INFILTRATION ANÆSTHESIA.--In this method anæsthesia is induced by injection of the drug directly into the tissues to be operated upon. This method acts by paralysing the sensory nerve-endings. Although the term anæsthesia is constantly used, it is, strictly speaking, an operative analgesia that is aimed at; it is a paralysis of the pain-conducting fibres, and not of those which conduct purely tactile sensations, the patient being often able to feel the contact of the fingers and instruments during the operation. True anæsthesia can be secured, but it is necessary to use considerably stronger solutions than those that are required for the production of analgesia. SOLUTION OF THE DRUG.--Novocaine is far superior to any other drug for infiltration anæsthesia. The strength for most purposes is ½ per cent., though some operators find ¼ per cent. quite satisfactory. In specially sensitive parts, such as the nose, throat, or mouth, 1 or even 2 per cent. solutions may be preferable. Sufficient sodium chloride should be added to prevent osmosis of the solution into the tissue cells. The most satisfactory preparation is:-- Novocaine 0·25, 0·5, 1 or 2 (¼ to 2 percent.) Normal salt solution (half 100·0 (·45 per cent. NaCl) strength) [Illustration: Fig. 51. All-Metal Syringe for Infiltration Anæsthesia.] Adrenalin is added to the solution in the proportion of 3 drops of 1 in 1000 adrenalin chloride to the ounce, and as much as 6 ounces of this preparation may be safely given. The novocaine solution can be boiled before use, but the adrenalin must not be added until after boiling. For private practice it is sometimes convenient to procure the novocaine in tabloid form of definite strength combined with sodium chloride. These tabloids are added to the necessary amount of water, and the whole boiled. The adrenalin can then be added. CHOICE OF SYRINGE.--The best form of syringe for infiltration anæsthesia is the all-metal syringe illustrated in Fig. 51. It ought to have a capacity of at least 10 c.c. The advantage of the all-metal syringe over glass syringes is that it can be safely sterilised by boiling, and does not get broken. Between the syringe and the needle is a metal segment which is curved so that the needle is set at an obtuse angle to the syringe. This renders the infiltration of the tissues at the proper depth much easier. The needles employed are the ordinary hypodermic needles which are sold in small tubes. The sizes which should be selected are 1 inch, and 3 or 3½ inches. The needle fits into a hole in a small metal mount, which screws on to the intermediate metal portion. This section also is attached to the syringe by a screw, and these screw attachments have the advantage of rendering leakage impossible.