140 THE CIRCULATION OF THE BLOOD AND LYMPH 



severe muscular work, when, as a matter of fact, by the aid of the 

 Rontgen-rays or by percussion of the chest, the volume of the heart 

 may be shown to be considerably increased. Tigerstedt, on the basis 

 of stromuhr measurements in animals, puts the ventricular output per 

 beat in man at 50 to 100 c.c. ; Plesch, on the basis of gasometric ob- 

 servations on man, at 59 c.c. Recently Krogh, using a gasometric 

 method based on the absorption of nitrous oxide gas in the lungs, found 

 that the minute volume during rest may vary between wide limits 

 (2-8 to 8'7 litres of blood per minute, corresponding, with a pulse-rate 

 of 70, to 40 c.c. to 120 c.c. per beat). During muscular work there is 

 a great and immediate increase, up to, it may be, 21-6 litres per minute. 

 These great Variations in the output of the ventricle depend primarily 

 upon variations in the rate of return of the blood to the heart by the 

 veins. According to Henderson, however, such great variations in the 

 output per beat as are postulated by the majority of physiologists who 

 have worked at the subject do not occur, and the fundamental variable 

 is the rate of the beat. 



In healthy persons in whom the pulse-rate is permanently much 

 below the normal (p. 107) the output of the ventricle per beat must, of 

 course, be correspondingly increased. In a man with a pulse -rate 

 always below 40 during rest in the sitting position, the flow in the hands 

 was found to be normal in amount, and all the signs of a normal delivery 

 of blood from the left ventricle were present. Here the output per 

 beat must have been twice the usual amount during rest. 



SECTION IV. THE HE ART- BE AT IN ITS PHYSIOLOGICAL 

 RELATIONS. 



So far we have been considering the circulation as a purely 

 physical problem. We have spoken of the action of the heart as 

 that of a force-pump, and perhaps to a small extent that of a suction- 

 pump too. We have spoken of the bloodvessels as a system of more 

 or less elastic tubes through which the blood is propelled. We have 

 spoken of the resistance which the blood experiences and the pressure 

 which it exerts in this system of tubes, and we have considered the 

 causes of this resistance, the interpretation of this pressure, and the 

 physical changes in the vascular system that may lead to variations 

 of both. But so far we have not at all, or only incidentally and very 

 briefly, dealt with the physiological mechanism through which the 

 physical changes are brought about. We have now to see that, 

 although the heart is a pump, it is a living pump ; that, although the 

 vascular system is an arrangement of tubes, these tubes are alive; 

 and that both. heart and vessels are kept constantly in the most 

 delicate poise and balance by impulses passing from the central 

 nervous system along the nerves. 



In many respects, and notably as regards the influence of nerves 

 on it, we may look upon the heart as an expanded, thickened and 

 rhythmically-contractile bloodvessel, so that an account of its 

 innervation may fitly precede the description of vaso-motor action 

 in general. 



