: A layer of adipose (fatty) tissue between the skin and muscle. It has less blood flow than muscle, allowing for slower, more sustained medication absorption (e.g., insulin or heparin).

: Highly vascularized tissue used for intramuscular (IM) injections. The rich blood supply enables rapid systemic absorption.

: The outermost layers used for intradermal (ID) injections. This area is highly immunological, making it ideal for allergy and tuberculosis testing.

Choosing an IM site requires identifying bony landmarks to avoid damaging deep structures like the sciatic nerve or brachial plexus. Intramuscular Injection - StatPearls - NCBI Bookshelf

: The target for intravenous (IV) injections, providing immediate delivery into the bloodstream for emergencies or rapid fluid replacement. Anatomical Landmarks for Intramuscular (IM) Sites