THE CHEMICAL CHANGES IN MUSCLE 241 



respiratory exchanges of the animal are determined (viz. its oxygen 

 intake and its C0 2 output), and the urine and faeces are carefully 

 analysed, in order to judge of the action of muscular work on the 

 carbon and nitrogen metabolism of the body. 



By one or other of these methods it has been found that the main 

 products of muscular activity are the same as those which are pro- 

 duced during the death of a muscle, viz. sarcolactic acid and carbon 

 dioxide. It was shown long ago by Helraholtz that when a muscle was 

 tetanised to exhaustion, the total amount of its watery extractives 

 diminished, while the amount of its alcoholic extractives increased ; 

 and there is no doubt that part of this difference is due to the formation 

 of lactic acid. The souring of muscle during activity can be easily 

 demonstrated by stimulating the muscle for some time and then 

 crushing a fragment of the excised muscle on litmus paper. The 

 litmus is at once turned red. Or we may inject a solution of acid 

 fuchsin under the skin of a frog, and the next day expose a sciatic 

 nerve and stimulate it for fifteen or twenty minutes. On skinning the 

 hind-legs a difference in colour will be at once apparent, the leg which 

 has been active being of a deep rose colour, owing to the action of the 

 acid on the fuchsin. 



Sarcolactic acid is not present in a free state in muscle, the acidity being, 

 like that of urine, due to the presence of acid phosphates. The sarcolactic 

 acid can be extracted from the muscle by means of alcohol. It is generally 

 separated in the form of the zinc sarcolactate, by boiling its partially purified 

 solution with zinc carbonate. Its presence may be tested for by means of 

 Uffelmann's reagent, which is made by the addition of ferric chloride to dilute 

 carbolic acid. The purple solution thus produced is at once changed to yellow 

 by the addition of even traces of lactic acid. 



A much more definite colour reaction for lactic acid has been introduced 

 by Hopkins. The test is carried out in the following way. About 5 c.c. of 

 strong sulphuric acid are placed in a test-tube together with one drop of saturated 

 solution of copper sulphate, which serves to catalyse the oxidation that follows. 

 To this mixture a few drops of the solution to be tested are added, and the 

 whole well shaken. The test-tube is now placed in a beaker of boiling water for 

 one or two minutes. The tube is then cooled under a water-tap, and two or 

 three drops of a very dilute alcoholic solution of thiophens (ten to twenty drops 

 in 100 c.c.) are added from a pipette. The tube is replaced in the boiling water 

 and the contents immediately observed. If lactic acid is present the fluid 

 rapidly assumes a bright cherry red colour, which is only permanent if the 

 tube be cooled the moment after its appearance. 



We get a similar formation of lactic acid in excised mammalian 

 muscles which are kept alive by an artificial circulation. We do not 

 know how far the formation of lactic acid occurs under normal circum- 

 stances in the living body. At all events, if lactic acid is produced 

 by the muscle in any quantity during some phase of its activity in 

 the normal animal, the greater part is further transformed (to C0 2 ) 

 before it leaves the body. If actual dyspnoea is present during muscular 



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