128 A MANUAL OF PHYSIOLOGY 



ventricle discharges more than 105 grm. of blood per second, or 

 87 grm. (80 c.c.) per heart-beat with a pulse-rate of 72. Putting this 

 result along with that deduced from the circulation-time, we can 

 pretty safely conclude that the average amount of blood thrown out 

 by each ventricle at each beat is not more than 70 or 80 c.c. Zuntz, 

 from the quantity of oxygen absorbed by the blood in the lungs, has 

 estimated the output at 60 c.c. But according to him this may be 

 doubled during 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. In the 

 middle of the eighteenth century, Passavant calculated -the output 

 at 46*5 grm., which is certainly too low. Tigerstedt puts it at 50 to 

 100 c.c.. Plesch at 59 c.c. 



The Relation of the Nervous System to the Circulation. 



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. 



The Relation of the Heart to the Nervous System. A very 

 simple experiment is sufficient to prove that the beat of the 

 heart does not depend on its connection with the central nervous 

 system, for an excised frog's heart may, under favourable con- 

 ditions, of which the most important are a moderately low 

 temperature, the presence of oxygen and the prevention of 

 evaporation, continue to beat for days. The mammalian heart 

 also, after removal from the body, beats for a time, and indeed, 

 if defibrinated blood be artificially circulated through the coronary 

 vessels, for several or even many hours. But although this 



