THE NERVOUS SYSTEM 249 



low level. Variations of the oxygen of the blood affect its 

 action surprisingly little. 



An important work of the nervous system and one which 

 is quite overlooked by the layman consists in the regula- 

 tion of blood-flow. In our previous description of the 

 plan of the circulation only its mechanical principles were 

 discussed : the heart was spoken of as a force-pump and the 

 vessels as elastic tubes. This is a proper presentation so 

 far as it goes, but we must now proceed to show that the 

 whole circulatory apparatus is living, and being so has the 

 biologic characteristic of adaptation. The changes l>y 

 which the ever-varying requirements of the body are met 

 are of two kinds, changes in the force and frequency of the 

 heart-beat and changes in the caliber of the small arteries 

 and veins. The former are brought about by the cardiac 

 nerves; the latter, by a department of the nervous mechan- 

 ism which we call the vasomotor system. 



It is a matter of familiar experience that the heart-rate 

 is subject to striking variations. From an average of per- 

 haps 65 per minute, when one is lying in comfortable re- 

 laxation, it can be driven to 150 or more, when one is 

 hurrying up a grade. Such a change under the influence of 

 muscular activity is evidently purposeful. The working 

 tissues need a swifter current of blood to supply them with 

 oxygen and to bear away their waste. The lungs must be 

 visited at shorter intervals to keep normal the gaseous com- 

 position of the blood. Muscular activity is not to be sup- 

 ported by increased breathing alone ; it is equally necessary 

 that there shall be acceleration of the circulation. As the 

 quick, deep breathing of one who is taking exercise be- 

 speaks a governing center for the muscles employed, so 

 the rapid beating of his heart suggests a central control of 

 that organ. 



Experimental proof of the central regulation of the heart 

 is ample and detailed. Branches of the nervous system 

 reach it from two distinct sources and are contrasted in 

 their effect upon it. One set of fibers is said to be inhib- 

 itory, the other to have an accelerator action. The terms 



