15 



Cnemical Regulation— 

 Endocrine Glanas 



Chemical Coordination 



The bodily activities of living organisms are so coordinated that every plant 

 and animal acts as a unit. Their chemical coordination is carried on mainly by 

 hormones, substances that are moved from one part of the body to another, 

 like messages in letters (Fig. 15.1). Contrasting with this, their nervous co- 

 ordination is achieved by cells with long processes over which changes (im- 

 pulses) move rapidly from one end to the other, like messages over a telegraph 

 wire. The relations of the endocrine and nervous systems are complex and 

 intimate. 



Hormones are usually concerned with gradual changes in the body: growth, 

 whether to usual or to dwarf or giant size, whether to normal form and sym- 

 metry or misshapen; the rate of metabolism, whether oxidation is rapid and 

 temperature high or vice versa; the reproductive functions, those of the sex 

 cells and the structures connected with them, and of the animal as a whole. 

 Together the nervous and endocrine systems carry on a cooperative enterprise, 

 creating in the body an internal environment that is sensitive and adjustable 

 to the world outside. 



Nature and Importance of Hormones 



Hormones are chemical compounds that activate, maintain, or depress the 

 functions of particular parts or the whole of an organism; they are liberated 

 directly into the blood often functioning far away from their point of origin. 

 The name hormone (Gr., hormon, exciting) was first used in 1903 by the 

 British physiologists, Bayliss and Starling, who applied it to a secretion of cer- 

 tain cells in the intestinal wall. Since then it has appeared that the action of 

 some hormones is depressing, while some others under certain circumstances, 

 excite activity and under others depress it. 



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