CHAPTER XIII. 

 THE DUCTLESS GLANDS. 



Introductory. We have no more than touched the very 

 fringe of the subject of metabolism, and yet we have learned 

 enough to impress us with the fact that although it is extremely 

 complicated, it is nevertheless under perfect control. It remains 

 for us to learn something regarding the nature of this control. 



If we take such a metabolic process as that which carbohy- 

 drates undergo, we should expect that the conditions which deter- 

 mine whether glycogen shall be formed or broken down would 

 be chemical in nature. We should expect, in other words, that 

 some change in the chemical composition of the blood either its 

 reaction or the amount of sugar in it, or the appearance in it of 

 some decomposition product of sugar would determine whether 

 or not glycogen should be mobilized as sugar. In muscular work, 

 for example, sugar is required by the contracting muscles, and 

 we find that the glycogen stores in the liver become very quickly 

 depleted to meet the demand. The question is, how do the mus- 

 cles transmit their requirements to the liver so as to cause this 

 organ to mobilize the dextrose ? Our natural assumption would 

 be that the active muscles cause some change to occur in the 

 blood and that it is this change which excites the liver cells. 

 Such a control of the metabolic activities of one tissue by prod- 

 ucts of the activity of another, transmitted between them by 

 way of the blood, is known as hormone control. We have already 

 become acquainted with it in connection with the control of cer- 

 tain of the digestive glands, particularly the pancreas (see 

 p. 72), and it is no doubt very largely by such a mechanism 

 that a given metabolic process becomes active or supressed, as 

 occasion demands. 



The hormones in such cases are in part the interim-diary prod- 

 ucts of metabolism, but besides these hormones others must exist 



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