198 



COMPARATIVE PHYSIOLOaY. 



the whole heing very suggestive of a galvanic battery) closely 

 resembles a muscle-nerve preparation or its equivalent in the 



normal body. The electric 

 organs experience fatigue ; 

 have a latent period ; their dis- 

 charge is tetanic (interrupted) ; 

 is excited by mechanical, ther- 

 mal, or electrical stimuli ; and 

 the effectiveness of the organs 

 is heightened by elevation of 

 temperature, and the reverse 

 by cooling, etc. 



MUSCULAR WORK. 



If during a given period 

 one of two persons raises a 

 weight through the same 

 height but twice as frequent- 

 ly as the other, it is plain that 

 he does twice the work ; from 

 such a case we may deduce the 

 rule for calculating work, viz., 

 to multiply the weight and 

 height together. 



The effectiveness of a given 

 muscle must, of course, depend 

 on the degree to which it shortens, which is from one half to 

 three fifths of its length ; and the number of fibers it contains 

 — i. e., upon its length and the area of its cross-section, taking 

 into account in connection with the first factor the arrangement 

 of the fibers ; those muscles in which the fibers run longitudinal- 

 ly being capable of the greatest total shortening. 



There is, as shown by actual experimental trial, a relation 

 between the work done and the load to be lifted. With double 

 the weight the contraction may be as great as at first, or even 

 greater ; but a limit is soon reached beyond which contraction 

 is impossible. This principle may be stated thus : The contrac- 

 tion is a function of the stimulus, and is illustrated by the 

 diagram below (Fig. 183). 



It has been shown experimentally that the chemical inter- 

 changes in a muscle, acting against a considerable resistance, 



Pig. 182.— The electi'ic-fieh torpedo, dissect- 

 ed to show electric apparatus (Huxley); 

 d, branchiee; c, brain; e, electric organ; 

 g, cranium; me, spinal cord; n, nerves 

 to pectoral fins ; nl, nervi laterales ; 

 np, branches of pneumogastric nerves 

 to electric organs; o, eye. 



