By the skin.—Although a number of drugs, notably mercury, are so readily absorbed by the skin of cattle as to render poisoning easy, medicines are not given in this way for their general or constitutional but only for their local effect.
Diseases of the skin and superficial parasites are treated or destroyed by applications in the forms of washes, ointments, dips, and powders. Liniments and lotions are applied to the skin for the relief of some near-lying part, such as a muscle, tendon, or joint. Blisters are applied to the skin for the purpose of obtaining the effect of counterirritation upon a neighboring region or organ. Cold water may be applied to the skin to reduce the temperature and to diminish congestion or inflammation in a superficial area or to reduce the temperature of the whole body. High fever and heat strokes are treated in this way.
By the tissue beneath the skin.—Hypodermic or subcutaneous injections are often made for the purpose of introducing a drug, reagent, or vaccine directly into the connecting tissue beneath the skin. Introduced in this way, the substance is quickly absorbed, none of it is lost, and its whole effect is obtained, often within a few minutes.
There are numerous precautions necessary in making a subcutaneous injection, most of which have to do with cleansing and sterilization. It is also important to select a proper site for the injection, so that blood vessels, joints, and superficial nerves, organs, or cavities may all be avoided. With due regard for the necessary precautions, there is practically no danger in such an injection, but it should be attempted only by those who are able to carry it through in a surgically clean way. Only certain drugs can be given subcutaneously, and dosage must be accurately graduated.
By the veins.—Certain medicines act most promptly and surely when introduced directly into the blood by injecting them into a vein, usually the jugular. Some vaccines and antitoxins are administered in this way. Intravenous injection should be practiced only by experienced veterinarians.