82 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



ventricle. Not all of the auricular contractions will then be followed 

 by ventricular contractions. This constitutes the condition of partial 

 heart-block. 



5. The Excised Heart. Raise the heart as a whole by the frenum, 

 and divide the aortse and venae cavae. Place the excised organ in a 

 watch-glass in some of its blood and cover it with another watch-glass. 

 Count the number of its beats, and note that it becomes soft and 

 flaccid during diastole and adjusts itself at this time to the surface 

 upon which it rests. Apply a drop of warmed saline solution to the 

 sinus. Note the increase in the rate of this segment, which, in turn, 

 brings about a similar change in the other portions of the heart. 



6. Isolated Segments of the Heart. The fact that a heart when 

 separated from the central nervous system continues to beat, shows that 

 it is automatically active. Cut transversely across between the sinus 

 venosus and the auricles. The sinus continues to beat, whereas the 

 rest of the heart ceases to beat for a time. This proves that the auricles 

 and ventricle are ordinarily activated by a wave of excitation derived 

 from the sinus. After a time the severed portion of the heart develops 

 a beat of its own and continues to contract rhythmically. 



Sever the ventricle from the auricles. . It ceases to beat, but may be 

 made to contract at any time by stimulating it, for example, by pricking 

 it with the point of a scalpel. It usually executes several beats. 



Cut off the apex of the ventricle, but preserve its basal portion. 

 The former remains quiescent, but reacts promptly to all stimuli. 

 Since the apex is free from nervous elements, this experiment is usually 

 cited to prove that the heart-beat is of myogenic origin. 



7. All-or-none Law. Suspend this preparation of the apical portion 

 of the ventricle by means of two silk threads between the writing lever 

 and the plate. Apply the hand-electrodes to it and stimulate it suc- 

 cessively with single induction shocks of different strength. Do you 

 observe a difference in the amplitude of its contractions? How does 

 striated and non-striated muscle tissue behave under these circum- 

 stances? Inquire into the reason for this difference. 



8. Isolated Strips of Ventricle. Fasten one pole of the quiescent 

 basal portion of the ventricle to the hook of a weight resting upon the 

 bottom of a beaker, and its upper pole to the hook and string of a writ- 

 ing lever. Counterpoise, so that this preparation is under the least 

 possible tension. Pour a solution of 0.7 per cent, sodium chlorid into 

 the beaker until it fully covers this preparation. Wait until the latter 

 shows continued activity (twenty to thirty minutes), and record suc- 

 cessive series of contractions at intervals of ten minutes. When the 

 contractions have weakened, add a few drops of a 1 per cent, solution of 

 calcium chlorid to the saline. The calcium stimulates cardiac muscle 

 (systole), and hence the individual contractions should again assume 

 their former amplitude. 



In small doses potassium favors the relaxation of cardiac muscle, 

 while in larger doses it brings about a continued diastole. To show this 



