ADVANCED EXPERIMENTAL PHYSIOLOGY 303 



to a myograph lever arid stimulate it with a single maximal break 

 shock. The contraction recorded on the drum will be no longer an 

 ordinary single contraction, but a series of tetanic twitches of abnormal 

 height and duration. Now remove the muscle, immerse it for ten 

 minutes in the solution containing the added calcium salt, and again 

 record its response to the same stimulus. A normal single contraction 

 will be obtained. It is clear that sodium salts, when acting alone on 

 skeletal muscle, have a powerful stimulatory effect, and that this can be 

 neutralised by adding a certain proportion of calcium salt. For this 

 reason ' normal ' saline solution is always made with tap-water instead 

 of with distilled water. Some tap-waters, however, do not contain 

 nearly enough calcium to bring about complete neutralisation of the 

 sodium salt. 



From the above experiments we learn certain facts of considerable 

 practical importance. We see that tissues are greatly affected by 

 changes in the osmotic pressure of the fluid surrounding them. Care 

 must therefore be taken not to expose the tissues of an animal or man 

 to fluids which are not isotonic with the blood-plasma. In man the 

 solution of NaCl isotonic with the blood-plasma is only just under 1 per 

 cent., and therefore differs widely in strength from the solution for a 

 frog ; it is very necessary to bear this in mind when injecting fluid into 

 veins or under the skin, and when irrigating the peritoneal cavity 

 during operations. We further see that, when isotonic solutions of 

 electrolytes are used, the tissues are by no means indifferent to the ions 

 in solution. A really ' normal ' saline solution would, therefore, be one 

 which contained the same salts in the same proportion as the animal's 

 own blood-plasma. Einger's l fluid is an attempt to make such a solu- 

 tion for the frog. Since in man it would often be difficult to obtain such 

 a solution when wanted, it might be preferable, instead of using an 

 imperfectly ' normal ' saline solution, to use an isotonic solution of a 

 non-conductor, such as dextrose. A 5*8 per cent, solution of dextrose 

 is isotonic with human blood-plasma. 



In all the above experiments it has been found that skeletal muscle 

 responds to the abnormal constant stimulus by an activity which is not 

 constant, but intermittent or rhythmical. This raises the question 

 whether the rhythmical contraction of the heart may not be the normal 

 response of that particular kind of muscle to the constant chemical 

 stimulus of the blood-plasma, and the same might be also partly true of 

 the rhythmical activity of the respiratory and vasomotor centres. 



1 A modified Ringer's solution contains NaCl '7 per cent., CaCl 2 '0026 per cent., 

 and KC1 '035 per cent. 



