704 STUDIES IN GENERAL PHYSIOLOGY 
nerve behaves in exactly the opposite way from the muscle. 
The latter contracts when taken out of the solution and ex- 
posed to the air, and relaxes when put back into the solution. 
If the nerve alone (without the muscle) be put into the 
solution, contractions of the muscles occur while the nerve is 
in the solution, and partial or complete relaxation is observed 
when the nerve is taken out. 
These experiments on the nerve give one the impression 
that the sodium-citrate solution and the solutions of the 
other sodium salts whose anions precipitate calcium stimu- 
late the nerve chemically. Albert Mathews has recently 
found that weak solutions of sodium salts can cause con- 
tractions of the muscle when the nerve alone is put into the 
solution, while the salts of the other metals can only produce 
contractions when their osmotic pressure is considerably 
higher than that of the tissues. I have confined my experi- 
ments chiefly to those sodium salts whose anions precipitate 
calcium. But I think I can show definitely that these salts 
are not the direct stimulus that calls forth the contractions 
of the muscle, but play only an indirect rdle, inasmuch as 
they make the nerve more sensitive for another kind of 
stimulus, either a mechanical- or a contact-stimulus. When 
the nerve alone has been put into a sodium-citrate solution 
(of 1 gram-molecule in about 10 liters) and the muscle has 
begun to contract powerfully, a gradual relaxation of the 
muscle is observed when the nerve is taken out of the solu- 
tion and allowed to hang in the air. But at any time the 
contractions and the final tetanus of the muscle will begin 
again when the nerve is brought into contact with any solid 
or liquid body, no matter whether it is a conductor or a non- 
conductor. As soon as the contact ceases and the nerve is 
surrounded by air again on all sides the muscle gradually 
relaxes. This can be repeated quite often with the same 
result. Among the substances whose contact causes con- 
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