318 ALIMENTARY CANAL 



of the intestinal musculature. The chyme is kneaded and its 

 surface broken by the rhythmic segmented contractions of the 

 circular muscles of the bowel. By this means (i) the various 

 digestive juices of the intestine are thoroughly mixed with the 

 chyme, (ii) fresh surfaces are exposed to the absorbing surfaces 

 of the wall and (iii) the capillary blood-vessels of the lining 

 membrane are compressed rhythmically, so helping to drive the 

 blood laden with the products of digestive activity on to the 

 liver, etc. 



The work of digestion, begun in the mouth and stomach, 

 is completed in the intestine. Carbohydrates are reduced 

 to single sugars and proteins are broken down to amino acids, 

 etc. In addition to this, the fats are attacked by lipase, which 

 resolves them into their component fatty acids and glycerol 

 (or other alcohol). In this process, the bile salts, by lowering 

 the surface tension at the fat-lipase interface, play an important 

 part. 



(c) Absorption seems to be a case of passage of material through 

 a membrane (q.v.). 



V. Faeces. The materials not absorbed by the intestine are 

 eliminated by the rectum as the faeces. One suggestive physico- 

 chemical fact about these excreta is the proportion of soap to 

 mass in their make up. It has been found that, normally, fat 

 forms approximately ^ of the faecal mass (dry). About 10 per 

 cent, of this fat is in the form of soap. This may be correlated 

 with the water-holding power of soaps and with their lubricating 

 properties. Somewhere about 80 per cent, of their contents is 

 water. This is somewhat remarkable, as both water, fatty acids 

 and soaps are readily absorbed from the gut. If one desires to 

 reduce the water content, calcium is exhibited. As we have 

 already seen (p. 82) calcium soaps are hard " dry " soaps. On 

 the other hand, the addition of easily dissociated sodium and 

 potassium salts leads to the formation of " softer " soaps and a 

 marked increase in the water content of the faeces. It is note- 

 worthy that the fat content (as soap) remains constant. That 

 unabsorbed fat is an excellent faecal lubricant is an axiom in 

 present-day prescribing when mineral oil (liquid paraffin), which 

 cannot be absorbed, is given to produce easy defaecation. 



For the final discharge of the waste alimentary contents a 

 simple kind of " touch button " mechanism is provided. The 

 act is initiated by a voluntary response (removal of inhibition) 

 to the stimulus produced by the stretching of the muscular wall 



