316 STRUCTURE AND LIFE HISTORIES 
the wound and caused gangrene, or inflammations, more 
fatal than the disease for which the operation was under- 
taken. When the discovery of bacteria, and of their 
universal presence, enabled men to understand these 
facts it was only a step to the practice of making the 
operating rooms, the surgical instruments, and the surface 
to be cut, absolutely aseptic or sterile, thus making possible 
the almost unbelievable achievements of antiseptic surgery. 
Only a step! But what a wonderful and all-important 
step for the human mind to take. The honor and credit 
for taking it belong chiefly to the famous English surgeon, 
Lord Lister. 
HELPFUL BACTERIA 
296. Bacteria and the Dairy. Not all bacteria, by 
any means, are harmful to man. Many of the practices 
of the dairy, for example, are dependent upon the action 
of bacteria. This, in part, is thought to explain the 
peculiarly delicious flavor of June butter. The souring 
of milk is caused by the action of substances produced 
by the bacteria present in all unsterilized milk. In 
fact the familiar flavor of milk is due in large measure 
to the presence in it of certain bacteria and the sub- 
stances they produce. When milk is obtained under 
strictly sanitary conditions ("certified" milk) it loses much, 
if not all, of its characteristic and familiar flavor. The 
ripening, flavoring, and other peculiarities of the various 
varieties of cheese are due in part to the fact that they 
ripen under the influence of different kinds of bacteria 
or molds. The "ripening" is caused, in part, by the 
action on the substance of the cheese of the enzymes 
peculiar to the various fungi or bacteria growing in it. 
