THE HEART 53 



certain solutions and drugs upon the beating ventricle. If a frog's 

 ventricle be placed in a weak solution of caustic soda (1 in 20,000 

 of normal saline) it relaxes less and less between the beats and 

 eventually stands still in systole, whereas if lactic acid (1-10,000 

 normal saline) be used the contractions become less and less, so 

 that finally the ventricle stops in a state of complete relaxation. 

 Similar results to that with alkali can be obtained by such drugs 

 as digitalis and veratrine, and to that of acid with muscarine. 

 Moreover, acid solutions antagonise the effect of alkaline on the 

 same preparation, as does muscarine that of digitalis. The heart 

 muscle, therefore, would appear to be possessed of a certain degree 

 of tonicity which, like the other properties of cardiac muscle, 

 probably varies in the different parts of the heart. 



Another peculiar form of tonicity has been observed by Fano 

 (quoted by Gaskell) upon the heart of Emys Europaea. If this 

 heart be clamped in the A-V groove, the auricles exhibit a 

 rhythmical variation of tone. A similar phenomenon has been 

 seen by Botazzi in the frog's heart. It apparently does not occur 

 in many cold-blooded animals. At present a satisfactory explana- 

 tion is wanting. 



Lastly, the heart possesses the properties of automaticity, 

 rhythmicity, and stimulus conduction. Do these properties reside 

 in the heart muscle itself or in the nervous tissue abounding in 

 the heart ? This is the vexed question which we discuss in the 

 following section. 



E. THE SITE OF ORIGIN AND THE MODE OF CONDUCTION OP 

 THE EXCITATORY WAVE 



By the site of origin is meant the kind of tissue, muscular or 

 nervous, in which the heart impulse arises. A long controversy 

 has raged around this point, and much fruitful research has been 

 the outcome. It is necessary to keep clearly in mind the difference 

 between the site of origin of the excitatory wave and the mode 

 of its conduction. These are frequently confused, and experiments 

 in regard to the site of origin of the excitatory wave are quoted 

 as evidence as to its mode of conduction. Certainly it must be 

 granted that if the excitatory wave be found to arise in one form 

 of tissue, it is highly probable that it will also be conducted by 

 that tissue ; but it is not necessarily the case. There are several 



