7 o8 PRINCIPLES OF GENERAL PHYSIOLOGY 



and Starling (1902, 1, pp. 336, 337) showed that the fall of blood pressure 

 could be prevented by extraction of the raucous membrane with absolute alcohol 

 before boiling with acid, and that acid extracts of desquamated epithelial cells 

 of the intestine had no action in depressing the blood pressure, although very 

 active on the pancreatic secretion. Recently Launoy and Oeschlin (1913) have 

 confirmed this conclusion in a completely convincing manner. Although the 

 substance acting on the pancreas is soluble in 90 per cent, alcohol, it is insoluble 

 in absolute alcohol. Thus, if concentrated aqueous solutions of secretin, prepared 

 in the usual way by the action of hydrochloric acid on the duodenal mucous 

 membrane, are poured into excess of absolute alcohol, a precipitate is obtained. 

 This process, several times repeated, results in the production of a white powder, 

 easily soluble in water, insoluble in absolute alcohol. As Fig. 251 shows, it ha* 

 a powerful secretory action but no depressor action on the blood pressure. On 

 the other hand, the alcoholic mother-liquors, concentrated, give a powder of 

 yellowish colour, also soluble in water, which has a powerful effect on the blood 

 pressure, together with a very small one on the secretion, no doubt due to small 

 amounts of secretin left unprecipitated. From the work of Dale and Laidlaw 

 (1910), it seems more than probable that the fall of blood pressure is due to 

 /i-iminazolylethylamine. 



As yet we have no definite knowledge of the chemical nature of secretin. It is 

 evidently an intensely powerful substance, but does not appear to have a very 

 complex structure, since it is diffusible through parchment paper. It is, naturally, 

 incapable of acting as an antigen, since the production of an anti-body in the 

 blood would be antagonistic to its function. The statement also applies to other 

 hormones. It was suggested by Bayliss and Starling that secretin is produced 

 by the action of acid on a precursor in the cells of the mucous membrane. To this 

 supposed precursor the name " prosecretin " was given. A certain amount of 

 discussion has since taken place as to whether secretin itself is not present in the 

 cells. The work of Stepp (1912) shows that it may occasionally be present in 

 small quantities, so that mere extraction with boiling water is sometimes 

 sufficient to obtain solutions of active secretin, as was indeed found by Bayliss 

 and Starling (1902, 1, p. 340). In most cases no such effect was obtained. In 

 certain of these cases it was found that the slightly alkaline opalescent solution, 

 obtained by boiling and filtering, contained a substance from which, by boiling 

 with acid, an active secretin was obtained. These results indicate that the cells 

 usually contain a precursor, but it is not surprising to find that it should some- 

 times happen that the secretin produced by action of acid, etc., on these cells has 

 not completely passed away into the blood stream. Stepp also comes to the 

 conclusion that, however prepared, secretin is one and the same substance. He 

 gives a method by which a permanent dry preparation can be obtained, which is 

 similar to that of Launoy and Oeschlin, but giving a better yield by the use of 

 ether to precipitate. 



The method of preparation of very active solutions, worked out by Dale and 

 Laidlaw (1912, 1), depends on the fact, that mercuric chloride precipitates 

 secretin as a mercury compound, soluble in dilute acids, insoluble in neutral or 

 weakly alkaline reaction ; it may, however, merely be held in adsorption by 

 a substance having these properties. In this method, a large amount of impurity 

 is stopped at the outset. It was easy to obtain a preparation of which 1 c.c. 

 produced 8-5 c.c. of juice. 



We may next consider briefly some results obtained by Lalou (1912, 1). 

 Bayliss and Starling noted the fact that solutions of secretin introduced into 

 the lumen of the gut failed to excite the pancreas. Hence the agents causing 

 the production of secretin, when they are introduced into this cavity, must act 

 directly on the cells and, at the same time, enable the secretin to pass into the 

 blood vessels. Lalou calls attention to the fact that various agents, such as 

 saccharose, urea, etc., produce secretin by action on the mucous membrane in vitro, 

 but do not excite pancreatic secretion in the living animal. It has not been 

 shown as yet, however, that such agents, which destroy the cells in vitro, really 

 produce secretin in them ia the living state, so that it seems to me that the 



