404 Chapter XIII 



effective ; or they may have to take up their abode in the cavities of 

 the animal organism, in order that they may be able to inundate it 

 with their poisons. We must here review briefly these natural barriers 

 to microbial invasion. 



The skin constitutes a protective covering of great importance in 

 [424] connection with the preservation against microbial invasion of the 

 delicate parts of an animal. In many of the lower and higher animals, 

 and even in man himself, the skin becomes the seat of a microbial 

 flora, often very abundant, in which may be found, in addition to 

 certain inoffensive organisms, other minute parasites more or less 

 harmful. The pyogenic cocci, staphylococci and streptococci, are 

 constantly found on the human skin, most frequently hidden in the 

 depths of the canals of the hair follicles. These micro-organisms 

 seize every favourable opportunity to attack the organism, producing 

 such local lesions of the skin as acne, pimples, boils, and erysipelas, 

 or even becoming generalised in the blood and tissues, as in the 

 septicaemias and pyaemias. To the skin, therefore, must be assigned 

 a very important function in the prevention of the invasion of 

 micro-organisms which are found constantly on the surface of the 

 body or which, along with all kinds of dirt, are brought there 

 accidentally. 



The skin is able to fulfil this protective function from the fact 

 that, in most animals, it is covered with a not very permeable layer 

 of some considerable thickness. In the majority of the Invertebrata, 

 of all classes, the surface of the body is clothed with a chitinous layer, 

 sometimes very thin and capable of folding and following all the 

 movements of the body; or again it may be impregnated with 

 calcareous salts and very hard, as in the case of the integument of 

 Insects and Crustacea, and the shell of the Mollusca. In all cases 

 this cutaneous sheath constitutes a formidable obstacle to the entry 

 of micro-organisms. Even in animals of very small size the thin 

 cuticle is effective in preventing any invasion by these parasites. 

 Thus the Sajyrolegniae, fungi so fatal to many aquatic animals, are 

 often quite unable to pass through this cuticular layer. In order to 

 pass this obstacle their germs must take advantage of some fissure 

 or wound, produced by other causes. Daphniae, too, may often be 

 observed to succeed in ridding themselves of the Monospora with 

 its needle-like spores by means of a mechanism which we have 

 already described in chapter vi. The white corpuscles of the blood 

 surround the spores of this parasite and transform them into an 



