H4 THE ENGINES OF THE HUMAN BODY 



when a man is sitting quietly his left ventricle throws 

 about 3 litres (5 pints) of blood into the aorta every 

 minute ; a heart going at the rate of seventy beats a minute 

 and throwing out 42 c.c. (i| oz.) per stroke will accom- 

 plish this amount. If the man gets up and sets out 

 walking at four miles an hour, then the output of blood 

 becomes four times as much ; if he runs upstairs the 

 output becomes seven times as much — namely, 2 1 litres 

 (37 pints) per minute. The total amount of blood in 

 a man's body is about 8 litres (14 English pints), thus 

 every half-minute our left ventricle has forced the whole 

 of the blood in our body through the aorta once and 

 begun on a second turn when we take violent exercise. 

 But how is it that the muscles succeed in making their 

 wants known to the heart ? How do they compel the 

 heart to increase its output according to their needs ? 

 The muscular engines of the body are started and stopped 

 by higher centres in the brain — the seat of will, but the 

 will has no power over the heart ; the upper centres of 

 the brain which control the muscular engines of the body 

 have no power to start or stop the heart. The impulse 

 that sets the muscular engines of the heart in action arises 

 within the musculature itself. But although the heart 

 receives no nerves from the higher or voluntary centres 

 of the brain, it is most richly connected with a lower 

 centre or nerve exchange situated in the medulla. Into 

 this centre stream nerves from the heart itself — particu- 

 larly from the left ventricle ; and from the aorta, which 

 represents the compression chamber of the heart. By 

 means of these nerve paths messages are being conveyed 

 to the medullary centre which give information regarding 

 the state of tension in the pump and the head of pressure 

 in its compression chamber. From the medullary centre 

 or centres stream out at least two sets of nerve paths to 

 the heart, by means of which the strokes of the pump can 

 be increased in number or extent or both. By some 

 means which we do not rightly understand, the voluntary 

 muscles can influence the work of the heart through the 

 medullary nerve centres. But there must be another 



