356 COMPARATIVE PHYSIOLOGY. 



As the partially digested mass (chyme) is passed on. into the 

 intestine as a result of the action of the alkaline bile, the para- 

 peptone, pepsin, and bile-salts are deposited. Certain of the 

 constituents of digestion are thus delayed, a portion of the pep- 

 sin is probably absorbed, either altered or unaltered, and pep- 

 sin is thus got rid of, making the way clear, so to speak, for the 

 action of trypsin. At all events, digestion in one part of the 

 tract is antagonized by digestion in another, but we must also 

 add supplemented. 



The fat, which had been but little altered, is emulsified by 

 the joint action of the bile and pancreatic secretion ■, a portion 

 is saponified, which again helps in emulsification, while an addi- 

 tional part, in form but little changed, is probably dealt with by 

 the absorbents. 



Pf oteid digestion is continued, and, besides peptones, nitro- 

 genous crystalline bodies are formed (leucin and tyrosin), but 

 under what conditions or to what extent is not known ; though 

 the quantity is likely very variable, both with the species of 

 animal and the circumstances, such as quantity and quality of 

 food; and it is likely also dependent not a little on the rate of 

 absorption. It seems altogether probable that in those that use 

 an excess of nitrogenous food more of these bodies are formed, 

 and thus give an additional work to the excreting organs, in- 

 cluding the liver. But the absence of albumin from healthy 

 faeces points to the complete digestion of proteids in the ali- 

 mentary canal. Plainly the chief work of intestinal digestion 

 is begun and carried on in the upper part of the tract, where 

 the ducts of the main glands are to be found. 



The contents of the intestine swarm with bacteria, though 

 these are probably kept under control, to some extent, by the 

 bile, the functions of which as an antiseptic we have already 

 considered. 



The removal of fats by the villi will be shortly considered. 

 The other products of digestion probably find their way into 

 the general circulation by the portal blood, passing through 

 the liver, which organ modifies some of them in ways to be 

 examined later. 



The valvulae conniventes greatly increase the surface of the 

 intestine, and retard the movements of the partially digested 

 mass, both of which are favorable. The peristaltic movements 

 of the small gut serve the obvious purpose of moving on the 

 digesting mass, thus making way for fresh additions of chyme 



