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 2 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. 
Vv. 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,, 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.HPO, 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 
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