INTESTINAL MOVEMENTS. 117 



vomiting impossible, as it cuts off both the commonest source of 

 stimulus going to the centre, and also the important efferent im- 

 pulses which cause the muscle coat of the stomach to contract 

 and to open the cardiac orifice. 



Movements of the Intestines. The muscular coats are 

 somewhat differently arranged in the small and the large intes- 

 tines, but have the same general relation to each other, viz., a 

 thin longitudinal layer lying externally, next the serous mem- 

 brane, and a layer of circular fibres considerably thicker lying 

 internally under the mucous membrane. In the large intestine 

 the external longitudinal fibres are collected into three bands 

 placed at equal distances one from another, which, being rather 

 shorter than the remainder of the intestine, throw the interme- 

 diate part into a series of pouches. 



It is in the small intestine that peristaltic motion of the most 

 typical kind occurs. A wave of contraction passes from the 

 pylorus along the circular fibres, so as to look like a broad ring 

 of constriction progressing slowly downwards. 



The longitudinal fibres at the same time contract so as to 

 shorten the piece of intestine immediately below the ring of con- 

 striction, and also cause a certain amount of rolling movement 

 of these loops of intestine which are free enough to move. 



This motion takes place periodically in proportion to the 

 amount and character of the contents of the intestine, the food 

 passing over the mucous membrane being to all appearance the 

 stimulus which normally calls forth and intensifies the action. 



The activity of the peristaltic movements varies with many 

 circumstances besides the contents of the intestines. Of these the 

 most noticeable is the amount and character of the blood flowing 

 through the vessels of the intestinal wall. Thus stoppage of the 

 blood current by tying the arteries or deficiency of oxygen and 

 excess of carbonic acid causes inordinate activity of the peristaltic 

 action. Direct irritation of the serous surface of the intestine, 

 with mechanical, chemical, or electrical stimuli causes also in- 

 crease in the movements of the intestine. 



The great activity of the motion observed when the abdominal 



