398 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



is a volume of blood the weight of which is 0.00027 of the weight of the 

 animal. 1 



A second way of attempting to ascertain the pulse- volume is to measure the 

 swelling and the shrinkage of the heart. This is called the " plethysruographic" 2 

 method. One application of it is as follows : The chest and pericardium of an 

 animal are opened, and the heart is inserted into a brass case full of oil. The 

 opening through which the great vessels pass is made water-tight by mechanical 

 means which do not impede the movement of the blood into and out of the 

 heart. The top of the brass case is prolonged into a tube, the oil in which 

 rises as the heart swells and falls as it shrinks. Upon the oil a light piston 

 moves up and down, and records its movements upon the kymograph. The 

 instrument is called a " cardiorneter." 3 



The average pulse-volume of the human ventricle has been very variously 

 estimated upon the basis of observations of various kinds made upon mam- 

 mals of various species. The figures offered range, in round numbers, from 

 50 to 190 cubic centimeters. If we assume the human pulse- volume to 

 weigh 100 grams, and the blood of a man who weighs 69 kilograms to weigh 

 5.308 kilograms, or T y of his body-weight, the pulse-volume will be about ^ 

 of the entire blood, and the entire blood will pass through the heart, from 

 the veins to the arteries, in only fifty-three beats that is, in less than one 

 minute. The speed with which a man may bleed to death if a great artery 

 be severed is therefore not surprising. 



The Work done by the Contracting Ventricles. Uncertain as is this 

 important quantity of the pulse-volume, the estimation of the work done by the 

 heart in maintaining the circulation must be based upon it, and upon the force 

 with which each ventricle ejects the pulse-volume. A small fraction of this 

 force is expended in imparting a certain velocity to the ejected blood ; all the 

 rest serves to overcome a number of opposing forces. The force exerted by 

 the muscular contraction is opposed by the weight of the volume ejected, and 

 by the strong arterial pressure, which resists the opening of the semilunar 

 valve and the ejection of the pulse-volume. Moreover, the elasticity of the 

 lungs tends at all times to dilate the ventricles, with a force which is increased 

 at each recurring contraction of the muscles of inspiration. Probably there is 

 also in the wall of the ventricle itself a slight elasticity which must be over- 

 come by the ventricle's own contraction in order that its cavity may be effaced. 

 The strong arterial pressure, with which the reader is already familiar, is by 

 far the greatest of these resisting forces in fact, is the only one of them 

 which is not of small importance in the present connection. 



Are we obliged to measure the force of the systole indirectly ? Can we not 

 ascertain it by direct experiment? Manometers of various kinds have been 

 placed in direct communication with the cavities of the ventricles. The fol- 



1 R. Tigerstedt: " Stud i en iiber die Blutvertheilung im Korper." Erste Abhandlung. 

 " Bestimmnng der von dem linken Herzen herausgetriebenen Blutmenge," Skandinavisches 

 Archiv fur Physiologic, 1891. iii. p. 145. 



* From irt.q0vafj.6s, enlargement. s C. S. Roy and J. G. Adami, op. cit. 



