224 INTESTINAL DIGESTION 



tion. Nearly all that can be said on this subject is that the action of 

 the bile seems to be auxiliary to that of the other digestive secretions. 



MOVEMENTS OF THE SMALL INTESTINE 



By the contractions of the muscular coat of the small intestine, the 

 alimentary mass is made- to pass along the canal, sometimes in one 

 direction and sometimes in another, the general tendency, however, 

 being toward the caecum ; and the partially digested matters that 

 pass out at the pylorus are prevented from returning to the stomach 

 by the peculiar arrangement of the fibres which constitute the pyloric 

 muscle. Once in the intestine, the food is propelled along the canal 

 by peculiar movements that have been called peristaltic, when the 

 direction is toward the large intestine, and antiperistaltic, when the 

 direction is reversed. These movements are of the character peculiar 

 to the non-striated muscular fibres ; they are slow and gradual, the 

 contraction enduring for a certain time and being followed by a corre- 

 spondingly slow and gradual relaxation. Both the circular and the 

 longitudinal muscular layers participate in these movements. 



So far as has been ascertained by observations on the human sub- 

 ject and warm-blooded animals, the regular intestinal movements are 

 excited by the passage of alimentary matters from the stomach through 

 the tube during the natural process of digestion. By a slow and 

 gradual action of the muscular coat of the intestine, its contents are 

 passed along, occasionally the 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 apparently are not continuous, and they depend in 

 some degree on 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 

 changing their position, mainly by the action of their longitudinal mus- 

 cular fibres, so that the force of gravity does not oppose the onward 

 passage of their contents so much as if the relative position of the 

 parts were constant. 



The gases found in the intestine have an important mechanical 

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

 stantly distended to the proper degree, thus avoiding the liability to 

 disturbances in the circulation and facilitating the passage of the ali- 

 mentary mass in obedience to the peristaltic contractions. They also 

 support the walls of the intestine and protect these parts against con- 

 cussions, in walking, leaping etc. The gases are useful, likewise, in 



