 with single induction shocks at intervals of five seconds. 

 Notice that the second beat is higher than the first, the third 

 than the second and so on until a maximum beat is obtained. 

 This is the "staircase " of Bowditch. 



372. Location of Motor Centers in the Frog's 

 Heart. Dissect out the entire heart of a frog, and note that 

 it continues to beat. Cut the heart vertically into three pieces, 

 so that the middle portion will contain the auricular septum, 

 in which lie the ganglionic cells. This portion continues to 

 beat while the right and left lateral parts do not beat spon- 

 taneously, but will respond with a single contraction if stimu- 

 lated. 



373. Effect of Temperature upon the Heart Beat. 

 Leaving the heart in its usual position insert a glass tube 

 into the oesophagus and allow it to project through -the 

 stomach. Pass water at different temperatures through the 

 tube and note the number of beats in each case. 



374. Bernstein's Experiment. Isoi^ated Apex. — Tie 

 a ligature around the ventricle about half way between its 

 apex and base, or compress it with a clamp or pair of forceps, 

 the object being to destroy physiological continuity but pre- 

 serving anatomical connection. Remove the ligature. The 

 physiologically isolated apex does not contract. This would 

 seem to indicate that the adult heart muscle is incapable of 

 spontaneous rhythmical contraction. If the bulbus arteriosus 

 is compressed, the pressure of blood in the ventricle rises and 

 is usually sufiScient to stimulate the apex strongly enough to 

 start it beating again. Remove the ligature and apply the 

 pressure to the bulbus and note the effects. 



