CHAPTER XVII. 



THE REGULATION OF THE HEART AND BLOOD- 

 VESSELS BY THE NERVOUS SYSTEM. 



The Need of Co-ordination. Eor the safe and harmo- 

 nious working of the circulatory apparatus it is obviously 

 necessary that there be some mode of mutual interaction 

 between the heart and the blood-vessels: if the heart beat 

 and the arteries relaxed or contracted, each without any 

 reference to the other, no orderly capillary flow culd 

 be maintained. To secure such a flow, the work done 

 by the heart and the resistance offered in the vessels must 

 at any given moment be correlated; so that the heart shall 

 not by too powerful action over-distend or perhaps burst 

 the small arteries, nor the latter contract too much and so, 

 by increasing the peripheral resistance, raise the aortic pres- 

 sure to a great height and increase unduly the work to be 

 done by the left ventricle in forcing open the semilunar 

 valves. / Again, the total amount of blood in the Body is 

 not sufficient to keep all its organs supplied with the 

 amount needful for the full exercise of their activity at 

 one time, and in the Body accordingly we never find all 

 its parts hard at work at the same moment. If when one 

 group of muscles was set at work and needed an extra 

 blood-supply, this was attained merely by increasing the 

 heart's activity and keeping up a faster blood-flow every- 

 where through the Body, there would be a clear waste of 

 force, much as if the chandeliers in a house were so ar- 

 ranged that when a larger flame was wanted at one burner 

 it could only be obtained by turning more gas on at all the 

 rest at the same time; besides the big tap at the gas-meter 

 regulating the general supply of the house, local taps at 



