A THREATENED FACTORY 215 



officials do their work, and they will do that according to 

 the nature of the task we throw upon them. 



The frontier guard of the great bowel has at its call an 

 ample service of police and sanitary officials. Behind or 

 below the epithelial defence, in the spaces between the 

 miniature test-tubes of the lining membrane, is mobilised 

 a great army of microscopic movable units which serve 

 as police and also as scavengers. All along the lining 

 membrane of the great bowel are scattered minute stations 

 —lymphoid follicles, little larger than a pin's head — which 

 serve as special stations for the intestinal police service. 

 In the lower .part of the small bowel and in the cascum 

 and appendix these stations are particularly abundant, 

 often closely crowded together in the form of extensive 

 establishments. Our health depends also on the adminis- 

 tration of police and scavenger services. Services of such 

 kinds form part of a human machine. 



Up to this point we have written as if all the intestinal 

 traffic was in one direction — passing from the alimentary 

 contents to the capillary field of the lining membrane. In 

 the lung we saw that a traffic took place in both direc- 

 tions : oxygen entered the blood, carbon dioxide left it. 

 There is also an export traffic — an excretion — across 

 the lining units of the great bowel. We have seen that 

 its interior is studded with hundreds of thousands of 

 minute test-tube glands. They are always busy — always 

 throwing out a secretion manufactured from the blood 

 circulating in the adjacent capillary field. The excretion 

 lubricates the surface of the bowel, but Nature would 

 never have established so extensive and elaborate a series 

 of retorts merely for the purpose of lubrication. Some 

 secret process of excretion forms one of the essential 

 functions of the great bowel, the exact nature of which 

 we do not yet know. 



There is another series of operations carried on by the 

 lining membrane of the great bowel to which the atten- 

 tion of every one is drawn from time to time. The 

 fermentative nature of the digestion carried on in the 

 great bowel must be attended by a free formation of 



