FUNCTIONS OF MUSCULAR COAT. 121 



size was exposed to the electric current, its diameter in from 

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

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

 rowing gradually increased, until the calibre of the tube be- 

 came 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 propelling 

 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 requirements of each, accord- 

 ing to various circumstances, but chiefly and most naturally, 

 according to the activity with which the functions of each 

 part are at different times performed. 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 di- 

 lating 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 equalizers stream, the smaller 

 arteries with muscular tissue add to these two purposes, that of 

 regulating and determining, according to its requirements, the 

 proportion of the whole quantity of blood which shall be dis- 

 tributed to each part. 



It must be remembered, however, that this regulating func- 

 tion of the arteries is itself governed and directed by the ner- 

 vous system. 



The muscular tissue of arteries is supplied with nerves 

 chiefly, if not entirely, by branches from the sympathetic sys- 

 tem. These so-called vasomotor nerves are again connected, 

 through the medium of ganglia, with the fibres from the sym- 



