There are many ways in which accidental infection may occur.  Take, for example, the case of a person who receives a cut on the face by being knocked down in a carriage accident on the street.  Organisms may be introduced to such a wound from the shaft or wheel by which he was struck, from the ground on which he lay, from any portion of his clothing that may have come in contact with the wound, or from his own skin.  Or, again, the hands of those who render first aid, the water used to bathe the wound, the handkerchief or other extemporised dressing applied to it, may be the means of conveying bacterial infection.  Should the wound open on a mucous surface, such as the mouth or nasal cavity, the organisms constantly present in such situations are liable to prove agents of infection.

Even after the patient has come under professional care the risks of his wound becoming infected are not past, because the hands of the doctor, his instruments, dressings, or other appliances may all, unless purified, become the sources of infection.

In the case of an operation carried out through unbroken skin, organisms may be introduced into the wound from the patient’s own skin, from the hands of the surgeon or his assistants, through the medium of contaminated instruments, swabs, ligature or suture materials, or other things used in the course of the operation, or from the dressings applied to the wound.

Further, bacteria may gain access to devitalised tissues by way of the blood-stream, being carried hither from some infected area elsewhere in the body.

The Antiseptic System of Surgery.—­Those who only know the surgical conditions of to-day can scarcely realise the state of matters which existed before the introduction of the antiseptic system by Joseph Lister in 1867.  In those days few wounds escaped the ravages of pyogenic and other bacteria, with the result that suppuration ensued after most operations, and such diseases as erysipelas, pyaemia, and “hospital gangrene” were of everyday occurrence.  The mortality after compound fractures, amputations, and many other operations was appalling, and death from blood-poisoning frequently followed even the most trivial operations.  An operation was looked upon as a last resource, and the inherent risk from blood-poisoning seemed to have set an impassable barrier to the further progress of surgery.  To the genius of Lister we owe it that this barrier was removed.  Having satisfied himself that the septic process was due to bacterial infection, he devised a means of preventing the access of organisms to wounds or of counteracting their effects.  Carbolic acid was the first antiseptic agent he employed, and by its use in compound fractures he soon obtained results such as had never before been attained.  The principle was applied to other conditions with like success, and so profoundly has it affected the whole aspect of surgical pathology, that many of the infective diseases with which surgeons formerly had to deal are now all but unknown.  The broad principles upon which Lister founded his system remain unchanged, although the methods employed to put them into practice have been modified.