560 THE MECHANICAL PROCESSES OF DIGESTION [CH. XXXVII. 



presence of food-material in the intestine, and especially of indi- 

 gestible food such as cellulose. 



(2) It may be influenced by impulses from the upper part of the 

 alimentary canal ; the mere taking of food will stimulate peristalsis 

 even in the large intestine also. This is most marked after abstinence 

 from food, and' the usual effect of breakfast as a stimulus for defeca- 

 tion is a familiar example. The mere taking of a glass of water 

 on rising will in many people have a similar effect. 



(3) It may be influenced by sensations and emotions; thus 

 movements are inhibited by pain, by the exposure of the peritoneum 

 to the air, or by handling the gut as in operations. Some emotions, 

 such as anger, will inhibit peristalsis ; others of a more pleasurable 

 kind, leading to what is popularly termed excitement, will increase it 

 and may even lead to diarrhoea. It is increased by muscular exercise, 

 though here no doubt the influence is partly the mechanical one of 

 the abdominal walls pressing about the intestinal loops. 



(4) It may be influenced by temperature, but here again we have 

 most knowledge in regard to the large intestine; a cold enema is 

 more efficacious than a warm one ; the latter is mainly sedative. 



(5) It may be influenced chemically. Drugs given for the relief 

 of diarrhoea or constipation act in various ways; some affect the 

 amount of secretion, and thus increase or decrease the fluidity of the 

 intestinal contents ; others act on the muscular tissue or its nerves, 

 and so influence the amount of peristalsis. Organic acids, including 

 the amino-acids, produced during digestion, will increase peristalsis. 

 The bile has a similar action, but only on the large intestine ; various 

 oils act in the same way ; certain gases do so also, but here again the 

 mechanical effect of distension is a factor to be reckoned with. A 

 vegetable diet stimulates peristalsis, partly for mechanical reasons 

 the presence of indigestible cellulose and formation of gas partly for 

 a chemical reason, namely, the production of organic acids. 



The pendulum movements differ from true peristalsis in being 

 myogenic ; that is, they are due to the rhy thmicality of the muscular 

 fibres themselves, and are propagated from one muscular fibre to 

 another. They are not abolished by cocaine or nicotine. (Starling.) 



MOVEMENTS OF THE LARGE INTESTINE. 



We have seen that in man the food begins to arrive in the 

 coecuin four and a half hours after it reaches the stomach ; when it 

 arrives in the caecum it contains 90 per cent, of water, together with 

 a small amount of the unabsorbed products of digestion of proteins, 

 fats, and carbohydrates. During its passage along the large intestine 

 these are absorbed, and most absorption appears to occur in the 

 caecum; the normal firm consistency of the faeces, which contain 



