152 THE CIRCULATION. 



five to ten seconds, became one-third less, and the area of 

 its section about one-half. On continuing the stimulus, 

 the narrowing gradually increased, until the calibre of the 

 tube became from three to six times smaller than it was at 

 first, so that only a single row of blood- corpuscles could 

 pass along it at once ; and eventually the vessel was closed 

 and the current of blood arrested. 



With regard to the purpose served by the muscular coat of 

 the arteries, there appears no sufficient reason for supposing 

 that it assists, to more than a very small degree, in pro- 

 pelling the onward current of blood. Its most important 

 office is that of regulating the quantity of blood to be 

 received by each part, and of adjusting it to the require- 

 ments of each, according to various circumstances, but 

 chiefly and most naturally, according to the activity with 

 which the functions of each part are at different times per. 

 formed. The amount of work done by each organ of the 

 body varies at different times, and the variations often 

 quickly succeed each other, so that, as in the brain for 

 example, during sleep and waking, within the same hour 

 a part may be now very active and then inactive. In all 

 its active exercise of function, such a part requires a larger 

 supply of blood than is sufficient for it during the times 

 when it is comparatively inactive. It is evident that the 

 heart cannot regulate the supply to each part at different 

 periods, neither could this be regulated by any general 

 and uniform contraction of the arteries ; but it may be 

 regulated by the power which the arteries of each part 

 have, in their muscular tissue, of contracting so as to 

 diminish, and of passively dilating or yielding so as to 

 permit an increase of, the supply of blood, according as 

 the requirements of the part may demand. And thus, 

 while the ventricles of the heart determine the total 

 quantity of blood to be sent onwards at each contraction, 

 and the force of its propulsion, and while the large and 

 merely elastic arteries distribute it and equalise its stream, 



