222 CIRCULATION IN THE ARTERIES. 



left ventricle of the heart by arteries and capil- 

 laries, and returned by the veins to the opposite 

 and right side of the heart, and again enters the 

 systemic circulation. Beside elasticity, arteries 

 are endowed with a greater or less degree of 

 contractility, by means of which they can narrow 

 their calibre. 



" Tonicity, or the tonic state, is no doubt a 

 species of contraction, as well as the more con- 

 spicuous and powerful action with which it alter- 

 nates ; but it is employed merely to maintain 

 equilibrium, not to cause motion, continuing 

 during sleep, when volition is in abeyance, and 

 occasioning no fatigue. When the nerves are 

 cut it ceases, and the muscles become flaccid." ' 



There is also, so to speak, a continuous mus- 

 cular as well as vascular current, caused by a 

 great number of contractions repeated at very 

 short intervals, and also excited by periodic con- 

 traction. Contractions caused by strychnine have 

 been known to follow each other with such ra- 

 pidity as to disrupt a muscular fibre, showing 

 the marvellous force of molecular action. 



Wollaston describes a remarkable sound which 

 is heard when the ear is applied over a muscle 

 during its action. Roget supposed this " susur- 

 rus," as he names the sound, was caused by a 

 sort of peristaltic motion of the fibrils. He sup- 

 posed, also, that the oscillations of the fibrils and 



1 Leidy's Quain, vol. i. p. 328. 



