126 The Vertebrate Organ Systems 



ments, it can be shown that the nerve also is not fatigued ; thus fatigue 

 must occur at some other site. The point at which it occurs has been 

 shown to be the junction between the nerve and muscle fibers; that is 

 the point where the muscle fibers are activated by the nerve. Prolonged 

 stimulation of the muscle itself will, naturally, eventually result in fatigue. 

 This appears to be due to the accumulation of lactic acid. 



The Effect of Training. — It is well known that exercise increases 

 the size of the muscles involved. This is due not to any increase in the 

 number of fibers, but to an increase in the size of the individual fibers. 

 This additional size is apparently due to a larger amount of sarcoplasm 

 between the myofibrils. 



In the trained individual, muscular efficiency is greater due in part, 

 at least, to the increased efficiency of the circulatory and respiratory 

 adjustments. The trained individual also tends to have fewer wasted 

 motions and better muscular coordination. 



ELECTRIC ORGANS 



It has been pointed out that muscles produce measurable amounts 

 of electrical current during their normal contractions. This particular 

 property has been capitalized upon by a few rather unique fish : the 

 electric eel of northern South America, the electric catfish of the Nile, 

 some rays, and a few other small forms. 



The electric eel is a large sluggish fish, which may attain 8 feet in 

 length and weigh nearly 40 or 50 pounds. It lives in fresh-water streams, 

 and utilizes these electric shocks for stunning or killing its prey. Up 

 to 500 volts have been recorded from this animal, and it is said that 

 horses have been killed when crossing streams where they live. The 

 electric organ of this eel occupies about four-fifths of the body. It is a 

 series of modified muscles arranged in layers in a manner more or less 

 resembling the alternating plates in a storage battery. 



