There are, however, circumstances which considerably limit its applicability. The danger of producing great local irritation, inflammation, or gangrene, is extremely slight, when due attention is paid to the choice of medicines, the proper introduction of them, and the state of the system. Non-irritant remedies, in a liquid form, may almost always be injected without the least inconvenience; or, if a slight irritation be produced, it subsides rapidly, without leaving any unpleasant effect. But substances in themselves irritant, or rendered so by their solid state interfering with absorption, may excite inflammation, ending in suppuration, or perhaps in gangrene, and should, therefore, never be employed in this way, in reference to their systemic influence. Besides, when the System happens to be in a state threatening erysipelas or gangrene; or when these conditions either exist, or a tendency to them prevails in any locality, the hypodermic method should be avoided, or used only under the most urgent circumstances, from the risk that would be incurred of exciting these affections.

In some rare instances, great alarm has been caused by the unexpectedly rapid and apparently dangerous action of medicines injected into the areolar tissue. Thus, morphia has sometimes operated, in the ordinary dose, with extreme violence, so as to threaten life itself, though I have not found recorded any absolutely fatal case from this, or any other medicine used in the same way. This occasional violence is ascribed, and probably with justice, to the penetration of a minute vein, and the consequent injection of the whole dose at once into the current of the circulation. Caution should, therefore, always be observed to avoid any visible blood-vessel; and, when the effect of the medicine is experienced immediately, the instrument should be at once withdrawn. It has been advised, in reference to this danger, to inject the medicine rather slowly, so that the process may be arrested, on the occurrence of violent symptoms, before the whole dose has been injected. In all instances, extreme caution should be observed not to administer excessive doses of poisonous medicines in this way; the danger being greater than when the medicine is swallowed, or given by enema; as, in either of these cases, it may be evacuated, while no such opportunity would be offered in the hypodermic method. Should alarming phenomena, however, result at any time from too rapid or copious absorption from the areolar tissue, a vacuum should be immediately produced, by the application of a cupping-glass over the puncture, so as to impede or prevent the continuance of the process. In the treatment of systemic diseases, it is often desirable to maintain an equable and steady impression by means of medicine. In this case, the exhibition of the remedy by the mouth would be preferable; as absorption is maintained continuously from the stomach; while, given by subcutaneous injection, the medicine acts more or less interruptedly, or by starts.

Operation. When circumstances do not indicate a special site for the operation, the arm near the insertion of the deltoid is perhaps as convenient a locality as can be chosen. When the operation is to be repeated, the place of puncture should be varied, so as to avoid irritating or inflaming the part. The pain from the operation is very slight, not exceeding that of the prick of a pin, and by the most sensitive soon in general comes to be disregarded. Sometimes it may for a time be very acute in consequence of the perforation of a cutaneous nerve; but this rarely happens.

The instrument recommended by Mr. Hunter is a small syringe, with a glass barrel and silver fittings, the piston of which is worked by means of a screw, each half turn of which expels half a minim. The pipe, which is attached by a screw, is of silver, with a hardened gold point, as sharp as a needle, and an opening on one side, near the point, through which the liquid is expelled. No incision or perforation is required, other than that made by the sharp point of the pipe, which is not more painful than the prick of a pin. The liquid to be injected having been introduced into the instrument, the skin is made tense, and the point inserted by a quick movement perpendicularly through the skin, after which any direction may be given to it that may be deemed desirable. Then the required number of minims is injected by as many turns of the screw; so that the dose may be regulated with the utmost exactness. After this the instrument is withdrawn, and, to prevent the escape of the liquid, the perforation is covered with a small piece of adhesive plaster. previously warmed for the purpose.

Different forms of the instrument have been recommended, all having in common the sharp perforated point. In one form, instead of measuring the amount of injected liquid by the turns of a screw, the piston is constructed in the ordinary manner, and the dose measured by a graduation of the barrel of the syringe, or of the piston itself. Instead of the hardened gold point, this is sometimes made of steel, and may be permanently affixed to the instrument, or attached by a screw, and removable at pleasure.

It is important that the point should not become blunted by oxidation or other cause; as the success of the operation, and the avoidance of subsequent injury, depends much on the perfect cleanness and sharpness of this part of the instrument. The syringe, too, should be thoroughly cleansed after each operation, especially when a different medicine is to be injected. A second dose should not be given until the effects of the first have subsided; or, if previously to this time, it should be reduced. Mr. Hunter thinks it advisable, as a general rule, to use a strong solution, so that the required dose may be thrown in by three or four turns of the piston. The quantity thrown in at once should never exceed thirty minims, and less than this will sometimes cause uneasiness by distension, and partially escape through the aperture. The whole process need not consume more than half a minute. It is very important to attend carefully to the operation; as, when the solution is very strong, a few turns of the screw, in excess, may produce serious results. Mr. Hunter recommends that not more than half the full ordinary dose of a narcotic medicine for a man, nor more than a third of it for a woman, should be injected at first; as the medicine operates rapidly, and with the entire force of the quantity introduced, the whole being absorbed.

The medicines given in this way should always be perfectly dissolved; and if, as sometimes happens in using the alkaloids, some acid is necessary for their complete solution, not more should be employed for the purpose than is absolutely required. If the solution is turbid, it should be filtered before being injected. Pure water is the best solvent, though tinctures may be used when deemed advisable. It is especially important that the tinctures should be freshly filtered, as, after long standing, they are apt to deposit a portion of their contents; and this is especially the case with laudanum. In the selection of medicines to be injected, it is desirable to avoid such as give precipitates with the alkaline chlorides and albumen, both of which they might encounter in the areolar tissue.

A use of the hypodermic method not yet referred to, has been suggested and put in practice by Dr. Eulenberg; namely, to facilitate laryngoscopy by the use in this way of morphia, or other medicine calculated to obtund the sensitiveness of the fauces. (Am. Journ. of Med. Sci., April, 1866, p. 436).

Still another application of subcutaneous injection has been brought to the notice of the profession by Dr. Luton, of Rheims, in France, under the name of parenchymatous substitution. He refers to the subcutaneous injection of irritating substances, with the view of exciting more or less irritation, which, according to the substances injected, may be of three grades; first, simple pain with insignificant local disorder, secondly, an irritative congestion, or, thirdly, inflammation with suppuration, etc. The remedial influence aimed at is the substitution of the new excitation for a pre-existing disease of the same part; a principle of therapeutical action identical with that already considered in this work under the name of supersession. (See page 51.) By the first degree of irritation he supersedes neuralgia and other painful affections; by the second, glandular diseases, acute or chronic, and especially strumous engorgements; and by the third, white swellings, local osteitis or periostitis, caries, etc. He obtains these gradations of effect by the injection of substances of various degrees of irritating power, from solution of chloride of sodium, alcohol, etc., which occasion rather severe pain, followed by local swelling, in general readily dissipated, through tincture of iodine, which causes non-suppurative inflammation, sometimes followed by atrophic absorption, to solution of nitrate of silver, sulphate of copper, etc., which give rise to phlegmons with suppuration. (Arch. Gen., Oct. 1863, p. 285.) It is perceived that this application of subcutaneous injection is wholly different from those of Dr. Wood and Mr. Hunter, the former of which aimed at the relief of pain by the local action of anodynes, the latter to the production of remedial impressions through the circulation. It is yet comparatively new, and the recorded experience is insufficient to justify, on that basis, a positive decision as to its merit.

In relation to the particular substances employed in hypodermic medication, their therapeutic application, and the precise dose and mode of exhibition, the reader is referred to the several medicines as treated of in the second part of this work.