292 LIFE : OUTLINES OF GENERAL BIOLOGY 



of acid by the cells lining the duodenum, that is to say, the beginning 

 of the small intestine. It is carried away by the blood-stream and 

 passes in due course round the body. But it seems to have no effect 

 any^vhere except in the pancreas, and, to a slight extent, in the 

 liver. It increases the secretion ol pancreatic juice, the most potent 

 of digestive juices, and this passes by the pancreatic duct into the 

 thiodenum, there promoting digestion. There is also to a less degree 

 a stimulus to the secretion of bile. When a meal is in process, or it 

 ma\' be in prospect, there is by means of the hormone secretine 

 a preparation for its digestion. This illustrates physiological corre- 

 lation; the mucous membrane of the food-canal and the pancreas 

 work together in vital partnership for the digestive good. 



How is a conclusion of this kind established, .seeing that secretine 

 is an invisible substance that leaves no track? The hormone was 

 obtained by boiling the mucous membrane with dilute hydrochloric 

 acid and afterwards neutralising and filtering. The hormone obtained 

 in the filtrate was then injected into the blood, and this was followed 

 by increased production of pancreatic juice. It was also known 

 that washing the duodenum with dilute mineral acid would pro- 

 voke increased secretion of pancreatic juice — doubtless through 

 the increased production of secretine. It may be mentioned that 

 another hormone analogous to secretine, but acting on the gastric 

 glands of the stomach, has been extracted (Edkins, 1906) from the 

 stomach lining towards the pyloric end. 



But the hormones that have been most studied are those of the 

 thyroid gland, the suprarenal capsule, the pituitary body, and the 

 reproductive organs. Before we consider these and others separately 

 can we form any cleaer picture of hormones in general? 



(i) In most of the glands that get rid of what they produce by 

 a duct, or on a free surface, the secretions are of the nature of 

 ferments or enz\'Tnes. They are probably of a protein nature and 

 they are easily destroyed by hot water. But hormones or "internal 

 secretions" of ductless glands, whatever they may be, are not 

 enzymes; and for the most part they are not destroyed even by 

 boiling. They resemble enzymes in being able to act in minimal 

 quantities; but they differ from enzymes and resemble vegetable 

 drugs (e.g. alkaloids) in the rapidity with which they exert their 

 influence. 



(2) Hormones are dialysable, and some of them have been isolated 

 in crystalline form. Adrenalin, which is produced by characteristic 

 cell-groups in the centre of the suprarenal capsules, has been 

 artificially synthesiscd; and so has thyroxin, the iodine-containing 

 hormone of the th\Toid gland. Since hormones pass readily through 

 the walls of the blood-vessels they must have a low molecular 

 weight. Although it contains iodine, thyroxin (like adrenalin) is 

 related to tyrosin (see Chemistry of the Celt). It is probable, how- 



