42 Transactions. — Miscellaneoiis. 



animal parasites. Age influences the predisposition of the 

 host. Young plants are more susceptible to the attack of 

 fungi than older ones ; and we all know how much more 

 prone children are to catch scarlatina, and measles, and 

 whooping-cough than adults, and how much more liable they 

 are to be affected with animal parasites. Again, as to pre- 

 ferences, in a consumptive family all the boys may escape and 

 the girls become consumptive, or vice versa, or all the mem- 

 bers of a family but one may escape. So, during an epidemic 

 of cholera or of typhus many individuals may be exposed to 

 infection, but the bacteria select by preference those whose 

 tissues are weakened, and so predisposed for their reception, 

 or which are unable to resist the invasion. Others, again, 

 accommodate themselves in various species of host, as the 

 tubercle bacillus for instance, which can live in man, mon- 

 keys, cattle, fowls, &c., just as the common mistletoe is able 

 to graft itself on trees so widely different as the apple-tree, the 

 willow, the oak, and the fir. Bacteria have also preferences 

 as to the part or tissue of their host, being found in that part 

 in which the conditions are most favourable to their life and 

 growth. The bacillus of cholera selects the intestines, the 

 spirillum of relapsing fever is found in the blood. In the 

 same way animal parasites choose their home in their host. 

 The Trichina spiralis, being introduced into the intestinal canal, 

 bores its way into the spaces between the fibres of the 

 muscles, while the liver-fluke finds out the channels leading 

 to the liver, where it sets up its home for the time being. 

 Bacteria obttiin entrance to the body with the air that we 

 breathe, with our food, and also through wounds. 



And now we have to consider, in what way can the pre- 

 sence of these minute organisms cause disease ? First, they 

 act as foreign bodies. It is an axiom in surgery that the pre- 

 sence of a foreign body is sufficient to produce inflammation. 

 A familiar example of this is the inflammation caused by a 

 grain of sand in the eye. Inflammation is necessary, however 

 inconvenient to the individual, and its use we shall see pre- 

 sently. As parasites, bacteria of course live at the expense of 

 the tissue in which they grow. If they merely absorbed the 

 juices of the part the changes effected by them would only 

 consist in lessening the nutritive substance, and the mischief 

 caused by them would be represented merely by the loss thus 

 occasioned. 



But they act as ferments, and it is this action especially 

 which is the cause of disease. And here it may be as well to 

 define what is meant by tlie word " ferment." A ferment is de- 

 fined to be a body which causes a change of composition in 

 organic compounds without itself forming any i^art of the 

 resulting products. The alcoholic and acetous fermentations 



