CHAPTER 30 



The Endocrine System 



The integration of the activities of tlie several parts of tlie higher, 

 more complex animals has been achieved by the evolution of two major 

 coordinating systems, the nervous system, discussed in the previous 

 chapter, and the endocrine system. The nerves and sense organs enable 

 an animal to adapt very rapidly— with responses measured in millisec- 

 onds—to changes in the environment. The swift responses of muscles 

 and glands are typically under nervous control. The glands of the 

 endocrine system secrete substances called hormones which diffuse or 

 are transported by the blood stream to other parts of the body and 

 coordinate their activities. The responses under endocrine control are 

 generally somewhat slower— measured in minutes, hours or weeks— but 

 longer lasting than those under nervous control. The long-range ad- 

 justments of metabolism, growth and reproduction are typically under 

 endocrine control. 



Endocrine glands secrete their products into the blood stream, 

 rather than into a duct leading to the exterior of the body or to one 

 of the internal organs as do exocrine glands, and hence are called duct- 

 less glands or glands of internal secretion. The pancreas is an example 

 of a gland with both endocrine and exocrine functions, for it secretes 

 enzymes which pass via the pancreatic duct to the duodenum and it 

 secretes hormones which are transported to other parts of the body in 

 the blood stream. In the toadfish the two parts of the pancreas are 

 anatomically separate. 



The term "hormone" was originated in 1905 by the British physi- 

 ologist E. H. Starling, who was studying the control of the exocrine 

 function of the pancreas by secretin, a substance produced in the duo- 

 denal mucosa. Starling defined a hormone as "any substance normally 

 produced in the cells in some part of the body and carried by the 

 blood stream to distant parts, which it affects for the good of the body 

 as a whole." Our rapidly increasing knowledge of the many different 

 hormones produced by both vertebrate and invertebrate animals and 

 by plants has led to the generalization that these are special chemical 

 substances, produced by some restricted region of an organism, which 

 diffuse, or are transported by the blood stream, to another region of 

 the organism, where they are effective in very low concentrations in 

 regulating and coordinating the activities of the cells. 



The hormones isolated and characterized to date have proved to 



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