ENDOCRINE CO-ORDINATION 119 



metabolic processes the circulatory system acts as a channel 

 through which can diffuse the substances known as hormones. 

 Up to the present no clear evidence of the operation of internal 

 secretion has been demonstrated outside the vertebrate series. 

 Accordingly, to illustrate the nature of endocrine regulation, 

 one cannot select a better instance than the discovery of 

 BayHss and Starling (1902), who first gave conclusive proof of 

 the functional role of hormones in the animal body. 



Secretin. — By the end of the nineteenth century it had 

 been established that pancreatic secretion was not entirely 

 prevented by severance of the nervous connections of the 

 gut, though it was well recognised that the secretion of the 

 pancreatic juice followed the signal provided by the entry of 

 the gastric contents into the small intestine. Having found 

 that the introduction of acid alone into the denervated gut was 

 adequate to elicit activity of the secretory cells of the pancreas, 

 Bayliss and Starling injected into the circulation an aqueous 

 acid extract of the mucous lining of the duodenum, thereby 

 activating the pancreas. Subsequent experiments showed 

 that the liberation from the enteric mucosa of a soluble product, 

 called by these authors secretin^ provides the immediate 

 stimulus to pancreatic secretion. Carried by the blood-stream 

 to the resting gland, " secretin " possesses the property of 

 producing secretory activity in the pancreatic cells ; the pro- 

 duction and translocation of the hormone is a mechanism by 

 which pancreatic activity is co-ordinated with the entry of food 

 into the small intestine. 



Nature 0! Chemical Co-ordination.— With this example before 

 us .we may proceed to define what is meant by endocrine or 

 chemical co-ordination and the kind of evidence on which one 

 can rely for proof of its existence. The essential character- 

 istics of a hormone are illustrated by secretin, in that a hormone 

 may be defined as a substance set free in the body by the 

 activity of a localised organ and capable of evoking a specific 

 response in tissues remotely situated from its seat of origin. 

 In one minor respect, however, the production of secretin 

 differs from that of some other well-established cases of 

 internal secretion in that the hormone is not produced from a 



