372 THE HUMAN BODY. 



unmixed. That of the crypts of Lieberkiihn is watery and 

 alkaline, and poured out more abundantly during digestion 

 than at other times. It has no special action on starches, 

 most proteids, or on fats; but is said to dissolve blood fibrin 

 and convert it into peptone, and it changes maltose into 

 grape sugar; so that this cane sugar is turned into a grape 

 sugar before being absorbed. Mucus is also formed and 

 poured out abundantly by the epithelium cells of the intes- 

 tinal lining membrane. It is more especially secreted during 

 fasting, and by its stickiness collects debris and keeps the 

 mucous membrane clean. 



Intestinal Digestion. Having considered separately the 

 actions of the secretions which the food meets with in the 

 small intestine we may now consider their combined effect. 



The neutralization of the chyme, followed by its conver- 

 sion into alkaline chyle, will prevent any further action of 

 the pepsin on proteids, but will allow the ptyalin of the 

 saliva (the activity of which was stopped by the acidity of the 

 gastric juice) to recommence its action upon starch. More- 

 over, in the stomach there is produced, alongside of the albu- 

 mose and true peptone, the parapeptone, which agrees very 

 closely with syntonin in its properties, and this passes into 

 the duodenum in the chyme. As soon as the bile meets 

 the chyme it precipitates the parapeptone, and this carries 

 down with it any peptones which, having escaped absorption 

 in the stomach, may be present; it also precipitates the pep- 

 sin. In consequence, one finds in an animal killed during 

 digestion, a granular precipitate over the villi, and in the 

 folds between the valvulge conniventes of the duodenum. 

 This is redissolved by the pancreatic secretion, which also 

 changes into peptone the proteids (usually a considerable pro- 

 portion of those eaten at a meal) which have passed through 

 the stomach unchanged, or as albumose or parapeptone. The 

 conversion of starch into maltose will go on very rapidly under 

 the influence of the pancreatic secretion. Fats will be split 

 up and saponified to a certain extent, but a far larger pro- 

 portion will be emulsified and give the chyle a whitish appear- 

 ance. Later cane sugar, which may have escaped absorption in 

 the stomach, and maltose will be converted into grape sugar 

 and absorbed, along with such salines as may, also, have hith- 

 erto escaped. Elastic tissue from animal substances eaten, 



