THE CIRCULATION OF THE BLOOD 161 



25. The regulation of the outflow from the arterial reservoir; 

 arterial tone. Wound around the walls of the arterial tubes, 

 especially the smaller arteries (arterioles) which deliver blood 

 from the arterial reservoir to the organs, are peculiar muscle 

 fibers. Their contraction diminishes the size and bore of the 

 tube, and, when they relax, the tube and its lumen become 

 wider. As a usual thing these smaller arteries are kept some- 

 where midway between extreme constriction and extreme 

 dilation. On a day of moderate temperature, for example, 

 the arterioles of the skin are moderately narrowed by this 

 action of their muscle fibers. During colder weather these 

 fibers contract more than usual and so lessen the size of the 

 tube, while during warm weather they relax somewhat and 

 widen it; but ordinarily they are never contracted to their 

 utmost nor are they often completely relaxed. 



This condition of sustained activity of the arterial muscles 

 is known as arterial tone, and in general any sustained activity 

 of a living cell is spoken of as tonic activity, or tone. Since, 

 as we have seen, the total quantity of blood in the body is 

 not enough to fill completely and distend all the blood ves- 

 sels when they are widened to their utmost, it follows that 

 the maintenance of arterial tone is essential to that overfilling 

 of the great arteries which supplies the driving force for the 

 flow of blood through the organs. If every arteriole were to 

 lose its tone, blood would flow out of the reservoir more 

 rapidly than the heart could possibly pump it in ; we should 

 have somewhat the same condition of affairs as if, in our arti- 

 ficial model (p. 144), the small nozzle which affords resist- 

 ance to the outflow were removed. Arterial pressure would 

 fall and, the driving force being thus removed, the blood 

 would remain at rest in the capillaries and veins of the 

 organs; the circulation would cease because blood would 

 not return to the heart to be pumped. The maintenance 

 of arterial tone is consequently no less essential to the 

 circulation than is the beat of the heart itself. 



