CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 479 



it high up with his hand or with his thumb and finger, and carrying 

 the squeezing action downwards along the length of the intestine, 

 he makes the passive intestine do very much what the circular 

 coat does actively, by contraction, in the living animal. 



The action of the longitudinal coat is perhaps not so clear; 

 but a contraction of the longitudinal coat taking place in any 

 segment of the tube would tend to draw the tube over the contents 

 lying immediately above, or below, the segment, very much as a 

 glove is drawn over a finger. And a succession of such contractions 

 travelling along the tube would lead to a movement of the contents 

 in the same direction. Were the circular coat absent a longitudinal 

 coat might by itself possibly suffice to propel the contents along the 

 tube. In the presence of the circular coat, the action of the longi- 

 tudinal coat in any segment of the tube, if taking place immediately 

 before the circular contraction would, by filling the segment with 

 contents, render the squeezing action of the circular coat more 

 efficient ; if taking place immediately after the circular contraction, 

 it would help in quickening the return of the tube to its normal 

 calibre, for the contraction of the longitudinal coat tends to shorten 

 and widen the segment, and thus would prepare it for new con- 

 tents. And it has been urged that in each segment of the canal 

 the longitudinal coat contracts while the circular coat is relaxed, and 

 the circular coat contracts while the longitudinal is relaxed. On 

 the other hand it is stated that the longitudinal coat may contract 

 whether the circular coat be contracted or no ; and it is argued 

 that the chief effect of the contractions of the longitudinal coat 

 is a gentle swaying to and fro of the contents of the tube. 



It must be remembered that the circular coat is always much 

 thicker than the longitudinal coat ; and we may infer that while 

 the chief work of driving the contents onward falls on the former 

 the latter assists the work in some other way. 



In the small intestine the tube is hung loosely and much 

 twisted so that many loops are formed, and in these the swaying 

 pendulum movements, attributed to contractions of the longi- 

 tudinal coat, may be observed. The contents of the loops are 

 moreover largely fluid ; hence the steady onward movement, such 

 as is seen when more solid contents pass along the straight and 

 somewhat firmly attached oesophagus, is complicated by movements 

 due to a loop being projected forward by the entrance of fluid from 

 above, or being dragged down by the weight of its new contents, 

 or, on the other hand, due to a loop being retracted by the driving 

 onward of its contents and the emptying of itself, and the like. 

 In this way a peculiar writhing movement of the bowel is brought 

 about, and the phrase 'peristaltic movement' is generally used to 

 denote this total effect of the contraction of the muscular coats ; 

 it will however be best to restrict the meaning to the progressive 

 contraction of the circular coat assisted, in most cases, by a similar 

 progressive contraction of the longitudinal coat. 



