34 RECENT DEVELOPMENTS OF THE 



in either case the poison is conveyed by particles, and is intro- 

 duced into the living tissues in the particulate form, whether 

 germs or not. As regards the method of conveyance of the 

 virus, and the various channels by which it may be introduced, 

 much has recently been made out. We know that the air around 

 us is constantly contaminated with living particles ; the floating 

 matter of the air is at all times capable of inoculating sterilised 

 test-tubes ; such air is introduced within the respiratory tubes, and 

 the alimentary canal in the processes of respiration and digestion. 

 But it has been pointed out by Dr. Lauder Brunton that the 

 contents of the alimentary canal are not part of the body ; a tube 

 may pass through a box, but the contents of the tube are not in 

 the box ; a barrier of epithelium lines all parts of the alimentary 

 tract, and the addition of a few more species of bacteria to those 

 always present in the digestive canal will, under conditions of 

 health, make little difference. 



Many of these, as is well known, will be rendered innocuous 

 by the process of digestion, as was illustrated by Dr. Klein when 

 he ventured to partake of a cholera-bacillus dinner ; others which 

 escape digestion will be hurried rapidly along with the stream and 

 not find any satisfactory nidus for germination. 



Again, in the case of the respiratory tract, the germ-laden air 

 will doubtless deposit some of its particulate burden on the 

 moistened mucous membrane, where the particles will be retained- 

 That this is the case was shown by the experiments of Tyndal, 

 who found that the residual air from the lungs was absolutely free 

 from particles which would reflect the electric beam, and this fact 

 is confirmed by the observations of Lister and others that, in 

 cases of Empyema with Pneumothorax, the air escaping from a 

 small opening in the periphery of the lung does not necessarily 

 convey any septic germs into the pleura. The particulate germs 

 will then rest on the surface of the mucous membrane of the 

 larger respiratory tubes. They wiU not penetrate the deeper parts 

 of the lung, the alveoli, where the epithelium is deficient or absent, 

 but will be propelled by ciliary action towards the outlet, and 

 either discharged by expectoration or find their way into the ali- 

 mentary canal. 



The protecting barrier of epithelium exists, then, along both 



