SEC. 4. CHANGES IN THE BEAT OF THE HEART. 



We have already discussed the more purely mechanical pheno- 

 mena of the heart. We have therefore in the present section only 

 to inquire into the nature and working of the mechanism (chiefly 

 at least nervous) by which the beat of the heart is maintained, 

 varied, and regulated. 



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, <fec.) 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 may be applied to the mam- 

 malian heart, but difficulties are introduced by the locomotion of the 

 heart caused by the movements of the lungs. 



2. Or, as in GaskelTs 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 intracardiac 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 

 force of the ventricular systole is brought to bear on the manometer, 



