266 HUMAN PHYSIOLOGY 



the heart and show, by means of stimulation, that it is 

 possible through them to increase the force and fre- 

 quency of its beating. The effect is not very striking. 



Cardiac Regulation. Summary. The heart has an 

 automatic tendency, particularly localized in the 

 auricles, which would determine its rate in the absence 

 of external influences. But the ordinary rate is not that 

 'which the auricles would spontaneously determine. It 

 is considerably slower than this and results from a tonic 

 inhibitory action exerted by the medulla through certain 

 fibers in the vagus . nerves. This tonic action may be 

 abated, causing the heart to quicken for a time, and may 

 then be resumed. .Finally, there is provision for a 

 moderate acceleration of the heart by impulses follow- 

 ing definite nerve paths. It will be seen that it is hard 

 to be sure whether a heart which is quickening its beat 

 is exhibiting the withdrawal of the vagus effect or the 

 active intervention of the accelerators. 



Vasomotor Control. It has been said that the large 

 arteries are essentially elastic tubes while the smallest 

 branches of the arterial tree are well provided with 

 contractile elements. The swelling of large arteries 

 occurs when the pressure of the blood within has been 

 raised and need not be referred to any living character- 

 istic of these vessels. The finer subdivisions (arterioles) 

 may contract against a rising pressure or dilate when 

 the pressure is falling. These possibilities exist only 

 because the vessels in question are largely composed 

 of living, contractile tissue. We may compare the large 

 trunks of the arterial system with rubber tubes and not 

 be seriously misled, but we cannot extend the com- 

 parison to the arterioles. 



The smooth muscle of the arterioles is reached by 

 numerous nerve fibers. Under their influence its tone 

 may be increased or diminished. Such are called vaso- 

 motor fibers and we often speak of vasomotor nerves. 

 Strictly speaking, what we mean by a vasomotor nerve 

 is usually a nerve in which vasomotor fibers abound; 



