SOAPS IN FMCES 419 



thcni into their coniponciit 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) Absorpti(jn 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 one-third of the fa?cal 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. 107) 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 (liqiud 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 

 of the rectum by the faeces. When the pressure of the faeces in 

 the rectum reaches a value of about 30 to 40 mm. Hg., there is a 

 call to defaecate. If no response be made, the call is not repeated 

 immediatelv, as the rectal walls relax and so lose their irritabilitv 

 to pressure. While the initiation is voluntary the act itself is 

 purely reflex like the other movements of the intestine. The 

 reflex contractions and relaxations are generally aided by voluntary 

 contraction of all the muscles which will increase abdominal 

 pressure. 



From a physico-chemical standpoint practically nothing can 

 be said of the mechanism of alimentary transport. While the 

 movements, etc., are apparent the underlying causes are com- 

 pletely hidden. No help so far is gi\'en by attempting to trace 

 the development of the higlily complex system of the vertebrate 

 from the apparently sim])le physico-chemical response of the 

 amoeba to contact with food (Tropisms, Chap. XXXIII.). 



27—2 



