An understanding of the basic anatomy of the skin and its underlying structures is helpful if medicines are to be injected efficiently. The outer layer of the skin consists of the epidermis or surface, beneath which is the thicker dermis consisting of living cells which are multiplying (Fig.4-4). The skin is attached to the underlying muscle by a combination of subcutaneous fat and fibrous connective tissue. An injection directly into the skin or the dermis (intradermal) requires a tiny needle less than 5mm long. This is used when testing for tuberculosis. Fat itself has a poor blood supply and an injection into fat is poorly absorbed. Abscesses are also more likely to develop. Subcutaneous injections must only be given where there is a minimal amount of fat.