66 EXPERIMENTAL PHYSIOLOGY. 



from that in the normal heart. A further tightening of the 

 ligature will again inhibit the beat. Explain results. 



Certain properties of cardiac muscle can be studied with 

 quiescent musc?e which cannot be studied in a heart beating 

 normally. 



Refractory Period. Arrange to stimulate ventricle with 

 weakest tetanizing current that is effective. Increase the 

 strength of the current and record contractions. How does 

 cardiac muscle differ from skeletal muscle in its response to 

 rapidly interrupted shocks? Is there any change in the rate 

 of response as the current is made stronger? 



All-or-none Law. Using the same preparation, make sure 

 that the beat of the auricle and ventricle is inhibited by the 

 first Stannius ligature. Arrange the induction coil for single 

 induction shocks. Apply the electrodes to the ventricle and 

 find the weakest break shock that will cause a contraction. 

 Record, then turn the drum about 2 mm. After 15 sec. record 

 the contraction in response to a slightly increased stimulus. 

 Turn the drum again and increase the stimulus a little more 

 and so on until the heart is receiving the maximal stimulus. 

 Does the height of the contraction under these conditions vary 

 with the strength of the stimulus? Compare with skeletal 

 muscle. An interval of 15 sec. is used because a short interval 

 would give the " staircase" effect. How is this effect to be 

 reconciled with the "all-or-none law"? 



(b) Second Ligature. Place a second ligature around the 

 heart exactly over the auriculo-ventricular groove. Tighten 

 this ligature so as to compress the heart between the ventricle 

 and the auricles. Note that the ventricle immediately begins 

 to beat, while the auricles continue in the relaxed condition. 

 Give a reason for this result. 



Determine whether it is possible to tetanize cardiac muscle 

 by stimulating with the tetanizing current. Explain your 

 results. 



Experiment 30. Study of the Turtle Heart. A turtle is 

 pithed in the same way as a frog. The plastron or ventral 

 portion of the shell is removed by the use of bone forceps and 

 scalpel. Operate so that hemorrhage is reduced to a minimum. 



