This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Subcutaneous Injection. Hypodermic Method. These names have been given to a plan of medication, first announced, in 1855, by Dr. Alexander Wood, of Edinburgh, consisting of the injection of remedial substances into the subcutaneous areolar or cellular tissue. Though apparently aware that medicines might be employed in this way with a view to their effects on the system at large, Dr. Wood confined his views practically to their local influence, and especially to the relief of neuralgic pains by anodynes injected into the areolar tissue, as near as possible to the seat of the affection. He no doubt believed that the relief obtained depended on the local action of the medicine. To Mr. Charles Hunter is due the credit of having generalized this plan of medication, He soon satisfied himself that the anodyne effect was obtained by injecting the medicine at a distance from the seat of pain, as well as in its immediate vicinity; and hence concluded that the medicine operated, not immediately on the painful nerve, but through the medium of the circulation, being absorbed into the blood, and carried with it to all parts of the system. He ascertained, moreover, that substances, employed in this way, exercised their ordinary remedial influence over systemic diseases; and the important fact was thus established, that the areolar tissue might be resorted to as an avenue for the introduction of medicines into the system, in all cases in which their relations with this tissue allowed them to be applied to it in a condition permitting their absorption in sufficient quantity, without injurious local effects. It must, nevertheless, be admitted that certain medicines, especially anodynes, such as morphia and aconitia, while they operate on the system generally through absorption, do exercise a peculiar influence in the neighbourhood of their application. This was proved by experiments of Dr. Eulenberg, who found that the sensibility of the parts into which morphia was injected, on one side of the body, was much more diminished than that of the corresponding parts on the other side. (Am. Joum. of Med. Sci., April, 1866, p. 432.) It would probably be a good practical rule, to inject the areolar tissue as near as possible to the Beat of the disease, whenever there is good reason to believe that this is exclusively local, and in the arm or any other convenient spot, without reference to the precise locality of the pain, when this depends on disorder of the nerve-centres, a morbid state of the blood, or some other systemic derangement.
After the discovery of the method of subcutaneous injection by Dr. Wood, it came rapidly into use, and is now almost universally resorted to under favouring circumstances. From the abundance of the capillaries distributed in the areolar tissue, it affords extraordinary facilities for absorption; and suitable remedies, applied in this way, act much more rapidly, and consequently with greater energy in equal quantities, than the same remedies introduced into the stomach or rectum, or employed endermically. One cause, no doubt, of the greater rapidity of the action hypodermically, is that substances taken up from the alimentary canal are for the most part conveyed through the portal reins into the liver, and are distributed through that organ before reaching the general circulation, while from the areolar tissue they probably enter immediately into the veins. Another advantage possessed by this method is that the medicine enters the circulation unchanged, and there exercises purely its legitimate effects on the system; while, when given by the stomach, it is liable to be modified by the action of the various and often powerful agents it finds in the alimentary passages; and, besides, the effects it produces directly on the stomach and bowels, which are often much disordered by it, complicate its proper systemic influ-Mr. Hunter states that the remedial effects of medicines, hypodermically administered, are often more permanent than those from the ordinary methods of exhibition. Thus, he has repeatedly effectually cured cases of disease, by injecting certain medicines into the areolar tissue, which had long resisted the same medicines given by the stomach; being relieved, indeed, for a time by the latter method, but returning constantly with unimpaired force. It has been asserted that narcotics, especially opiates, are less apt to disturb the brain, than when given by the mouth.
Notwithstanding these various advantages of the hypodermic method. the number of remedies to which it is applicable is limited. Many art-altogether unsuited to it on account of the large quantity required to operate; others from their insolubility; and other, again, from their irritant properties, which at the same time occasion local inflammation, and interfere with absorption. It is now well known that crystalline bodies in solution are diffused through animal membrane much more readily than gelatinous or glue-like substances (the colloids of Graham). which pass with great difficulty if at all. Hence it may be inferred that active principles of medicines, capable of assuming crystalline forms, are much better adapted than others not crystallizable, to this mode of administration, as likely to find easier admission into the capillaries; and the fact is, that the vegetable alkaloids, as morphia, atropia, quinia, etc., are peculiarly fitted for subcutaneous injection.
The following circumstances are peculiarly favourable to this method of medication. It is called for in cases of great severity, where the prompt relief of pain is required, or immediate interference may be neces -sary to save life; and, under these circumstances, it may either be depended on as the chief agency, or may be used as auxiliary to the ordinary measures. As examples, may be adduced the excruciating pain attendant on the passage of a urinary calculus,and a case of pernicious fever in which life may depend on the prompt and efficient action of quinia. When the stomach rejects all suitable remedies, or the patient obstinately refuses or is unable to swallow them, subcutaneous injection is a most valuable resource. When, moreover, remedies by the mouth and rectum have been used without satisfactory results, there is a strong indication for this method. Indeed, it may be resorted to in almost all instances, otherwise not unsuitable, in which it may be preferred by the patient.