CHAPTER LIII 

 THE MECHANISMS OF DIGESTION (Cont'd) 



THE MOVEMENTS OF THE INTESTINES 



The length of the small intestine and the size of the cecum of the 

 large intestine vary considerably in different animals. In the carnivora, 

 such as the cat, the small intestine is relatively short; in the herbivora, 

 relatively long. Thus, it is three times the length of the body in the cat, 

 and four to six times in the dog ; whereas in the goat and sheep, it may 

 be nearly thirty times the length of the body. In the carnivora the 

 cecum is either absent or rudimentary, whereas in those herbivora which 

 do not have a divided stomach the cecum is very large and sacculated, 

 as is also the colon. The reason for the great size in herbivora is! that 

 practically the whole of the digestion of cellulose takes place in this 

 part of the gut. This digestion, as we shall see later, does not depend 

 on any secretion poured forth by the animal itself, but upon the action 

 of bacteria and of certain enzymes (cytases) that are taken with the 

 vegetable food. 



Movement of the Small Intestine 



The movements of the small intestine have been studied (1) by the 

 bismuth subnitrate and x-ray method, (2) by observing them after open- 

 ing the abdomen of an animal submerged in a bath of physiologic saline 

 at body temperature, (3) by observing the changes in pressure produced 

 in a thin-walled rubber balloon inserted in the lumen of the gut and 

 connected with a recording tambour (Fig. 160), and (4) by excising 

 portions of the intestine and keeping them alive in a bath of saline solu- 

 tion at body temperature, through which oxygen is made to pass. 



THE SEGMENTING MOVEMENTS 



When a suitably fed animal is placed on the holder for examination 

 by the x-ray method, no movement in the intestinal shadows is generally 

 observed for some time. The first movement to appear is the breaking of 

 one of the columns of food into small segments of nearly equal size. 

 Each of these segments again quickly divides, and the neighboring 

 halves suddenly unite to form new segments, and so on, in a manner 



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