ALIMENTATION AND DIGESTION 125 



masses of food will be seen to undergo division into small 

 segments, obviously produced by a series of constrictions of 

 the walls, as shown in Fig. 60. The next moment these are 

 replaced by a second series of constrictions between the first. 

 Each segment is thus divided into two, and the neighboring 

 halves of these segments fuse. The next moment the second 

 series of constrictions is replaced by the first, and this process 

 continues at times for many minutes with no change in the 

 general position of the food mass. These divisive, or segment- 

 ing, movements occur from twenty to thirty times a minute, 

 and it has been estimated " that a slender string of food 

 may commonly undergo division into small particles more 

 than a thousand times while scarcely changing its position 

 in the intestine." 



32. Peristalsis. Every now and then a ring of constriction, 

 instead of being confined to one place, moves onward, push- 

 ing the contents of the tube before it for a short distance 

 (two or more inches). A contraction of this kind is called 

 peristaltic. The effect produced is much the same as when 

 the contents of a rubber tube are emptied by squeezing it 

 along between the thumb and finger. 



Thus each consignment of chyme delivered from the 

 stomach immediately receives its share of pancreatic juice 

 and of bile, and the final transformation of the digestible 

 foods takes place as the whole is driven from time to time 

 along the intestine by peristaltic contractions, the efficiency 

 of the contact of the food with the digestive juices, as well 

 as its exposure to the absorbing surfaces, being greatly 

 enhanced by the agitation produced by the movements of 

 constrictive division carried out by the circular muscles be- 

 tween periods of peristaltic activity. The efficiency of digestion 

 and absorption depends as much on the movements carried out 

 by the muscular coat as on the chemical processes effected by 

 enzymes and other constituents of the digestive juices. Digestion 

 is always a cooperation of chemical and mechanical work. 



