318 ESSENTIALS OF PHYSIOLOGY. 



the boiled fluid till it is almost neutral, the proteins are precipitated, 

 and a protein-free nitrate may be obtained. Injection of the nitrate 

 into a blood-vessel of another animal excites the production of pancreatic 

 juice. From this experiment it is obvious that the hormone is not 

 destroyed by acid or by boiling ; it is, however, readily destroyed 

 by alkalies, Secretin is absorbed by the blood, directly, and does not 

 normally reach the lumen of the intestine ; and its introduction into 

 the duodenum does not lead to a flow of pancreatic juice. 



The epithelial cells of the intestinal wall form in the first instance 

 a precursor of secretin, called prosecretin. Secretin its.elf is freely 

 soluble in water and alcohol, but an extract of intestinal mucous 

 membrane made with either of these fluids contains neither secretin nor 

 prosecretin. The latter substance, therefore, is insoluble in water and 

 alcohol, and is converted into secretin on boiling with dilute hydrochloric 

 acid. 



Prosecretin is most abundant in the duodenum, it occurs also to a 

 considerable extent in the jejunum, and to a less degree in the ileum ; 

 but near the junction of the small and large intestines it is formed in 

 very small amount. 



The amount of secretin formed in the body, as shown by the volume 

 of pancreatic juice secreted, varies with the nature of the food. After 

 a meal of bread or meat the flow of juice is more abundant, and 

 reaches its maximum more rapidly, than after a meal of milk. The 

 reason of this difference is that meat or semi-digested bread stimulates 

 the production of gastric secretin and thus causes a large flow of gastric 

 juice, whereas milk is a less efficient stimulus to gastric secretion. As 

 a result, more acid reaches the duodenum after a meal of bread or meat 

 than after the ingestion of milk, and therefore more secretin is produced 

 in the former case than in the latter. On the other hand, the fatty 

 acids formed by the action of gastric lipase on milk are converted into 

 soaps in the duodenum. Soaps stimulate the production of secretin, 

 and the delayed maximal production of pancreatic juice after a meal of 

 milk may be explained by a second formation of secretin in this way. 



The Changes in the Pancreas which accompany Secretion. The 

 pancreas is a compound tubular gland, and it contains, in addition to 

 the ordinary secretory tubules, clumps of cells which do not stain 

 deeply with the ordinary dyes, and which are known as cell-islets 

 (fig. 130). These islets are supposed to be concerned with the formation 

 of an internal secretion, and their function will be discussed in con- 

 nection with metabolism. The secretory tubules of the pancreas are 

 lined by a single layer of columnar cells, each of which shows two 

 zon es an outer, which stains with basic dyes such as hsematoxylin or 



