SUMMARY OF DIGESTION. 63 



How, then, is this to be explained? Assuredly there is no other 

 source from which these bodies can come than the one indicated the 

 common salt, and yet it seems to be totally inadequate. 



I think that this difficulty ?s rather imaginary than real. Things are 

 so arranged that a limited quantity of salt can produce unlimited quanti- 

 ties of gastric juice and bile ; for the former, associated with the food it 

 has digested, scarcely escapes from the pyloric valve before it encounters 

 the bile and pancreatic juices discharging into the duodenum, and through 

 the length of the upper portion of the small intestines these secretions, 

 together with the food they have acted upon, are brought into complete 

 contact. The reproduction of chloride of sodium is therefore constantly 

 taking place in intestinal digestion, and it returns back to the system 

 through the absorbents. Again it undergoes decomposition, its acid re- 

 appearing in the gastric juice, and its alkali in the pancreatic juice and 

 bile. By thus using a small amount over and over again, great effects 

 can be produced, and it is then only necessary to restore those small por- 

 tions that are wasted in carrying out the general scheme. 



In the low-pressure marine steam-engine we have an example of the 

 same kind. A certain quantity of water is vaporized in the boiler and 

 condensed in the engine ; pumped back into the boiler to be vaporized, 

 arid then recondensed in the engine. Comparatively little is required to 

 supply the wants of the machine, and long voyages can be made with 

 only as much water as will compensate for the necessary waste arising 

 in the working. 



For the sake of presenting the consideration of the function of diges- 

 tion with clearness, it is customary to leave out of consider- Stomach di 

 ation the subordinate actions taking place both in the stom- gestion is his- 

 ach and intestine. This, however, involves a certain amount JeftlnafdLes- 

 of error, since respiratory or non-nitrogenized digestion oc- tion is caiorifa- 

 cuf s in the former cavity, and nutritive or nitrogenized in the 

 latter. Nevertheless, there can be no doubt that if our view is restricted 

 to the more imposing characters, we are justified in accepting the dogma 

 that " stomach digestion is histogenetic or nitrogenized, and intestinal 

 digestion is calorifacient." 



Under the most comprehensive point of view, examining the action of 

 the entire digestive tract from the mouth to the rectum, we General sum . 

 discover a recurrent periodicity. In the mouth, the transi- mary of diges- 

 tory digestion taking place is wholly expended upon the ca- 

 lorifacient food ; in the stomach it is the nutritive portion which is chiefly 

 attacked ; in the duodenum there is a return to the calorifacient, and in 

 the ccecum of animals a resumption of the nutritive. This last is less 

 apparent in man, for in him the ccecum exists only in a rudimentary state, 

 represented by the appendix vermiformis. 



