CO-ORDINATED MUSCULAR CONTRACTIONS 



491 



nervous system. Such motor impulses may arise through reflexes, through 

 automatic activity of the nerve center, or by higher cerebral origin associated 

 with conscious psychic effort. In either case the apparatus consists of one 

 or more central neurones, an anterior-horn motor cell, and the muscle itself. 

 Conscious or voluntary effort may be taken as a type. 



Simple contractions are possible to human muscles, but undoubtedly 

 tetanic contractions are the rule. If one holds the arm out at right angles to 



FIG. 331. a, Frog's gastrocnemius muscle stimulated with four induction shocks per 

 second, showing complete relaxation between stimuli; b, same muscle stimulated eight times 

 p^r second, showing partial relaxation between stimuli (incomplete tetanus); c, same 

 muscle stimulated twelve times per second, showing development of an almost complete 

 tetanus. 



the trunk, the movement requires the continuous or tetanic contraction of the 

 deltoid and the series of extensor muscles. If the arm is retained in the 

 extended position long enough, extreme fatigue is felt and presently one can 

 no longer maintain the position. Yet, if the muscles involved are immediately 

 stimulated directly with an electric current, they contract, showing that such 

 exhaustion as exists is not wholly due to the muscle. 



Mosso's ergograph was devised for the specific purpose of studying the 

 character of fatigue of voluntary effort. This apparatus is adapted to the 



