IMMUNITY AGAINST MICROBES. 107 



experiments of Tschistowitsch^ demonstrated the fact that they 

 belong to the group of Phagocytes^ and are therefore probably 

 derived from ordinary lymphocytes. These cells remove particles 

 of foreign matter which have entered the air-passages, and 

 carry them to the neighbouring lymphatic glands, where they 

 are again arrested and often remain permanently. In the 

 bronchial glands of London cats, for instance, which always 

 contain a large quantity of charcoal, the particles of charcoal, 

 dust, &c., are contained in the interior of large mono-nucleated 

 epithelioid cells which are real Macrophages. 



The free surface of vertebrata, like the free surface of in- 

 vertebrata, is everywhere in contact with a large number of 

 micro-organisms. Nevertheless, the internal organs of the 

 body, the liver, kidneys, spleen, &c., contain no parasites, and 

 it follows that there must be some mechanism preventing the 

 entrance of micro-organisms into a healthy animal's tissues, or 

 some means of destroying them should they gain admittance. 



The skin of all animals is an impenetrable barrier to the 

 entrance of micro-organisms, and, to all appearances, the latter 

 do not make an attempt to penetrate through the same into 

 the tissues. It is plain that micro-organisms cannot easily 

 force their way through several layers of hard epithelium cells, 

 such as go to form the outer skin of most mammals or cover 

 the surface of the lips and tongue ; but it is evident also that 

 micro-organisms cannot penetrate through a single layer of 

 epithelium cells. In sections passing through the walls and 

 contents of the intestines of any animal, an appalling number 

 of microbes are seen lining the free border of the layer of 

 epithelium cells, but none are found in or between the latter. 

 The epithelium cells therefore, together with the cementing 

 substance between them, form a satisfactory barrier to the 

 entrance of microbes. 



Matters, however, are not quite so simple as they appear, for 

 in some organs, of healthy animals even, a struggle takes place 

 during every hour of the day between micro-organisms and 

 mesodermic cells. 



1 Tschistowitscli, ' Auu. de I'lnstiiut Pasteur,' July, 1889. 



