THE STORY OF NINETEENTH-CENTURY SCIENCE 



In reality, however, they had learned only half the 

 story, as Bernard himself proved only a few years later 

 by opening up a new and quite unsuspected chapter. 

 While experimenting in 1858 he discovered that there 

 are certain nerves supplying the heart which, if stimu- 

 lated, cause that organ to relax and cease beating. As 

 the heart is essentially nothing more than an aggrega- 

 tion of muscles, this phenomenon was utterly puzzling 

 and without precedent in the experience of physi- 

 ologists. An impulse travelling along a motor nerve 

 had been supposed to be able to cause a muscular con- 

 traction and to do nothing else; yet here such an im- 

 pulse had exactly the opposite effect. The only tenable 

 explanation seemed to be that this particular impulse 

 must arrest or inhibit the action of the impulses that 

 ordinarily cause the heart muscles to contract. But the 

 idea of such inhibition of one impulse by another was 

 utterly novel, and at first difficult to comprehend. 

 Gradually, however, the idea took its place in the cur- 

 rent knowledge of nerve physiology, and in time it came 

 to be understood that what happens in the case of the 

 heart nerve-supply is only a particular case under a very 

 general, indeed universal, form of nervous action. Grow- 

 ing out of Bernard's initial discovery came the final un- 

 derstanding that the entire nervous system is a mechan- 

 ism of centres subordinate and centres superior, the 

 action of the one of which may be counteracted and 

 annulled in effect by the action of the other. This ap- 

 plies not merely to such physical processes as heart- 

 beats and arterial contraction and relaxing, but to the 

 most intricate functionings which have their counterpart 

 in psychical processes as well. Thus the observation of 

 the inhibition of the heart's action by a nervous impulse 



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