254 THE HUMAN BODY. 



called an "internal bleeding," due to the accumulation of so 

 much blood in the vessels of the abdomen that not enough is 

 left over for the supply of the brain and other parts. 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 should be pro- 

 vided merely by increasing the heart's activity and keeping 

 up a faster blood-flow everywhere throughout the Body, 

 there would be a clear waste much as if the chandeliers 

 in a house were so arranged that when a larger flame was 

 wanted at one burner it could only be obtained by turn- 

 ing 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 each burner are required which regulate 

 the gas-supply to each flame independently of the others. A 

 corresponding arrangement is found in the Body. Certain 

 nerves control the calibre of the arteries supplying different 

 organs and, when the latter are set at work, cause their arte- 

 ries to dilate and so increase the amount of blood flowing 

 through them, while the general circulation elsewhere re- 

 mains practically unaffected. The resting parts at any mo- 

 ment thus get enough blood to maintain their healthy nutri- 

 tion and the working parts get the larger quantity required to 

 make good used-up material and to wash out wastes : as certain 

 organs come to rest and others are set in activity, the arteries 

 of the former narrow and of the latter dilate; in this way the 

 distribution of the blood in the Body is undergoing constant 

 changes, parts which at one time contain much blood at an- 

 other having but little. In addition, then, to nervous organs 

 regulating the work of the heart and the arteries with refer- 

 ence to one another, we have to consider another set of vascu- 

 lar nerves which govern the local blood-supply of different 

 regions of the Body. How important this is may be illus- 

 trated by considering what happens when the surface of the 

 Body is exposed for some time to cold. The skin normally 

 contains much blood, brought to it in part to be cooled ; but 

 under the supposed conditions the loss of heat would soon be 

 so great as to be harmful did not small arteries of the skin 

 contract, as is indicated by its pallor, and thus lessen the 

 blood-flow through it. This contraction is not chiefly, if at 

 all, due to direct action of the cold on the vessels, but to the 

 stimulation of cutaneous afferent nerves which excite a nerve- 



