This method, the introduction of which we owe to Dr. Alexander Wood of Edinburgh, possesses great advantages.

It consists in the injection of a solution of a remedy under the skin. Absorption takes place from the subcutaneous cellular tissue rapidly, and it is much less likely to be modified by altered conditions of the organism than absorption from the stomach and intestine. For in the intestinal canal there is not only the condition of the circulation to be taken into consideration, but the fulness or emptiness of the stomach and intestine, the condition of their epithelial covering and of their nervous supply, and the state of the liver. These conditions may not only delay but entirely prevent absorption.

The advantages of the hypodermic method, therefore, are 1st, certainty of effect, and 2nd, rapidity of action.

As absorption of a drug takes place so much more rapidly from the subcutaneous cellular tissue than from the stomach, a less quantity is excreted during the process of absorption, and consequently a smaller quantity of the drug is required (p. 38 et seq.). But absorption does not take place with equal rapidity from all parts of the intercellular tissue. The vascularity of this tissue, and the rate of absorption from it, are greater on the temples and breast than on the back, and greater on the inner than on the outer surface of the arms and legs.

As the liquids used for hypodermic injection are usually concentrated solutions of powerful poisons, it is important that neither more nor less than the quantity previously determined upon should be administered. The syringe consists of two parts (Fig. 159), a glass barrel in which a piston plays airtight, and a hollow needle which fits tightly on to the end of the syringe either with or without a screw. The bore of the needle being very fine it is apt to get choked by rust, or by crystals of the substance last employed for injection forming within it, and rendering it impermeable and useless. In order to avoid this it should be carefully washed with water each time it is used, and a small piece of thin wire kept constantly in it during the intervals of use, or, better still, a little oil drawn into the bore of the needle. When the syringe has not been used for some time, the packing of the piston is apt to shrink, so that it will no longer either suck in fluid or drive it out of the barrel efficiently. This may often be remedied to a great extent by soaking the syringe for a short time in warm water and driving the piston up and down in it. If this is insufficient the piston may be taken out, and sufficient thread wound round it to make it work. Care must be taken also that the needle fits tightly on the syringe, and that no leakage takes place at the junction. The liquid to be injected should contain no solid particles which may obstruct the needle, and if any such should be present, the fluid may be filtered through clean blotting-paper.

Fig. 159.   Syringe for hypodermic injection.

Fig. 159. - Syringe for hypodermic injection.

The exact quantity required, and no more, should then be drawn up into the syringe and injected. Some syringes have a small screw upon the piston, so as to stop its movement at any required point. With such a syringe the barrel may be filled quite full of the solution, and the required quantity injected by forcing the piston down until it is stopped by the screw. The advantage of this arrangement is that if any leakage should occur, the screw may be moved further up, and an additional quantity of solution injected without the necessity of withdrawing and reintroducing the needle under the skin. If all proper precautions be taken, however, the necessity for such a procedure will rarely arise.

Convenient places for injection are the outside of the arm near the deltoid, the fore-arms, or the thighs. In order to avoid the risk of introducing the needle into a vein, the injection should not be made over a vein visible through the skin. The skin should be pinched up between the finger and thumb, the needle pushed directly through it, and then passed onwards a little way obliquely in the subcutaneous cellular tissue.

Objections to Hypodermic Injections. - The chief objections are, (1) the pain caused at the time by the introduction of the needle, or by the drug itself after its injection, (2) the inflammation which either the needle or the drug may give rise to subsequently, (3) the scars which may be left by the frequent repetition of the injection, (4) the danger of communicating a specific or contagious disease, (5) the danger of injecting the drug directly into a vein, and thus producing a dangerous or fatal effect from the too rapid entrance of the drug into the circulation. With a little care these untoward results may be almost entirely avoided. If the needle is well sharpened the pain of introducing it is very slight, and may be still further lessened by making the patient take several deep breaths in rapid succession before the injection is made. If the patient is excessively sensitive, partial or complete anaesthesia of the part may be produced by cold or by carbolic acid (p. 204).

The solutions should always be perfectly free from solid particles and should be as neutral and bland as possible. Metallic salts have their irritating properties diminished or removed when combined with albumen or with an alkaline citrate or tartrate so as to form double salts.

By washing the syringe and needle thoroughly out with carbolic acid, the danger of conveying any specific or contagious disease is rendered very slight, and it may be completely avoided by heating all parts of the syringe in a spirit-lamp before using them. The syringe employed by Koch in his experiments on the effects of micro-organisms in producing disease (Fig. 159) is admirably adapted for this purpose, as all parts of it can be readily heated, and the padding upon the piston, which is more likely to retain infective matter than any other part of the syringe, can be renewed each time that the instrument is employed. In order to prevent pain or inflammation being caused by the solution injected, care should be taken that its reaction is as nearly as possible neutral, and that the quantity should not be great. The smart which follows the injection is lessened by rubbing the finger gently over the part so as to distribute the fluid in the subcutaneous tissue. If it is necessary to employ such large quantities as half a drachm or a drachm, as may be the case with ergot, it is better not to inject the solution under the skin but into the substance of a muscle, such as the gluteus maximus.

Cicatrices are not apt to follow injection if the precautions already mentioned have been taken, and if the injections are not made too frequently at the same point.