456 PERISTALTIC MOVEMENTS. [BOOK n. 



on the special movements of the several parts of the alimentary 

 canal. 



The muscular coat of the alimentary canal consists as we have 

 seen of two layers, separated more or less distinctly by a sheet of 

 connective tissue, an outer thinner longitudinal layer, and an 

 inner thicker circular layer; and a similar arrangement obtains in 

 nearly all the muscular hollow tubes of the body, except the 

 arteries, in which the muscular elements are present not so much 

 for the purpose of driving the blood onward as for the sake of 

 regulating the irrigation. 



The action of the circular coat is fairly simple. A contraction 

 starting at any part travels onwards in the same direction, generally 

 downwards, that is to say from a part nearer the mouth to a part 

 nearer the rectum, for a greater or less distance, the circularly 

 disposed bundles contracting in sequence. The result is a 

 narrowing or constriction of the tube which, travelling more or 

 less slowly along the tube, drives the contents onwards ; when a 

 butcher empties the intestine of a slaughtered animal by squeezing 

 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. We can hardly imagine that the two coats would contract 

 at the same time, since they would tend to neutralize each other's 

 action. Indeed we may probably go farther and assume that in 

 each segment of the canal first the longitudinal coat contracts while 

 the circular coat is relaxed, and that then the circular coat contracts 

 while the longitudinal relaxes. When we come to deal with 

 respiration we shall meet with a similar double antagonistic and 

 successive action between inspiratory and expiratory muscles ; we 

 shall further see reason to think that the processes which start 

 the expiratory act tend to check or inhibit the inspiratory act 



