PRACTICAL EXERCISES 819 



muscle-curve is obtained. The commutator is now arranged for stimu- 

 lation of the central end of the nerve, and another muscle-curve taken. 

 Vertical lines are drawn through the points where the two curves just 

 begin to separate out from the abscissa line. The interval between 

 these lines corresponds to the time taken by the nerve-impulse to travel 

 along the nerve from the central to the distal pair of electrodes. Its 

 value in time is given by the tracing of the tuning-fork. The length of 

 the nerve between the two pairs of electrodes is now carefully measured 

 with a scale divided in millimetres, and the velocity calculated (p. 793). 

 17. Chemistry of Muscle. Mince up some muscle from the hind-legs 

 of a dog or rabbit (used in some of the other experiments), of which 

 the bloodvessels have been washed out by injecting 0*9 per cent, salt 

 solution through a cannula tied into the abdominal aorta until the 

 washings are no longer tinged with blood. To some of the minced 

 muscle add twenty times its bulk of distilled water, to another portion 

 ten times its bulk of a 5 per cent, solution of magnesium sulphate. 

 Let stand, with frequent stirring, for twenty-four hours. Then strain 

 through several folds of linen, press out the residue, and filter through 

 paper, (i) With the filtrate of the watery extract make the following 

 observations : 



(a) Reaction. To litmus-paper acid. 



(b) Determine the temperatures at which coagulation of the various 

 proteins in the extract takes place, according to the method described 

 on p. 9.* Put some of the water}' extract in the test-tube, and heat 

 the bath, stirring the water in the beakers occasionally with a feather. 

 Note at what temperature a coagulum first forms. It will be about 

 47 C. Filter this off, and again heat; another coagulum will form at 

 56 to 58. Filter, and heat the filtrate; a third slight coagulum may 

 be formed at 60 to 65 C., but this represents merely a residue of the 

 myosinogen which was left in solution at the previous heating. A 

 fourth precipitate (of serum-albumin) will come down at 70 to 73. 

 Saturate some of the watery extract with magnesium sulphate ; a large 

 precipitate will be formed, showing the presence of a considerable 

 amount of globulin. Filter off the precipitate and heat the filtrate; 

 coagulation will again occur at very much the same temperatures as 

 before, although the total amount of precipitate will be less. Note in 

 particular that there is still some precipitate at 47 to 50. Paramyo- 

 sinogen possesses some of the characters of both globulins and albumins, 

 for it is partially but not entirely precipitated by saturation with 

 magnesium sulphate, and is not precipitated by sodium chloride. 



(2) (a) Test the reaction of the magnesium sulphate extract. It 

 will usually be faintly acid to litmus. 



(b) Heat some of it. Precipitates will be obtained at the same tem- 

 peratures as in (i) (b), but those at 47 to 50 and 56 to 58 will be 

 more abundant. Of the two, that at 47 to 50 will usually be the 

 larger when time is given for it to come down and the heating is gradual. 



(c) Dilute some of the magnesium sulphate extract with three times, 

 another portion with four times, and another with five times, its volume 

 of water in a test-tube, and put in a bath at 40 C. Coagulation or 



* It should be remembered that the temperature of heat-coagulation of 

 any substance is by no means an absolute constant. It depends on the 

 reaction, the proportion and kind of neutral salts present, perhaps on the 

 strength of the protein solution and the manner of heating. A solution oi 

 egg-albumin, e.g., can be coagulated at a temperature much below 70 when 

 it is heated for a week. Small differences in the temperature of heat-coagula- 

 tion, unless supported by well-marked chemical reactions, are not enough 

 to characterize protein substances as chemical individuals. 



