SUPPORT, MOTION, AND SENSATION 339 



striated and nucleate, resembling skeletal muscles in being capable 

 of rapid, powerful contractions, but unlike other muscles by reason 

 of their regular automatic contraction and relaxation. 



Muscular Contractions. That muscle contraction is stimu- 

 lated by a nerve impulse in the living animal has long been proved, 

 but this is as far as we can at present safely go, for in seeking a physico- 

 chemical explanation of what actually happens within the cell itself 

 we are treading upon dangerous ground. At the present time there 

 does not appear to be an accepted theory that accounts completely 

 and satisfactorily for muscle contraction. 



Certain things, however, are definitely known. In the first place, 

 muscles shorten when they contract. Under the microscope, the light 

 and dark bands so readily seen in striated muscle appear to exchange 

 places. In reality, the light bands have become dark and the dark 

 ones light so that there has been no actual exchange of position but 

 only a change in physical make-up. Chemically, muscular action is 

 due to a series of complex chemical reactions which imdergo a number 

 of complicated changes, yielding in the end specific amounts of lactic 

 acid. It is known that the shortening of the muscle fibers occurs 

 before and independeyiily of the formation of the acid and therefore it is 

 difficult to believe that the two are unrelated. When muscular activ- 

 ity is prolonged, or when it is carried out under conditions implying a 

 lessened supply of oxygen, there is an accumulation of so-called waste 

 products, especially of lactic acid. According to Hill (1923) experi- 

 ments on man caused an increase of from 29 to 104 mg. of lactic acid 

 per 100 cc. of blood in the case of violent exercise carried on for one 

 and a half minutes. This large increase in acid has been interpreted 

 as meaning that the supply of oxygen to the contracting muscles was 

 inadequate. Even with increased respiration and circulation, lactic 

 acid accumulated in the muscles and was given off to the blood, thus 

 creating an "oxygen debt" to the muscles. This phenomenon is 

 associated with the condition of fatigue and has been studied in ath- 

 letes, especially track men, where it was found that an accumulation 

 of lactic acid hinders muscular relaxation. In races the intake of 

 oxygen is of course determined . by the efficiency of the lungs and 

 heart. In long distance running the athlete reaches an equilibrium 

 between his oxygen intake and lactic acid production. In short races 

 he may breathe but once or not at all and so builds up a large oxygen 

 debt. In such cases a state of exhaustion may be reached in a few 

 seconds. 



