CHAPTER VIII 



COORDINATION AND CONTROL: 

 (2) THE ENDOCRINE GLANDS 



We have just examined one of the two chief coordinating mechanisms 

 of the human body — the nervous system. The other is a system of chemical 

 coordination that depends for its functioning upon the circulation of the 

 blood. The agents are the hormones, or chemical messengers. These are 

 special substances, many of them proteins, which are carried in the blood 

 stream to all parts of the body and produce appropriate responses in the 

 proper tissues and organs. Numerous different hormones have already 

 been discovered. They are all produced in ductless glands that pour their 

 secretions directly into the blood that flows through them. Such glands, 

 lacking ducts, are called endocrine glands (Greek, endon, "within," and 

 krino, "to separate"), or glands of internal secretion. 



A good illustration of hormone action is furnished by secretin. As we 

 learned in Chap. IV, this is a substance produced by the mucosa cells 

 of the duodenum. Under the stimulus of the acid chyme from the stomach, 

 secretin is liberated into the capillaries of the intestinal wall and is carried 

 by the blood throughout the body. It has no effect upon most of the body 

 tissues and organs, but when it reaches the capillary network of the 

 pancreas, the liver, and the gall bladder, secretin 1 stimulates these organs 

 to immediate activity. The pancreas begins to pour its secretion into the 

 intestine, the liver and gall bladder to discharge bile, and the enzyme- 

 producing cells in the intestinal wall to liberate their products. 



Bayliss and Starling discovered the existence and function of secretin 

 in 1902. They had found that the intestine, liver, and pancreas continued 

 to liberate their secretions whenever food entered the intestine from the 

 stomach, even after every conceivable nervous connection had been 

 severed. They considered the possibility that the hydrochloric acid of 

 chyme, absorbed into the blood stream, might act as the stimulus respon- 

 sible, but they found that injecting dilute hydrochloric acid into the 

 blood produced no effect. When, however, they scraped mucosa from the 

 duodenal wall of an experimental animal, ground it up with sand and 



1 Here used in the wide sense, to include the substances that cause the gall bladder 

 to contract and the intestinal mucosa to secrete. 



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