§4.111 DIGESTIVE GLANDS 119 



active substances, but whether any one of these is a true hormone 

 can only be tested by physiological experiments. Further "it must 

 be remembered that even when an endocrine gland has been 

 identified as producing a hormone, and an active extract has been 

 obtained, no proof has been given in the case of gut hormones, 

 that the extract contains the unaltered physiological hormone". 



Hydrochloric acid secretion by stomach glands 



Mammalia. Although secretin was the earliest of the gut hor- 

 mones to be identified, gastrin wdll be taken first, as it is the first 

 to act when food passes down the alimentary tract and reaches the 

 pyloric end of the stomach. A flow of acid from oxyntic cells and 

 pepsin from the gastric glands in the fundus or cardiac region of 

 the stomach is stimulated by the release of gastrin into the 

 circulation from cells situated in the mucosa lining the pyloric 

 region of the stomach (Fig. 4-1). Much evidence pointed in this 

 direction before the action of the hormone was conclusively 

 established in 1948 (Grossman et aL). Their proof lay in experi- 

 ments on dogs, and followed a classical pattern. Operations in two 

 stages provided animals with two stomach pouches separated from 

 the rest of the alimentary canal, devoid of nerve supply, and with 

 the original blood supply replaced from the vascular tissue (e.g., 

 mammary gland) into which the pouch is grafted in a subcutaneous 

 position. There the changes in secretion within the pouch can be 

 measured by collection through a fistula to the exterior (at 4, Fig. 

 4-2). Whether it was the fundus or the pylorus of the stomach 

 w^hich was moved in this way, the results were essentially the 

 same. When a small balloon w^as inserted into the pyloric pouch 

 and inflated, an increased secretion from the fundus pouch and a 

 marked increase in its acidity followed (Fig. 4-3). Since there was 

 no nerve connection, transmission of stimulus from one part to the 

 other must have been through the circulation. It was observed that 

 the reduction in blood flow, following the severing of the original 

 blood supply to the cutaneous graft, reduced the response. At the 

 same time, the fact that the stimulus to the pyloric region was only 

 mechanical distension ruled out the possibility that the humoral 

 agent carried in the blood was a secretagogue, derived directly or 

 indirectly from the digestion of food in the stomach. This evidence 



