The use of antiseptics internally is limited by the resistance of the organism itself, as already mentioned (p. 102). In the stomach antiseptics are used for the purpose of preventing decomposition, and by thus lessening the production of irritating products they diminish irritation of the stomach and arrest vomiting. Those which are chiefly employed for this purpose are creasote, carbolic acid, sulpho-carbolates, salicylic and sulphurous acids. In the intestine antiseptics are useful in arresting putrefaction, and thus preventing the harm caused locally to the intestine by the products of decomposition as well as the injury due to their subsequent reabsorption. They therefore tend to check diarrhoea and dysentery. It is probably to its antiseptic action that corrosive sublimate owes its curative power in cases of infantile dysentery, and it is not improbable that the beneficial action of calomel is due to a similar action, for it has been found by Wassilieff greatly to retard the decomposition due to low organisms.

The great danger which may arise from an exceedingly minute portion of septic matter renders great caution necessary on the part of those who might, by a little indiscretion, convey it from one to another. Thus a number of years ago a medical man was nearly driven mad by an epidemic of puerperal fever which he had in his practice : one patient dying after the other. In order to get rid of any infection, he burnt all his clothes and went away for three months. During his absence everything went well. On his return the epidemic again broke out: on careful investigation he found the only thing he had forgotten to burn was his gloves, and these had acted as a reservoir of infection. The hands, imperfectly cleansed in the first instance, had conveyed the septic matter into the gloves, and there it remained, re-infecting the hands every time the gloves were put on. In the same way a thermometer may prove a cause of continual infection unless the thermometer be carefully washed, and, if necessary, disinfected, each time it is used and before it is put into the case. In a similar manner it has been found that gonorrhoeal matter may remain in the vagina and infect several persons without the woman herself ever suffering. One of the best antiseptics for disinfection in such cases is permanganate of potassium. This may be used to wash out abscesses, if there is any fear of danger from absorption of carbolic acid; and also as a lotion for ulcers or wounds about the mouth, the urethra, or anus, where the carbolic acid might be too irritating; as is evident from Koch's experiment, however (vide p. 92), a solution of the strength ordinarily used - one per cent., i.e. four grains to the ounce - is not sufficient to destroy the septic organism, although one of five times the strength will do so.

Another way in which septic poisoning may be produced is by the introduction of a catheter into the bladder, where this cannot be completely emptied naturally on account either of paralysis, enlarged prostate, or stricture. So long as the contents of the bladder have not come in contact with any foreign matter they may remain in the bladder for some time without undergoing decomposition, but if a dirty catheter should be passed, and thus a few organisms introduced into the bladder, decomposition may set up in the urine and septic poisoning ensue. A solution of carbolic acid in oil is sometimes trusted to for the disinfection of catheters, but, as Koch's experiments (p. 96) show that such a solution has little or no antiseptic power, the catheters should be disinfected by a strong solution of carbolic acid in water, and afterwards oiled before their introduction.

Antiseptics are employed externally in order to destroy microbes before their entrance into the body, and are administered internally with a like object, or for the purpose of at least preventing the free development and multiplication of the microbes.

They are employed externally in surgical operations, with the object of destroying any organisms which might find a nidus in the wound, and there give rise to the formation of poisonous substances. Both these substances and the bacteria themselves will not only have an injurious local action in the wound, but by undergoing absorption may prove injurious or fatal to the organism as a whole. The antiseptic plan of treatment has been empirically practised in a limited manner for a very long period without its principle being recognised : for the well-known Friar's balsam has antiseptic properties. It is to Lister that we owe the introduction of such a mode of treatment, not based upon mere empiricism, but upon scientific knowledge. The reason why it had fallen into disuse probably was that some of the antiseptic substances used for dressing wounds in the Middle Ages were irritants as well as antiseptics. Those who employed them did not know the reason why they were beneficial, and supposed that their virtue was due to their irritating properties. The ointments were accordingly made more and more irritating : and thus more harm than good was done, until they were discarded by Ambrose Pare. The antiseptic most commonly employed is carbolic acid. Not only are all the instruments to be employed disinfected by a watery solution, but the operation itself is conducted under a spray of the dilute acid, so as to render innocuous any organisms which may be present in the air. The wound is then covered with an antiseptic dressing. Whenever this requires to be removed it must always be done under the spray. The reason of these great precautions is obvious : if any germs, however few, gain an entrance they will soon multiply and prove as deadly as a great number, the only difference being one of time.