CHAPTER 13 



THE ENDOCRINE SYSTEM 



There are two great coordinating systems within the body whose 

 function is to integrate the action of all the various parts into a unified 

 whole. One of these systems, the nervous, which exercises control by 

 means of neurons, has already been considered. The other is the 

 endocrine which exercises its control by means of chemical secretions. 

 The endocrine system is composed of glandular masses of tissue scat- 

 tered throughout the body. These particular glands are ductless, that 

 is, they empty their products directly into the blood stream. In 

 contrast to them are the exocrine glands, such as those of the digestive 

 system, which deliver their products to a specific point by means of 

 ducts. The products of the endocrine glands are known as hormones, 

 a term derived from the Greek w^ord hormaein meaning to excite. Al- 

 though this is not an entirely correct name — for often enough the hor- 

 mones actually act as depressants — it does emphasize their powerful 

 efifects. As these hormones are exceedingly potent, a very small 

 amount may cause tremendous effect upon the physiology of the cells. 



Since the products of the endocrine glands are carried in the 

 blood stream, they come in contact with all parts of the body. This 

 is in striking contrast to the glands with ducts which deliver their prod- 

 ucts directly to the part to be affected. Some endocrines are very 

 general in their action, affecting all cells of the body; others are quite 

 specific. Thus thyroxin, elaborated by the thyroid, exercises control 

 over the metabolism of every cell of the body, while secretin, produced 

 in the wall of the intestine, affects only the cells of the pancreas and 

 liver. 



While a large body of knowledge about many of these glands has 

 been accumulated, their relationships are so complex that much re- 

 mains to be learned. In fact there are some endocrine glands so poorly 

 understood that their secretion or function is not at all certain. The 



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