MECHANISM OF SECRETION. 23 



is well known, for example, that the function of digestion is 

 seriously disturbed when the act is too prolonged, from the 

 habitual ingestion of an excessive quantity of food. With 

 regard to the pancreas this fact has been demonstrated in 

 the most satisfactory manner. The experiments of Bernard 

 and others have shown that this organ is peculiarly suscep- 

 tible to irritation ; and when a tube is fixed in its duct, after 

 a time the flow of the secretion may become constant, leav- 

 ing no intervals for repose of the gland. "WTien this occurs, 

 the fluid discharged loses the character of the normal secre- 

 tion and is found to possess none of its peculiar diges- 

 tive properties. 1 In one or two instances in which the irrita- 

 tion of the tube introduced into the pancreatic duct did not 

 produce a constant secretion, the fluid, which was discharged 

 intermittently in the normal way, possessed all its physio- 

 logical properties. 2 



From the considerations already mentioned, it is evident 

 that the secretions, as the rule, are formed by the epithelial 

 structures of the glands. There has been a great deal of 

 speculation with regard to the mechanism of this action of the 

 cells. As we before remarked, this question cannot be con- 

 sidered as settled. It does not seem probable that the cells 

 are ruptured during secretion and discharge their contents 

 into the ducts, for under these circumstances we should 

 expect to find some of their structure in the secreted fluid ; 

 whereas, aside from accidental constituents, the secretions 

 are homogeneous, and do not contain any formed anatomical 

 elements. There is no good reason for supposing that this 

 action takes place, and that more or less of the glandular 

 epithelium is destroyed whenever secretion occurs ; and, in 

 the present state of our knowledge, we can only assume that 

 the secreting cells induce catalytic transformations in the 

 organic elements of the blood and modify transudation, with- 

 out pretending to understand the exact nature of this process. 



1 See vol. ii., Digestion, p. 337. 



2 BERNARD, Memoire sur le pancreas, Paris, 1856, p. 46. 



