2i 4 THE ENGINES OF THE HUMAN BODY 



little more than one-fourth of the surface which the small 

 bowel presents to its contents, is still a very considerable 

 area. There are none of the contrivances for increasing 

 the absorbing surface — duplications of the lining mem- 

 brane, upgrowths in the form of villi — such as we saw in 

 the small bowel. The surface of the lining membrane is 

 covered by the usual kind of paving — mucus-forming 

 units, even more active in maintaining a lubricating 

 covering than in the small bowel. The lining mem- 

 brane is set with minute test-tube glands. These also 

 throw their products, which, so far as we know at 

 present, are mainly of a lubricating kind, on the surface 

 of the membrane. Under the paving epithelium and 

 between the test-tube glands is a capillary field not so 

 richly fed by arteries and drained by veins as in the small 

 bowel, but yet of an extent that convinces us an important 

 work is carried on in the lining of the great bowel. The 

 paving epithelial units are the active agents in extracting 

 the available products set free in the alimentary contents 

 of the great bowel ; they pass the fruits of their labours 

 on to the neighbouring capillary field. From there the 

 products are carried to the liver to undergo further treat- 

 ment before being issued in the daily ration of tissue-fuel. 

 We know that absorption proceeds most actively in the 

 first part of the great bowel. When the chyme enters 

 the caecum it is fluid ; by the time it reaches the trans- 

 verse colon it has become reduced to a paste, and in that 

 condition it remains throughout the remaining stages of 

 its journey. 



It is manifest that the pavement of the great bowel is 

 a living barrier or screen interposed between a mass of 

 matter seething with bacterial products, good and bad, 

 and a film of moving blood — the blood contained in the 

 capillary field of the lining membrane. The living pave- 

 ment units have to serve not only as a frontier guard but 

 also as a customs barrier at which all the traffic which 

 enters the kingdom of the body is submitted to scrutiny 

 and accepted or rejected according to the nature of its 

 mission. Clearly our health depends on how the customs 



