574 DIGESTION. 



as well as to the character of the food; a very luscious article sooner 

 cloying than one that is less so. A due supply of liquid with solid ali- 

 ment also enables us to prolong the repast with satisfaction. 



As the stomach, when distended, presses upon the different viscera 

 and upon the abdominal parietes, it is obvious, that it must experience a 

 proportionate reaction. An interesting question consequently arises; 

 to determine the causes, which oppose the passage of the food back along 

 the oesophagus, as well as through the pylorus. M. Magendie 1 found, 

 in his vivisections, that the lower portion of the oesophagus experiences, 

 continuously, an alternate motion of contraction and relaxation. The 

 contraction begins at the junction of the two upper thirds with the 

 lowest third ; and is propagated, with some rapidity, to the termination 

 of the oesophagus in the stomach. Its duration, when once excited, is 

 variable ; the average being, at least, half a minute. When thus con- 

 tracted, it is hard and elastic, like a cord strongly stretched. The relaxa- 

 tion, that succeeds the contraction, occurs suddenly and simultaneously 

 in all the contracted fibres ; at times, however, it appears to take place 

 from the upper fibres towards the lower. In the state of relaxation, the 

 oesophagus is remarkably flaccid; forming a singular contrast with that 

 of contraction. This movement of the oesophagus is, according to M. 

 Magendie, 2 under the dependence of the eighth pair of nerves. When 

 these nerves were divided in an animal, the oesophagus was no longer 

 contracted. Still it was not relaxed. Its fibres, deprived of nervous 

 influence, were shortened with a certain degree of force; and the canal 

 remained in a state intermediate between contraction and relaxation. 



The lower part of the oesophagus of the horse, for an extent of eight 

 or ten inches, is not contractile in the manner of muscles. M. Ma- 

 gendie 3 found, when the eighth pair of nerves was irritated, or the 

 parts were exposed to the galvanic stimulus, that no contraction was 

 produced. The oesophagus of that animal is, however, highly elastic ; 

 and its lower extremity is kept so strongly closed, that for a long time 

 after death, it is difficult to introduce the finger; and considerable pres- 

 sure is required to force air into it. M. Magendie considers this arrange- 

 ment to be the true reason, why horses vomit with such difficulty as to 

 occasionally rupture the stomach by their efforts. The alternate mo- 

 tions of the oesophagus, which we have described, oppose the return of 

 the food from the stomach. The more the. organ is distended, the more 

 intense and prolonged is the contraction, and the shorter the relaxation. 

 The contraction commonly coincides with inspiration ; the time at which 

 the stomach is, of course, most strongly compressed. The relaxation 

 is synchronous with expiration. 



The pylorus prevents the alimentary mass from passing into the 

 duodenum. In living animals, whether the stomach be filled or empty, 

 this aperture is constantly closed by the constriction of its fibrous ring, 

 and the contraction of its circular fibres ; and, so accurately is it closed, 

 that, if air be forced into the s.tomach from the oesophagus, the organ 

 must be distended, and considerable exertion made to overcome the 

 resistance of the pylorus. Yet, if air be forced from the small intes- 



1 Precis, &c., ii. 82. 2 Ibid., ii. 18. 3 i^rf., & 19. 



