THE MUSCULAR MECHANISMS OF DIGESTION." 293 



themselves, we find that these, together with the movements of the stomach 

 and of the small and large intestines right down to the anus are more or less 

 alike, and may be described under the general name of " peristaltic" move- 

 ments. We have already in 88 spoken of these, but it may be well to 

 consider them briefly again under a general aspect, before dwelling 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 regula- 

 ting the irrigation. 



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

 at any part, travels onward in the same direction, generally downward, 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 se- 

 quence. The result is a narrowing or constriction of the tube which, travel- 

 ling more or less slowly along the tube, drives the contents onward ; 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 downward along the length of the intestine, he makes the passive in- 

 testine 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 con- 

 traction 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 con- 

 tents along the tube. In the presence of the circular coat, the action of the 

 longitudinal 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 quicken- 

 ing 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 contents. 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 pro- 

 cesses which start the expiratory act tend to check or inhibit the inspiratory 

 act and vice versa; and very possibly a like see-saw of stimulation and in- 

 hibition obtains in the muscles of the alimentary canal. 



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, 

 either in the way which we have suggested or in some other way. 



In the small intestine the tube is hung loosely and much twisted, so that 

 many loops are formed ; the contents, moreover, are largely fluid. Hence, 



