198 



THE OUTPUT OF THE HEART. 



[BOOK i. 



from I. flows back by \v to the right auricle, while the blood from the 

 carotid flows into II. by 2a. When a certain quantity has escaped 

 into II., the action is reversed, and I. is once more rilled ; and so on. 



FIG. 43. DIAGRAM OF STOLNIKOW'S APPARATUS. 



In this way the quantity of blood which the heart delivers, its ' output ' 

 during a given time can be measured ; the quantity discharged at a 

 single beat can similarly be determined. By means of recording floats 

 in I. and II., a graphic record of the output may also be obtained. 



The other methods are plethysmographic ( 104) in nature. The 

 volume of the heart changes only with the volume of its contents, 

 for we may neglect, in the first instance at least, as insignificant the 

 changes of volume due to changes in the amount of blood held by the 

 coronary system, and we may wholly neglect the changes of volume due 

 to changes in the quantity of lymph present in the cardiac tissues. 

 An increase in the volume of the heart means that more blood is flowing 

 into it than is leaving it, a decrease that more is leaving it than is 

 flowing into it. Hence, if we measure the diminution of volume which 

 takes place during the systole, this gives us the volume of blood dis- 

 charged by the two ventricles during that systole, the effect of changes 

 in the auricles being neglected ; and since the two ventricles discharge 

 equal quantities, half this will give us the quantity of blood discharged 

 by the left ventricle during the systole. 



In the method of Tigerstedt and others the pericardial cavity is 



