66 SCIENTIST 



sleepy. ) A night or two later he again had the same dream. 

 Taking no chances this time, he arose immediately, went 

 to the laboratory, and performed the experiment which was 

 to bring him a Nobel Prize some fifteen years later. First 

 he arranged an artificial circulation to and from the heart 

 of a frog so that he could collect the fluid that emerged 

 from it. He then took this fluid and placed it in a second 

 heart and recorded its rate. The fluid coflected from heart 

 A under normal circumstances resulted in no change in 

 heart B. Fluid taken from heart A during stimulation of 

 its vagus nerve, however, resulted in marked slowing of 

 heart B. 



The way was now clear for other workers of less imagi- 

 nation but comparable skill to demonstrate that this was 

 not an isolated phenomenon, unique to frogs and vagus 

 nerves, but common to a wide variety of other nerves and 

 other species. It would take a very large book indeed even 

 to outline all the experiments that have been done to show 

 that the vagus nerve in all species, both warm- and cold- 

 blooded, does indeed liberate acetylcholine; that not only 

 the vagus nerve but other nerves of the same class which 

 supply the blood vessels, the gut, the bladder, and a number 

 of other organs do the same thing; and that certain other 

 nerves which have opposite effects hberate another set of 

 substances. 



A httle later it was shown that the nerves to the muscles 

 which move the body also work through acetylchoUne, and 

 that the transmission of nerve impulses from cell to cell in 

 some parts of the nervous system also involves the same 

 important chemical. It now seems likely that there are a 

 number of different "chemical mediators," each one of 

 which may have a particular sort of function in the nervous 

 system. Forty years after Loewi the hunt is still on to ex- 



