Immunity of the skin and mucous membranes 421 



in these Invertebrata. As the physiology of digestion in these animals 

 is as yet little understood, it is at present impossible to define clearly 

 the conditions which regulate these phenomena. In any case, it is 

 very probable that the soluble digestive ferments destroy the micro- 

 organisms and prevent them from growing in the intestinal content 

 Otherwise it is difficult to explain why the larvae of clothes-moths, 

 which live in old dusty textile fabrics, where the germs of bacteria are 

 not wanting, present a digestive canal from which micro-organisms 

 are entirely absent. The digestive juices, adapted to digest wool 

 and even wax, are evidently capable also of digesting the bodies of 

 micro-organisms. In other insects, which feed on vegetables and on 

 substances less difficult to digest, micro-organisms develop in the 

 intestinal content, as in many of the higher animals. Insects often 

 have their intestine lined by a very delicate chitinous membrane 

 which offers no obstacle to the absorption of the products of digestion, 

 but prevents the micro-organisms from reaching the epithelial layer. 

 We have here a defensive apparatus against microbial invasion, which 

 must be the more useful because this membrane is thrown off and 

 renewed at each moult, thus enabling the insect to rid itself at one 

 swoop of a large number of its microscopic inhabitants. 



In the Vertebrata the canal of the pancreas and that of the small 

 intestine are always populated by a greater or smaller number of 

 micro-organisms, amongst which bacilli predominate. We know the 

 great difficulty experienced every time we wish to make experiments 

 on the pancreatic digestion outside the animal body. The digestive [442] 

 fluid, alkaline and containing many bacteria, is soon transformed 

 into a microbial purge. We are then obliged to have recourse to 

 antiseptics to arrest this development and to bring into prominence 

 the digestive role played by the soluble ferments of the pancreas. 

 This well-known fact may be used as an argument against the 

 existence of any kind of bactericidal power in the small intestine 

 of higher vertebrates. Even in those animals which are distin- 

 guished by the remarkable poorness of their intestinal flora, we fail 

 to reveal the presence of bactericidal substances. The Crustacea, 

 e.g. the crayfish, and certain worms, such as the Ascaris, contain few 

 micro-organisms in their intestine. The former feed on putrescent 

 substances, the latter inhabit the small intestine of man and animals, 

 populated by myriads of bacteria. It might be supposed that, under 

 these conditions, the intestinal content must contain a mass of micro- 

 organisms or, if that be not the case, that it must contain some 



