PHYSICAL SUPPORT AND MOVEMENT 99 



may be prevented by certain poisons, and the muscle still be able to 

 contract. It seems necessary to conclude that the energy comes from 

 decomposition of organic phosphates; such phosphates are known to 

 release energy with almost explosive speed when they are decomposed. 

 The organic phosphates must be reconstituted, ready for the next con- 

 traction, and the energy for this reconstitution comes from oxidations. 

 The oxidations are thus accessory phenomena; instead of furnishing the 

 energ}^ for the contraction itself, they provide for the restoration of the 

 phosphates, and the latter on decomposition furnish the energy for 

 contraction. 



The mechanism of the contraction itself is probably the sudden 

 folding of long protein molecules arranged lengthwise in the myofibrils. 

 Since the most abundant protein in muscle is myosin, this may be the 

 responsible agent. Myosin extracted from muscle exercises a strong 

 catalytic action on the decomposition of organic phosphates, and this 

 action may be a part of the contraction process. 



A muscle in which there is no more organic phosphate nor glycogen, 

 and in which much lactic acid has been accumulated, is incapable of 

 contraction; it is "fatigued." In living animals as distinguished from 

 laboratory preparations, however, the common source of fatigue is 

 not in the muscle itself, but between the muscle fibers and the nerve 

 which delivers the commands to contract. Some substance there, at 

 the junction of nerve with muscle, experiences a change in response 

 to repeated stimulation such that it no longer transmits the stimulus 

 or does so more weakly. The nerve fiber still conducts, and the muscle 

 is still able to contract. The nature of the failure of the junction is not 

 known. 



References 



Carlson, A. J., and V. Johnson. The Machinery of the Body. University of 

 Chicago Press. (Pp. 345-360.) 



Mitchell, P. H. A Textbook of General Physiology. McGraw-Hill Book Com- 

 pany, Inc. 3d Ed. (Chap. II, muscle contraction.) 



