210 PHYSIOLOGY 



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, 



FIG. 71. A. Tracing of the contraction of a frog's sartorius, poisoned with veratrin, 

 in response to a momentary stimulus. The time-marking indicates seconds. 



B. Tetanic contraction of normal sartorius in response to rapidly interrupted 

 stimuli. (The duration of the stimulus is indicated by the words ' on ' and 

 ' off.') It will be noticed that the two curves are practically identical. (Miss 

 BUCHANAN.) 



so that single induction shocks may cause tetaniform 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 con- 

 taining 0-5 per cent. NaCl, 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 regu- 

 larity 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 laboratory should always be 

 Excitation ma & Q w ^h tap water, containing 

 calcium salts. 



FIG. 72. Tracing of the contraction of a . Potassium salts, although form- 



muscle poisoned by the injection of a strong ing SO important a constituent of 



solution of veratrin, showing the double ^ ^ f mil snip apr a mii^rlp 



contraction due to unequal poisoning of l ' 16 ' a 



different fibres. (BIEDERMANN.) poisons, quickly and permanently 



destroying its irritability. If a 



muscle be transfused with normal fluids containing minute traces of potas- 

 sium 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 percent. 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. 



