DIGESTION 31 



starch into sugar ; it also emulsifies the fat, dividing the globules into 

 extremely small drops, which it tends to saponify or spUt into fatty 

 acids and gl3'cerine. 



{d) Into the beginning of the small intestine the bile from the liver 

 also flows, but it is not of great digestive importance, being partly 

 of the nature of a waste product. It has a very important action in 

 lowering surface tension so that the fatty constituents of the chyme 

 can form a finely divided emulsion, readily attacked by the digestive 

 ferment from the pancreas, and it also aids in the absorption of the 

 digested fat by the cells lining the intestine. In some animals it is 

 said to have shght power of converting starch into sugar ; by its 

 alkalinity it helps the action of the trypsin of the pancreas (which, 

 unlike pepsin, acts in an alkaline fluid) ; and it is said to have various 

 other qualities. 



(e) In addition to the liver and the pancreas, there are on the walls 

 of the small intestine a great number of small glands, which secrete a 

 juice which seconds the pancreatic juice ; this contains the ferment 

 erepsin, which completes the sphtting of peptones into amino-acids, 

 and ferments which spht the more complex sugars, such as cane sugar. 

 The digested material is in part absorbed into the blood, and the 

 mass of food, still being digested, is passed along the small intestine 

 by means of the miiscular contraction of the walls known as peristaltic 

 action. It reaches the large intestine, and its reaction is now distinctly 

 acid by reason of the acid fermentation of the contents. The walls of 

 the large intestine contain glands similar to those of the small intestine, 

 and the digestive processes are completed, while absorption of water 

 also goes on ; so that by the time the mass has reached the rectum, it 

 is semi-sohd, and is known as faeces. These contain the indigestible 

 and undigested remnants of the food, especially cellulose ; residues 

 of the secretions of the digestive glands ; and enormous numbers of 

 bacteria (mostly dead) from the large intestine, with the products of 

 their activity. 



The digestive processes of Invertebrates are, in a general 

 way, similar : for instance, an alternation of acid and 

 alkaline reactions in digestion is common, though not so 

 well marked as in Vertebrates. There is, however, 

 much adaptation to the diet, both in the structure of the 

 alimentary canal and in the ferments secreted. Un- 

 necessary ferments are dispensed with ; for example, 

 many carnivorous insects have lost the power to digest 

 starch. New ferments are evolved -to digest substances 

 of which other groups can make no use ; thus the clothes- 

 moth larva can digest the very resistant keratin of hair. 

 Cellulose and some other insoluble carbohydrates, which 

 constitute the hard parts of plants, can be digested by some 

 protozoa, and by snails and perhaps a few other families ; 

 but in many groups, for instance in some wood-boring 



