THE MECHANICAL RESPONSE OF MUSCLE 235 



stimulation will recover. The recovery is hastened by passing a 

 stream of blood, or even of salt solution, through the blood-vessels 

 of the muscle. Recovery in a muscle outside the body is never 

 complete. 



The phenomena of fatigue probably depend on two factors : 



(1) The consumption of the contractile material or the substances 

 available for the supply of potential energy to this material. 



(2) The accumulation of waste products of contraction. Among 

 these waste products the lactic acid is probably of great importance. 

 Fatigue may be artificially induced in a muscle by ' feeding ' it with 

 a dilute solution of lactic acid, and again removed by washing- 

 out the muscle with normal saline solution containing a small per- 

 centage of alkali. After a certain time the mere removal of waste 

 products by means of an artificial circulation of salt solution becomes 

 inadequate to restore contractile power to the muscle. In this case 

 the muscle can be made to contract once more by supplying it with 

 fresh food material, as by the circulation of serum or diluted blood. 



THE ACTION OF SALTS 



The action of sodium salts on muscle is of considerable interest. 

 We are accustomed to use a 0-6 per cent, solution of NaCl as a ' normal 

 fluid ' to keep muscle preparations moist. If, however, the solution 

 be made with distilled water, it has a distinctly excitatory effect 

 upon the muscle, so that single induction shocks may cause tetani- 

 form contractions. The same excitatory effect is still better marked 

 with solutions of Na 2 C0 3 . If a thin muscle, such as a frog's 

 sartorius. be immersed in a solution containing 0-5 per cent. Nad, 

 0-2 per cent. Na 2 HP0 4 , and 0-04 per cent. Na 2 C0 3 (Biedermann's 

 fluid), the muscle enters into a series of frequent contractions, so that 

 it may wriggle from side to side, or may even ' beat ' for a time with 

 the regularity of heart-muscle, though at a much greater rate. 



This excitatory action of sodium salts is neutralised by the addition 

 of traces of calcium salts. Hence the normal saline used in the labora- 

 tory should always be made with tap water, containing calcium salts. 



Potassium salts, although forming so important a constituent of 

 the ash of muscle, act as muscle poisons, quickly and permanently 

 destroying its irritability. If a muscle be transfused with normal 

 fluids containing minute traces of potassium salts, it at once shows 

 all the signs of fatigue, signs which may be removed by washing out 

 the potassium salts by means of 0-6 per cent. NaCl solution. It is 

 possible that the setting free of potassium salts may be one of the 

 factors involved in the development of the normal fatigue of muscle. 



