CHEMICAL AND MECHANICAL STIMULATION 165 



proportions in which they are present in those 

 liquids. Practically, . a 0.6 per cent solution of 

 sodium chloride has commonly been employed, 

 in the case of the frog. Such a solution is said 

 to be isotonic, i. e. neither giving nor taking 

 water from the tissue. That it is not perfectly 

 indifferent appears from this experiment. 



"Normal Saline." Allow a sartorius muscle 

 to stand half an hour in normal saline solution 

 (0.6 per cent NaCl). Kecord its contraction in 

 response to a maximal break induction current. 

 In place of a simple twitch, a prolonged contrac- 

 tion of abnormal height and duration will usually 

 be secured. 



Importance of Calcium. Place the " normal 

 saline " sartorius in 0.6 per cent sodium chloride 

 solution containing 10 per cent of saturated solu- 

 tion of calcium sulphate. After ten minutes 

 record the maximal b*eak contraction. 



The abnormal contraction will have disap- 

 peared. 



Constant Chemical Stimulation may cause Peri- 

 odic Contraction. Place a sartorius muscle in a 

 solution of 5 grams NaCl, 2 grams Na 2 HP0 4 , and 

 0.4 gram ISra 2 C0 3 in one litre of distilled water. 



Usually rhythmic contractions are seen. All 

 contractile substance shows a tendency to peri- 

 odic contractions in response to a constant stimu- 



