DISTRIBUTION 25 



barrier for the external surface of the mammal ; the 

 nasal filtering of the inspired air and the ciliary action 

 in the air passages sterilise the inspired air and protect 

 the lungs : but the alimentary tract is only lined by a 

 mucous membrane which is thin, delicate, moist, ab- 

 sorbing and in continuous contact at many points with 

 hordes of hostile bacteria. The alimentary lining is 

 clearly the anxious frontier, at some points being 

 weaker than at others. At the fauces come first the 

 fresh organisms in food and drink and also from the 

 dusty or spray-laden atmosphere, and here they colon- 

 ise and multiply. The oesophagus is safer^ as material 

 does not remain in contact with its walls. In the 

 stomach the acid secretions of the cardiac portion 

 numbs or destroys the bacteria, though the activity is 

 less pronounced towards the pylorus. The alkaline 

 contents of the small intestine are readily decomposable 

 and swarm with micro-organisms. In the lower half of 

 the duodenum and upper part of the jejunum the bac- 

 teria are still not very active but towards the lower 

 end of the ileum the longer incubation causes increase 

 in their numbers and activity. Here the frontier is 

 greatly threatened. The caecum with it's very infective 

 fluid contents is in like case, but the walls of the colon 

 and rectum are less menaced, for the inspissation of the 

 fseces does not favour bacterial activity. 



It is in these dangerous localities — the fauces, the 

 ileum and the caecum— that the subepithelial lymphoid 

 tissue is developed (Fig. 5). Though widely scattered, 

 this tissue is not insignificant in amount. If the human 

 faucial, naso-pharyngeal and lingual tonsils, the soli- 

 tary and the aggregated lymphatic nodules, and the 

 vermiform appendix were massed together, they would 

 form an organ of some bulk. 



It should be noted, however, that the subepithelial 

 lymphoid tissue does not form a complete covering for 



