280 GRAPHIC RECORD OF HEART-BEAT. [BOOK i. 



The Development of the Normal Beat. 



154. The heart of a mammal or of a warm-blooded animal 

 generally ceases to beat within a few minutes after being removed 

 from the body in the ordinary way, the hearts of newly born 

 animals continuing however to beat for a longer time than those 

 of adults. Hence, though by special precautions and by means of 

 an artificial circulation of blood, an isolated mammalian heart may 

 be preserved in a pulsating condition for a much longer time, our 

 knowledge of the exact nature and of the causes of the cardiac 

 beat is as yet very largely based on the study of the hearts of 

 cold-blooded animals, which will continue to beat for hours, or 

 under favourable circumstances even for days, after they have 

 been removed from the body with only ordinary care. We have 

 reason to think that the mechanism by which the beat is carried 

 on varies in some of its secondary features in different kinds of 

 animals : that the hearts, for instance, of the eel, the snake, the 

 tortoise and the frog, differ in some minor details of behaviour, 

 both from each other and from the bird and the mammal ; but 

 we may, at first at all events, take the heart of the frog as 

 illustrating the main and important truths concerning the causes 

 and mechanism of the beat. 



In studying closely the phenomena of the beat of the heart it 

 becomes necessary to obtain a graphic record of various movements. 



1. In the frog or other cold-blooded animal, a light lever may be 

 placed directly on the ventricle (or on an auricle, <fcc.) and changes of 

 form, due either to distension by the influx of blood, or to the systole, 

 will cause movements of the lever, which may be recorded on a 

 travelling surface. The same method as we have seen may be applied 

 to the mammalian heart. 



2. Or, as in Gaskell's method, the heart may be fixed by a clamp 

 carefully adjusted round the auriculo- ventricular groove while the apex 

 of the ventricle and some portion of one auricle are attached by threads 

 to horizontal levers placed respectively above and below the heart. 

 The auricle and the ventricle each in its systole pulls at the lever 

 attached to it ; and the times and extent of the contractions may thus 

 be recorded. 



3. A record of endo-cardiac pressure may be taken in the frog or 

 tortoise, as in the mammal, by means of an appropriate manometer. 

 And in these animals at all events it is easy to keep up an artificial 

 circulation. A cannula is introduced into the sinus venosus and another 

 into the ventricle through the aorta. Serum or dilute blood (or any 

 other fluid which it may be desired to employ) is driven by moderate 

 pressure through the former; to the latter is attached a tube connected 

 by means of a side piece with a small mercury manometer. So long 

 as the exit-tube is open at the end, fluid flows freely through the 

 heart and apparatus. Upon closing the exit-tube at its far end, the 



