236 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



then, under normal conditions, is the stomach protected from corrosion by its 

 own secretion ? The question has given rise to much discussion, and in reality 

 it deals with one of the fundamental properties of living matter, for the 

 same question must be extended to take in the non-digestion of the small 

 intestine by the alkaline pancreatic secretion, the non-digestion of the digestive 

 tracts of the invertebrates, and the case of the unicellular animals in which 

 there is formed within the animal's protoplasm a digestive secretion which 

 digests foreign material, but does not affect the living substance of the cell. 

 In the particular case under consideration namely, the protection of the 

 mammalian stomach from its own secretion explanations of the following 

 character have been offered : It was suggested (Hunter) that the " principle 

 of life" in living things protected them from digestion. This suggestion 

 cannot be considered seriously at the present day, since it implies that living 

 matter is the seat of a special force, the so-called " vital principle," different 

 from the forms of energy acting upon matter in general. Appeals of this 

 kind to an unknown force in explanation of the properties of living matter 

 are not now permissible in the science of physiology. Moreover, it was 

 shown by Bernard that the hind leg of a living frog introduced into a 

 dog's stomach through a fistula undergoes digestion. The same thing will 

 happen, it may be added, if the leg is put into a vessel containing an artificial 

 gastric juice at the proper temperature. Bernard's theory was that the epithe- 

 lium of the stomach acts as a protection to the organ, preventing the absorp- 

 tion of the juice. Others believe that the mucus formed by the gastric mem- 

 brane acts as a protective covering ; while still another theory holds that the 

 alkaline blood circulating through the organ saves it from digestion, since it 

 neutralizes the acid of the secretion as fast as it is absorbed, and it is known 

 that pepsin can digest only in an acid medium. None of these explanations 

 is sufficient. The last explanation is unsatisfactory because it does not explain 

 the immunity of the small intestine from digestion by the alkaline pancreatic 

 juice, or the protection of the infusoria from their own digestive secretion. 

 The mucous theory is inadequate, as we cannot believe that by this means the 

 protection could be as complete as it is ; and, moreover, this theory does not 

 admit of a general application to other cases. The epithelium theory simply 

 changes the problem a little, as it involves an explanation of the immunity of 

 the living epithelial cells. It is well known that in the dead stomach the 

 epithelial lining is no longer a protection against digestion, so that we are led 

 to believe that there is nothing peculiar in the composition of epithelial cells, 

 as compared with other tissues, to account for their exemption under normal 

 conditions. When we come to consider all the evidence, nothing seems clearer 

 than that the protection of the living tissue is in every case due to the proper- 

 ties of its living structure. So long as the tissue is alive, it is protected from 

 the action of the digesting secretion, but the ultimate physical or chem- 

 ical reason for this property is yet to be discovered. In the case of the 

 mammalian stomach it is quite probable that the lining epithelial cells are 

 especially modified to resist the action of the digestive secretion, but, as has 



