DIGESTION OP POOD. 321 



which is an antecedent of the final product, which we term a 

 ferment. It is now customary to speak of these changes as 

 constructive (anabolic) and destructive (katabolic), though we 

 have already pointed out (page 258) that this view is, at best, 

 only one way of looking at the matter, and we doubt if it may 

 not be cramping and misleading. 



We must also urge caution in regard to the conception to 

 be associated with the use of the terms " resting " and " active " 

 stage. It is not to be forgotten that strictly in living cells 

 there is no absolute rest — such means death ; but, if these terms 

 be understood as denoting but degrees of activity, they need 

 not mislead. It is also more than probable that in certain of 

 the glands, or in some animals, the processes go on simultane- 

 ously; the protoplasm being renewed, the zymogen, or mother- 

 ferment, being formed, and the latter converted into actual fer- 

 ment, all at the same time. 



The nature of secretion is now tolerably clear as a whole; 

 though it is to be remembered that this account is but general, 

 and that there are many minor differences for each gland and 

 variations that can scarcely be denominated minor for dififerent 

 animals. Evidently no theory of filtration, no process depend- 

 ing solely on blood-pressure, will apply here. And if in this, 

 the best-studied case, mechanical theories of vital processes 

 utterly fail, why attempt to fasten them upon other glands, as 

 the kidneys and the lungs, or, indeed, apply such crude concep- 

 tions to the subtle processes of living protoplasm anywhere or 

 in any form ? 



It is somewhat remarkable that an extract of a perfectly 

 fresh pancreas is not proteolytic ; yet the gland yields such an 

 extract when it has stood some hours or been treated with a 

 weak acid. Those facts, together with the microscopic appear- 

 ances, suggested that there is formed a forerunner to the actual 

 ferment — a zymogen, or mother-ferment, which at the moment 

 of discharge of the completed secretion is converted into the 

 actual ferment. We might, therefore, speak of a pepsinogen, 

 trypsinogen, etc., and, though there may be a cessation in the 

 series of processes, and no doubt there is in some animals, this 

 may not be the case in all, or in all glands. 



Secretion by the Stomach.— The glands of the stomach differ 

 in most animals in the cardiac and pyloric regions. In those 

 of the former zone, both central (columnar) and parietal (ovoid) 

 cells are to be recognized. It was thought that possibly the lat- 



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