290 HANDBOOK OF PHYSIOLOGY. 



Functions. The secretion of Brunner's glands is said to be able to 

 convert proteids into peptones, and that of Lieberkiihn's is believed to 

 convert starch into sugar. To these functions of the succus entericus 

 the powers of converting cane into grape sugar, and of turning grape 

 sugar into lactic, and afterwards into butyric acid, are added by some 

 physiologists. It also probably contains a milk-curdling ferment (W. 

 Roberts). 



The reaction which represents the conversion of cane sugar into 

 grape sugar may be represented thus: 



80,.H 1 ,0 11 + 2H,0 = C 15 H, 4 0,, + C,,H,,0 12 



Saccharose water Dextrose Laevulose 



The conversion is effected probably by means of a hydrolytic fer- 

 ment. (Inversive ferment, Bernard.) 



The length and complexity of the digestive tract seem to be closely 

 connected with the character of the food on which an animal lives. 

 Thus in all carnivorous animals, such as the cat and dog, and pre-emi- 

 nently in carnivorous birds, as hawks and herons, it is exceedingly short. 

 The seals, which, though carnivorous, possess a very long intestine, 

 appear to furnish an exception; but this is doubtless to be explained as 

 an adaptation to their aquatic habits, their constant exposure to cold re- 

 quiring that they should absorb as much as possible from their intestines. 



Herbivorous animals, on the other hand, and the ruminants espe- 

 cially, have very long intestines (in the sheep 30 times the length of the 

 body), which is no doubt to be connected with their lowly nutritious 

 diet. In others, such as the rabbit, though the intestines are not ex- 

 cessively long, this is compensated by the great length and capacity of 

 the caecum. In man, the length of the intestines is intermediate be- 

 tween the extremes of the carnivora and herbivora, and his diet also is 

 intermediate. 



Summary of the Digestive Changes in the Small Intestine. 



In order to understand the changes in the food which occur during 

 its passage through the small intestine, it will be well to refer briefly to 

 the state in which it leaves the stomach through the pylorus. It has 

 been said before, that the chief office of the stomach is not only to mix 

 into an uniform mass all the varieties of food that reach it through the 

 oesophagus, but especially to dissolve the nitrogenous portion by means 

 of the gastric juice. The fatty matters, during their sojourn in the 

 stomach, become more thoroughly mingled with the other constituents 

 of the food taken, but are not yet in a state fit for absorption. The 

 conversion of starch into sugar, which began in the mouth, has been in- 

 terfered with, if not altogether stopped. The soluble matters both 

 those which were so from the first, as sugar and saline matter, and the 

 gastric peptones have begun to disappear by absorption into the blood- 

 vessels, and the same thing has befallen such fluids as may have been 

 swallowed wine, water, etc. 



