706 STUDIES IN GENERAL PHYSIOLOGY 



nerve is taken out of the sugar solution and brought into 

 contact with solid bodies, it gives rise to stronger contrac- 

 tions. But, as was to be expected, the nerve loses this irri- 

 tability again when put into | NaCl or Na citrate solution. 

 In such a solution water will enter the muscle and restore 

 the original condition, and only later will the entrance of 

 citrate ions show its effect. 



It now remains to be seen how far these facts can throw 

 light upon the heart-beat. The fact that a heart which has 

 ceased to beat in a solution often begins to beat again when 

 taken out of the solution reminds us of the contact-reaction 

 of muscle described above. 



V. SUMMARY 



1. Certain salt solutions (1 gram-molecule in 8 or 10 

 liters) bring about an apparently new form of irritability in 

 muscles, which may be called provisionally contact-irrita- 

 bility. A muscle that has been treated in this way will 

 contract powerfully when it passes from the salt solution to 

 air, CO 2 , oil, sugar solution, etc., or from glycerin solutions, 

 sugar solutions to air. 



2. The salts whose solutions produce this form of irrita- 

 bility are (with one exception) sodium salts, whose anions 

 are liable to precipitate calcium, namely: 



Sodium fluoride Na 2 HP0 4 Sodium citrate 



Sodium carbonate Sodium oxalate Sodium tartrate 



3. If the nerve alone (without the muscle) be put into 

 one of these salt solutions (1 gram-molecule in 8 or 10 liters), 

 the muscle begins to twitch in about five minutes and finally 

 goes into tetanus. If the nerve be taken out of the solu- 

 tions, the contractions cease. Although this seems to indi- 

 cate that the salts or their ions stimulate the nerve directly, 

 it can be shown that they only modify or increase the irrita- 

 bility of the nerve. For when the same nerve is brought 



