354 PEACTICAL PHYSIOLOGY 



Xanthin is dioxypurin: 



HN C=0 



0=C C-NH 



HN C N^ 



Lastly trioxypurin, which occurs in muscle only in traces, is uric 

 acid, C 5 H 4 N 4 O 3 . (See Chapter XVIII. p. 422.) 



Hypoxanthin and xanthin result in part from the breakdown of the 

 nuclein present in the muscle, but their amount is normally so large 

 compared to the amount of nuclein present that this cannot be their 

 sole source ; the other source of supply is at present unknown. It is 

 probable that they normally give rise to uric acid, since it is found 

 that some time after muscular exercise the uric acid output of the 

 urine is considerably increased. (For the isolation of these bodies 

 from muscle extract see end of chapter.) 



Lactic Acid (C 3 H 6 3 ). This variety of lactic acid differs from that 

 obtained by the fermentation of lactose, which does not rotate the 

 plane of polarised light. The lactic acid of muscle, often termed 

 sarcolactic acid, rotates the plane of polarised light to the right. This 

 behaviour depends upon a difference in position of the various side 

 chains in relation to the central carbon atom. (Cf. carbohydrates, p. 280.) 

 The amount of lactic acid increases very much during the death of a 

 muscle, and also during muscular activity. These points can be shown 

 by the following experiments : 



(a) To some of Uffelmann's reagent (a mixture of ferric chloride and 

 carbolic acid) add some of the muscle extract provided. This probably 

 contains lactic acid from the dying muscle ; if it does the violet 

 colour of Uffelmann's reagent will be turned to yellow by the lactic 

 acid present. 



(b) Hopkins' Test for Lactic Acid. Take about 5 c.c. of strong sulphuric 

 acid in a dry test tube, add 1 or 2 drops of a solution of muscle 

 extract, 2 or 3 drops of saturated solution of copper sulphate. Warm 

 in boiling water for about two minutes ; cool and add a few drops of 

 alcoholic thiophene solution (20 minims in 100 c.c. alcohol), and warm 

 gently. With lactic acid a cherry red colour develops. 



(c) Take a pithed frog which has been kept on ice for half an hour. 

 Quickly cut off the muscles of one hind limb ; cut off the other limb at 

 the pelvic girdle, and stimulate electrically until irritability is nearly 

 lost. Cut off the muscles. Treat both sets of muscles as follows: 

 Grind with cold absolute alcohol and sand, filter, evaporate the alcohol, 

 dissolve in water, heat with a little animal charcoal, filter, evaporate, and 



