CHAPTER XX. 

 SECRETION AND EXCRETION. 



(See also Chapter XIII, The Enzymes.) 



The Glands. 



THE lymph and the blood current discharge into the glands, 

 where they are modified in a specific manner. The result of this 

 selective action is the secretion which pours from the ducts of the 

 various glands. 



The cause of secretion is one of the most debated of physiolog- 

 ical questions. None of the blood colloids are found in the secre- 

 tions, but the crystalloids, which are also contained in the blood, 

 occur in abundance. The freezing point depression gives us an idea 

 of the total crystalloid content. This shows that the crystalloid 

 content (expressed in osmotic pressure) of the saliva is always lower 

 than that of the blood; the osmotic pressure of gastric juice and bile 

 may equal that of the blood; milk has approximately the same 

 osmotic pressure as blood and fluctuates with it in this respect. 

 Sweat and especially urine, on the contrary, may either have a lower 

 osmotic pressure than the blood or exceed it. If secretion were only 

 an ultrafiltration of the blood through the gland filters, it would be 

 incomprehensible how an excretion could occur with a lower osmotic 

 pressure than the blood, or with one that is higher, as sweat and 

 urine. The conditions are especially complicated by reason of the 

 fact that the relative crystalloid content of various secretions may 

 be absolutely different from what they are in the blood plasma. 

 Thus, for instance, milk contains from 4 to 5 per cent milk sugar, 

 whereas the blood shows only from 0.08 to 0.12 per cent grape sugar; 

 urine contains disproportionately more urea than other salts, whereas 

 the amount of urea in the blood is entirely insignificant. We thus 

 find, besides producing specific substances (bile, pepsin, ptyalin, etc.), 

 that the individual glands have a specific activity in relation to the 

 lymph crystalloids, which some biologists even to-day prefer to assign 

 to the inexplicable physiological functions, and thus to remove it 

 from physical and chemical study. As yet colloid chemistry has no 

 better explanation to offer. However, it seems probable to me that 

 we shall, with its help, come to a solution of these questions. 



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