REGULATION OF ENVIRONMENT 73 



The heart itself could not act as the prime regu- 

 lator of the general circulation rate without pro- 

 ducing great variations in the arterial blood pressure, 

 so as to drive the blood at varying rates through 

 the resistance of the arterioles. These great variations 

 do not normally exist, as is easily shown by measure- 

 ments of the blood pressure. Nor would primary 

 regulation of the blood flow by the heart be of much 

 use, since any regulation brought about in this way 

 would apply to all parts of the body alike, whereas the 

 increased or diminished requirements for blood are 

 purely local, according as one part or another of the 

 body is in a state of greater or less functional activity. 



The heart is known to be provided with two sets of 

 nerve fibres through which its action is controlled, and 

 which reach it as branches of the vagus and the sym- 

 pathetic nerves. The vagus fibres, when excited, exer- 

 cise an inhibitory action, reducing both the frequency 

 and the strength of the heart beats. The very signifi- 

 cant discovery of this inhibitory action was made 

 known by the brothers Weber in 1845. Excitation of 

 the sympathetic fibres, discovered by von Bezold in 

 1862, increases the frequency and strength of the 

 heart beat. 



The inhibitory influence of the vagus fibres is at 

 once increased reflexly if the blood pressure in the 

 aorta (the great artery leaving the heart) rises, and 

 diminished if it falls. As an additional preventive to 

 excessive arterial blood pressure there is a further 

 nervous connection through which excessive rise of 

 blood pressure causes reflex dilation of the arteries 



