CHAP, i.] DIGESTION. 287 



way down. But contractions may, and in all probability oc- 

 casionally do, begin at various points along the length of the 

 intestine. In the living body the intestines have periods of rest, 

 alternating with periods of activity, the occurrence of the periods 

 depending on various circumstances. 



With regard to the causation of the peristaltic movements of 

 the intestine, this much may be affirmed that they may 'occur, as 

 in a piece of intestine cut out from the body, wholly indepen- 

 dently of the central nervous system ; and the only nervous elements 

 which can be regarded as essential to their development are 

 the ganglia of Auerbach or those of Meissner in the intestinal 

 walls. Though the movements can readily be excited by stimuli, 

 applied either to the outside, or, more especially, to the inside of 

 the intestine, they are probably at bottom automatic. The 

 presence of food, especially of food in motion, may at times act 

 as a stimulus, and may in all cases be a condition affecting the 

 nature and extent of the movement; but cannot be regarded as 

 the real cause of the action. When any body is introduced into 

 the intestine, a contraction at first occurs, but soon passes off as 

 the intestine becomes accustomed to the presence of the body. 

 There is no reason why the intestine should not become equally 

 accustomed to the presence of food; and, as a matter of fact, 

 peristaltic movements are often absent when the intestines arc 

 full. The presence of food bears about the same relation to the 

 movements of the intestine, that the presence of blood bears to the 

 beat of the heart. Both are favouring but not indispensable 

 conditions: in both cases the action can go on without them. 

 We may add that just as the tension of a muscle increases up 

 to a certain extent the amount of its contraction, and a full heart 

 beats more strongly than an empty one, so distension of the 

 intestine largely increases peristaltic action. Hence in cases of 

 obstruction of the bowels, the movements become distressing by 

 their violence. 



Among the chief circumstances affecting peristaltic action 

 may be mentioned in the first place the condition of the blood. 

 A lack of oxygen or an excess of carbonic acid in the blood excites 

 powerful movements. This is well seen in asphyxia, and the 

 powerful post-mortem peristaltic movements witnessed on opening 

 a recently-killed animal, as well as those which frequently occur 

 when in the living body, the blood-stream is cut off by com- 

 pression of the aorta, are probably due to the deficiency of oxygen 

 or the accumulation of carbonic acid in the blood and tissues of the 

 intestinal walls. Conversely, saturation of the blood with oxygen, 

 as in the peculiar condition known as apncea (see chapter on 

 Respiration), tends to check peristaltic movements. 



In the second place, peristaltic action is largely influenced by 

 nervous influences passing along the splanchnic and vagus nerves. 

 The movements will go on after section of both these nerves ; but 



