sinus and auricle while the heart is in the body and full of 

 -o that sinus can be distinguished fnnn auricles when excised. 



Separate ( I ) the two auricles fnnn the ventricle. ( 2) the auricles 

 from each other, (^) the tip of the ventricle- from the base, and 

 determine in each ca-c if the piece of heart isolated is automatically 

 rhythmic and its rate. In which region is the beat fastest? Are 

 the now automatically beating regions independent of each other in 

 rhythm. 



b. Repeat the above experiment with the frog's heart Used in 

 experiment 262. Does the conns arteriosus of a frog's heart beat 

 when i-olated? 



II. IMIYSIOLOCY ( )!' HEART MUSCLE. 



jdo. Graphic record. Use the frog's heart still beating in the 

 body. I'ass a bent pin. to which has been fastened a fine wire, 

 through the tip of the ventricle. Fasten the wire t> the heart 

 lever by wax, and adjust the lever on the support against a slow 

 moving drum. Record the contractions. 



jfij. Refractory period and compensatory pause. Place die 

 signal magnet in the primary circuit of the inductorium and arrange 

 the latter for single induced shocks. Attach one wire from the 

 -ecoiidary posts of the inductorium to the heart lever and place 

 another about the auricles. Record the normal beat of the heart 

 on a slow moving drum, and stimulate at various phases of the beat 

 with make or break shocks. From your record determine at what 

 period the heart is non-irritable (i.e., refractorv toward stimuli). 

 .Vote the compensatory pause. At what pha-e is the maximum 

 extra-contraction obtained? Is there any difference in the latent 

 I eriod- of the extra contractions? Trv tetani/ing. Result? 



jf'cS. . /// contractions are ina.viinal. Inhibit the heart by a 

 ligature or a ( laskell clamp placed at the auriculo-ventricular junc- 

 tion. Find the least strength of stimulus that will cause the ventricle 

 to contract. Increase the -trength of the -timnlu-, but do not 

 stimulate oftencr than mice in ten seconds (to avoid the stair-ca-c 

 contractions described below). Record the contractions. 1 )oes the 

 force of ventricular contraction remain the same, notwithstanding 

 the increased stimulus? I low i- this expressed in words? 



Jin,. Stair-case effect or treppe. Find the lea-t stimuli!- that 

 will cause the ventricle t contract. Repeat tin- minimal Stimulus 

 after everv relaxation, recording the contractions on a slow moving 

 drum. I low dues tin- n ult agree with the above experiment? 



82 



