1918] Bovard: Nervous Impidses in the Earthworm 131 



Transmission by Reinforced Stimuli 



There is one other point of great importance in the analysis of 

 locomotion in the earthworms and one which has not been heretofore 

 mentioned. This is the variability in the rate of the impulse along 

 the cord. Experiments have shown that the transmissions over short 

 distances are much faster than those over longer distances, and this 

 agrees with a phenomenon easily observable in the movements of 

 worms, i.e., the dying out of waves of contraction. One can watch a 

 Avave of contraction start down the length of the worm and become 

 more and more feeble until it is lost at the middle region. The distance 

 the wave runs seems to depend on the force of the wave at the start. 

 A strong wave runs further than one with a weak start. A glance back 

 at the charts of the speeds of impulses passing through the etherized 

 portion of a worm will show that there is a great variability. One 

 has but to observe a single worm under the experimental conditions 

 to become convinced of this without the figures. 



Any theory that accounts for locomotion must take into considera- 

 tion the short unit system of the nervous system, the transmission of 

 locomotor impulses over long sections of the cord, and the variability 

 in rate of the speed of these impulses. 



Friendlander (1894) likened the locomotor mechanism to a system 

 of telegraphic relays. Each contraction of the circular muscle elongated 

 the segment and stretched the longitudinal muscle. This stretching 

 caused a stimulus to pass along the nerves to the cord, where a reflex 

 gave a contraction of the longitudinal muscle. The contraction of the 

 longitudinal gave the pull which caused the circular muscle to contract 

 and so on down the length of the worm, each segment with its own 

 reflex, but progression of the wave of contraction due to the pull of 

 contracting parts on succeeding segments. 



A short unit nervous system is all that is necessary for such an 

 explanation. But when transmission of locomotor impulses can pass 

 along the cord this relay system in each segment is not sufficient. If, 

 however, we suppose that the association fibers transfer stimuli from 

 one ganglion to the next, then we have a means for explaining Bieder- 

 mann's experiment. One of the characteristics of this transmission 

 was that it varied considerably in rate. When the worm was in an 

 excited state or stimulated, the impulses passed through an etherized 

 section faster than otherwise. If we suppose that with each contrac- 

 tion reflexes are set up in each segment and that these stimuli entering 

 the cord reinforce the locomotor stimuli passing along in the short 



