444 THE RABBIT. PHYLUM CHORDATA 



of rings ; the rectum is a narrower tube about two and a half 

 feet long, in which faecal pellets can be seen. 



DIGESTION 



The function of the alimentary canal is to digest and absorb 

 the food (p. 7). It is partly broken down mechanically by the 

 teeth, but the main attack is chemical. The walls of the stomach 

 and small intestine, and certain special glands, produce enzymes, 

 which hydrolyse the various classes of foodstuff. Carbohydrates 

 are first acted on in the mouth by an amylase (from amylum = 

 starch), formerly called ptyalin, in the saliva or spittle. This is 

 secreted by four pairs of salivary glands ; the parotids, behind 

 the angles of the jaws, the submaxillaries between the angles, 

 the infraorbitals below the eyes, and the sublinguals inside the 

 dentaries. Amylase converts starch and glycogen to maltose, 

 a disaccharide.^ In the small intestine another amylase, produced 

 by the pancreas, continues the breakdown of starch and glycogen, 

 and a series of disaccharases secreted by the intestinal wall 

 break down cane, malt and milk sugars to their appropriate 

 hexoses. The digestion of protein begins in the stomach, where 

 pepsin hydrolyses it to proteoses and peptones. The walls of the 

 stomach also secrete much hydrochloric acid, giving in man a 

 pH of I. Pepsin can only act in a strongly acid medium. In the 

 intestine the protein meets more enzymes. Trypsin, which is 

 secreted by the pancreas in the inactive form of trypsinogen, 

 which combines with enterokinase in the intestinal juice to give 

 the active substance, is a mixture of enzymes. Between them they 

 break down the proteins to dipeptides and amino-acids. In the 

 intestinal juice is another mixture of enzymes, called erepsin, 

 which finishes the work by changing polypeptides and dipeptides 

 to amino-acids. The intestinal contents are approximately neutral 

 in reaction. Fat is broken up physically into very fine droplets 

 by the bile, and partially hydrolysed to glycerol and free fatty 

 acid by lipases from the stomach, pancreas, and small intestine. 



Absorption of the digested food takes place in the small 

 intestine. Hexoses, amino-acids, and fatty acids go into the 

 blood capillaries and so by the portal vein to the liver ; unsplit 



* The saliva of the rabbit contains only a low concentration of amylase. So 

 far as is known, high concentrations are present only in primates and elephants, 

 and many mammals, especially the carnivores, have little or none. 



