256 INTESTINAL DIGESTION. 



action being reversed for a time, until the indigestible residue, mixed with a 

 certain quantity of intestinal secretion, more or less modified, is discharged 

 into the caput coli. These movements are apparently not continuous, and 

 they depend in some degree upon the quantity of matter contained in different 

 parts of the intestinal tract. Judging from the movements in the inferior 

 animals after the abdomen has been opened, the intestines are always chang- 

 ing their position, mainly by the action of their longitudinal muscular fibres, 

 so that the force of gravity does not oppose the onward passage of their con- 

 tents as much as if the relative position of the parts were constant. There 

 are no definite observations concerning the relative activity of the peristaltic 

 movements in different portions of the intestine ; but from the fact that the 

 jejunum is constantly found empty, while the ileum contains a considerable 

 quantity of pultaceous matter, it would seem that the movements must be 

 more vigorous and efficient in the upper portions of the canal. 



The gases which are found in the intestine have an important mechanical 

 office. They are useful, in the first place, in keeping the canal constantly 

 distended to the proper degree, thus avoiding the liability to disturbances in 

 the circulation and facilitating the passage of the alimentary mass in obedience 

 to the peristaltic contractions. They also support the walls of the intestine 

 and protect these parts against concussions, in walking, leaping etc. The 

 gases are useful, likewise, in offering an elastic but resisting mass upon which 

 the compressing action of the abdominal muscles may be exerted in straining 

 and in expiration. 



There can be hardly any question that the normal movements of the in- 

 testine are due principally to the impression made upon the mucous mem- 

 brane by the alimentary matters, to which is added, perhaps, the stimulating 

 action of the bile. It is difficult to determine with accuracy what part the 

 bile plays in the production of these movements, from the fact that the nor- 

 mal action of the intestine is not easily observed. In the case of intestinal 

 fistula so often referred to, when food was introduced into the lower portion of 

 the canal, there was at first an abundant evacuation every twenty-four hours ; 

 but subsequently it became necessary to use enemata. As there was no com- 

 munication between the lower and the upper portions of the intestine, this 

 fact is an evidence that the peristaltic movements can take place without the 

 action of the bile. 



The vigorous peristaltic movements which occur soon after death have 

 been explained in various ways. It has been shown that these movements 

 are not due to a lowering of the temperature or to exposure of the intestines 

 to the air. The latter fact may be easily verified by killing a rabbit, when 

 vigorous movements may be seen through the thin, abdominal walls, even 

 while the cavity is unopened. According to Schiff, the cause of these exag- 

 gerated movements is diminution or arrest of the circulation. By compress- 

 ing the abdominal aorta in a living animal, he was able to excite peristaltic 

 movements in the intestine as vigorous as those which take place after death ; 

 and on ceasing the compression, the movements were arrested. 



The nerves distributed to the small intestine are derived from the sym- 



