2!ts AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 

 just been said, they lose this property as soon as they undergo the change 

 from living to dead structure. The digestion of the living frog's leg in 

 gastric juice, and similar instances, do not affect this general idea, since, as 

 Bernard himself pointed out, what happens in this case is that the tissue is 

 first killed by the acid and then undergoes digestion. On the other hand, 

 \i minister has shown that a living frog's leg is not digested by strong pan- 

 creatic extracts of weak alkaline reaction, since under these conditions the 

 tissues are not injured by the slightly alkaline liquid. When it is said that 

 the exemption of living tissues from self-digestion is due to the peculiarities 

 of their structure, it must not be supposed that this is equivalent to referring 

 the whole matter to the action of a mysterious vital force. On the contrary, 

 all that is meant is that the -tincture of living protoplasmic material is such 

 that the action of the digestive secretion is prevented, possibly because it is not 

 absorbed, this result being the outcome of the physical and chemical forces 

 exhibited by matter with this peculiar structure. While a statement of this 

 kind is not an explanation of the facts in question, and indeed amounts to a 

 confession that an explanation is not at present possible, it at least refers the 

 phenomenon to the action of known properties of matter. 



General Remarks upon Gastric Digestion. — From the foregoing 



