injection, introduction of a fluid into the body, usually by means of a needle and syringe. The material injected may be a test substance (as in determining allergic sensitivity or immunity to a disease), an anesthetic, a therapeutic drug, a nutrient (in cases where intravenous feeding is necessary), blood, or blood plasma (see blood transfusion). An intracutaneous injection is the introduction of a small amount of fluid between the skin layers. A subcutaneous injection is directed to the tissues under the skin. When quicker absorption of a drug is required, an intramuscular injection may be used; even more rapid action is obtained by injection into a vein (intravenous). In certain emergencies involving the heart, such as cardiac arrest, an intracardiac injection can penetrate directly into a chamber of the musculature of the heart. Anesthetics are sometimes injected into the spine. In an injection by means of a jet injector gun, fluid penetrates through the skin by means of air pressure and there is no visible puncture mark. Micro-injections can be made into fetuses, even individual cells, with tiny capillary injectors.