194 



LIFE S BEGINNING ON THE EARTH 



the effect is to surround the wire near one end with a narrow tube. (Fig. 

 76.) This end is then charged in the usual manner. Owing to the narrow 

 tube around it, it remains charged and continuously sends traveling charges 

 along the wire at regular intervals. 



Fig. 76. Set-up for Generating Rhythmic Discharges 



The upper end of the wire is enclosed in a narrow glass tube. When 

 this end is charged as described in Figure 75, the end enclosed in the tube 

 remains charged and continuously sends out rhythmic waves. One wave 

 follows another at regular intervals. 



This is an example of a "pacemaker." A similar mechanism exists in 

 the upper part of the heart where a bundle of nerve fibers connected with 

 the sinus node sends out impulses eighty times a minute. p]ach impulse 

 initiates one regular contraction of the heart. It is remarkable that with a 

 simple iron wire immersed in acid and surrounded by a glass-tube a regular 

 sequence of traveling electrical waves can be produced which resemble 

 closely the automatic impulses by which the heart is kept in motion. 



9. THE PRODUCTION OF ELECTRIC CHARGES AND OF ELECTRIC 

 CURRENTS IN LIVING TISSUE 



We find valuable clues as to the nature of the nerve im- 

 pulse by comparing the nerve-fiber with a battery plate 

 drawn out to a wire, but there must also be distinctions. 

 Ordinary commercial batteries work by means of (heir 

 metallic plates, but nerves and other living tissues contain 



