284 HAND-BOOK OF PHYSIOLOGY. 



The reaction which represents the conversion of cane sugar into grape 

 sugar may be represented thus: 



3C 10 H M O n + 2H 5 = O.^.O,, + O.^O,, 



Saccharose Water Dextrose Lsevulose 



The conversion is probably effected by means of a hydrolytic ferment. 

 (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-eminently 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 requiring that 

 they should absorb as much as possible from their intestines. 



Herbivorous animals, on the other hand, and the ruminants especially, 

 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 excessively long, 

 this is compensated by the great length and capacity of the caecum. In 

 man, the length of the intestines is intermediate between 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 a 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 con- 

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

 fered 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. 



The thin pultaceous chyme, therefore, which during the whole period 

 of gastric digestion, is being constantly squeezed or strained through the 

 pyloric orifice into the duodenum, consists of albuminous matter, broken 

 down, dissolving and half dissolved; fatty matter broken down and 

 melted, but not dissolved at all; starch very slowly in process of conversion 

 into sugar, and afi it becomes sugar, also dissolving in the fluids with which 



