138 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1934 



mitted through the circulation as to coordinate by chemical trans- 

 port the activities of the body as a whole. Unification by chemical 

 means must today be recognized as a fundamental aspect of all 

 such organisms. In all of them it is true that the nervous system 

 has pride of place as the highest seat of organizing influence, but 

 we know today that even this influence is often, if not always, 

 exerted through properties inherent in chemical molecules. It is 

 indeed most significant for my general theme to realize that when 

 a nerve impulse reaches a tissue the sudden production of a definite 

 chemical substance at the nerve ending may be essential to the re- 

 sponse of that tissue to the impulse. It is a familiar circumstance 

 that when an impulse passes to the heart by way of the vagus nerve 

 fibers the beat is slowed, or, by a stronger beat, arrested. That is, 

 of course, part of the normal control of the heart's action. Now 

 it has been shown that whenever the heart receives vagus impulses 

 the substance acetylcholin is liberated within the organ. To this 

 fact is added the further fact that, in the absence of the vagus in- 

 fluence, the artificial injection of minute graded doses of acetyl 

 choline so acts upon the heart as to reproduce in every detail the 

 effects of graded stimulation of the nerve. Moreover, evidence is 

 accumulating to show that in the case of other nerves belonging to 

 the same morphological group as the vagus, but supplying other 

 tissues, this same liberation of acetyl choline accompanies activity, 

 and the chemical action of this substance upon such tissues again 

 produces effects identical with those observed when the nerves are 

 stimulated. More may be claimed. The functions of another group 

 of nerves are opposed to those of the vagus group; impulses, for 

 instance, through certain fibers accelerate the heart beat. Again a 

 chemical substance is liberated at the endings of such nerves, and 

 this substance has itself the property of accelerating the heart. "We 

 find then that such organs and tissues respond only indirectly to 

 whatever nonspecific physical change may reach the nerve ending. 

 Their direct response is to the influence of particular molecules with 

 an essential structure when these intrude into their chemical 

 machinery. 



It follows that the effect of a given nerve stimulus may not be 

 confined to the tissue which it first reaches. There may be humeral 

 transmissions of its effect, because the liberated substance enters 

 the lymph and blood. This again may assist the coordination of 

 events in the tissues. 



From substances produced temporarily and locally and by virtue 

 of their chemical properties translating for the tissues the messages 

 of nerves, we may pass logically to consideration of those active sub- 

 stances which carry chemical messages from organ to organ. Such 

 in the animal body are produced continuously in specialized organs. 



