152 AN AMERICAN TENT-BOOK OF PHYSIOLOGY. 



also accessary. Sodium chloride must bo present to preserve the osmotic 

 equilibrium between contractile tissue and surrounding liquid. As phrased 



by Loci), it may be assumed that the sodium, calcium, and potassium ions 

 must exist in definite proportions in the tissue which is expected to show 

 rhythmical activity. Only so long as these proportions are preserved does 

 the tissue possess such physical properties and such labile equilibrium as to 

 be capable of rhythmical processes or contractions. 



The Excitation-wave. — The change in form which constitutes what com- 

 monly is called the cardiac contraction is preceded by a change in electrical 

 potential, supposed to be a manifestation of the unknown process by which the 

 heart-muscle is excited to contract. Both the contraction and the electrical 

 change sweep over the heart in the form of waves, and it has become the cus- 

 tom to speak of the electrical change as the excitation-wave. It should not be 

 forgotten, however, that this usage rests merely on an assumption, for the real 

 nature of the excitation is still a mystery. The contraction-wave begins nor- 

 mally at the great veins, travels rapidly through the auricle, and, after a dis- 

 tinct interval, spreads through the ventricle. The excitation-wave, which pre- 

 cedes and is the cause of the contraction, probably takes the same course, 1 and 

 in fact it is possible to show that the change in electrical potential actually 

 begins under normal conditions at the great veins and passes thence over the 

 entire heart. But this sequence is not invariable. The ventricle under abnor- 

 mal conditions has been seen to contract before the auricle, the normal sequence 

 of great veins, auricle, and ventricle being reversed. 2 The energy of the ven- 

 tricular muscle-cell may, therefore, be discharged by an excitation arising 

 within the ventricle itself. Evidence of this is afforded also by the experi- 

 ment of Wooldridge, who isolated the ventricles by drawing a silk ligature 

 tightly about the auricles at their junction with the ventricles, completely 

 crushing the muscle and nerves of the auricle in the track of the ligature with- 

 out tearing through the more resistant pericardium. This experiment was 

 repeated the following year by Tigerstedt, who devised a special clamp for 

 crushiug the auricular tissues. Both observers found that the auricles and 

 ventricles continued to beat. The rhythm, however, was no longer the 

 same. The ventricular beat was slower than before and was independent of 

 the brat of the auricle. Thus the ventricle, no longer connected physiologically 

 with the auricle, develops a rhythm of its own, an idio-ventricular rhythm. It 

 seems improbable that the very small part of the auricular tissue which cannot 

 be included in Wooldridge's ligature for fear of closing the coronary arteries 

 should be able to maintain the ventricular contractions. 



Independent contraction is said to be secured by properly regulated excita- 

 tion of the cardiac end of the cut vagus nerve. Stimuli of one second duration 

 applied to the vagus at intervals of six to seven seconds arrest the auricles 

 completely, but do not stop the ventricles, except during the second of stimu- 

 lation. The ventricles, now dissociated from the auricles, beat with a rhythm 



1 Bottazzi : Lo sperimentale, 1 898, li. No. 2. 



1 Recently studied by En^elmann : An hiv fur die gesammte Physiologic, 1895, lxi. p. 275. 



