PERIODIC MOLECULAR MOVEMENT. 223 



the accompanying sound are constant, but that 

 they are greatly increased during the contraction 

 of the muscle. 1 



There is, we may infer, a similar periodic and 

 continuous excitation transmitted through the 

 nerve-fibres, though it may not have been de- 

 tected on account of the delicacy of the molecular 

 arrangement. 



" The cilia, or hair-like processes on the epithe- 

 lium, execute a lashing motion when not acting 

 very briskly, but when in a state of very rapid 

 excitation their motion is like that of the waving 

 of a field of wheat in the wind, or of swiftly run- 

 ning water. The undulation or, as it may be, 

 the current always moves in the same direction 

 in the same parts. The impulse which the cilia 

 communicate to the fluids, or other matter in 

 contact, maintains a continuity of motion and 

 direction. Thus in the wind-pipe the mucus is 

 conveyed always upward toward the larynx." 



We may assume then, from the continuity of 

 this periodic molecular movement (which is al- 

 ways in one direction in the same parts), that 

 the periodic electric excitation, having its prime 

 impulse in the heart, is transmitted in a closed 

 circuit through the vascular, nervous, and mus- 

 cular systems of the human body ; and that the 

 molecular vibration excited in each individual 

 organ is distinctively its own. 



1 Leidy's Quain, vol. i. pp. 226, 227. 



