998 DIGESTION 



to find that the contents of the lower small intestine may become acid, in fact, this 

 acidity may on occasions invade higher segments without, however, materially 

 impairing pancreatic digestion. The latter, as we have seen, does not require an 

 especially high alkalinity. 



(c) On Proteins. Some bacteria are capable of splitting proteins into amino- 

 acids, liberating during this process such substances as indol (CgHrN), scatol 

 (CgHgN), and phenol (CeHeO). These animo-acids are further reduced by them 

 into their corresponding amine bases by the process of decarboxylation which con- 

 sists in removing carbon dioxid from their carboxyl (COOH) group. In this way, 

 leucine may be converted into its base iso-amylamine, as follows : 



iS 3 ">CH-CH 2 -CH-NH 2 .COOH = SS'NcH-CH.CHrNH, + CO 2 



L/Xls/ ^3/ 



This base, and especially the oxyphenylethylamine derived from 

 tyramine, possesses a pressor action similar to that of adrenalin. 

 The former substance is a constituent of ergot. It is also of interest 

 to note that the enzymes of fungi, such as those affecting grasses and 

 fruits, are capable of decarboxylizing some of these bases. In spite 

 of the formation of the aforesaid acids, however, the contents of the 

 large intestine become alkaline. This change is due to the fact that 

 some of the bacteria generate ' ammonia which again neutralizes the 

 organic acids. 



CHAPTER LXXXIV 

 THE MECHANICS OF DIGESTION 



A. MASTICATION AND DEGLUTITION 



General Consideration. In those animals in which digestion is 

 chiefly intracellular, the chemical processes necessitate a mechanical 

 manipulation of the food which purposes to effect its reduction into 

 smaller masses and its steady onward movement, so that it may be 

 successively subjected to the different secretions. Leaving out of 

 consideration the celenterata, in which the digestive and vascular 

 systems are still incompletely separated, as well as the echinodermata, 

 in which this separation is complete, it may be said that the arth- 

 ropoda are the first to present an alimentary canal which shows 

 definite variations in its caliber, corresponding to the stomach, and 

 small and large intestines of the higher animals. Glandular organs 

 are placed along this canal which seem to be homologous with the 

 salivary glands and the liver-pancreas of the higher forms. Possibly 

 the simplest alimentary system among the vertebrates is presented 

 by the fishes. It consists of a stomach, the glands of which furnish 

 an acid proteolytic secretion, and a fully differentiated intestine with 

 a series of digestive fluids possessing different actions. 



The alimentary canal of birds exhibits several peculiarities, such as 



