Hypodermis
The hypodermis is the layer beneath the dermis, made largely of fat and connective tissue. Also called the subcutis or subcutaneous layer, it cushions and insulates, and it anchors the skin to the structures underneath.
The deepest of the layers drawn on any cross-section of skin, and the one that gets least attention, largely because most of dermatology happens above it.
It is composed chiefly of fat, held in compartments by strands of connective tissue that run down from the dermis and anchor the skin to whatever lies beneath, whether that is muscle or bone. Larger blood vessels and nerves pass through it on their way up.
Its functions follow from its composition. It cushions, absorbing pressure that would otherwise reach deeper structures. It insulates. It stores energy. And it allows skin to move over the tissue below rather than being fixed to it, which is why skin on the back of the hand can be lifted and skin on the palm cannot.
Its thickness varies enormously by site and between people, which accounts for a good deal of the difference in how the same procedure feels in different places.
Dermatology touches on it mainly at the margins: some inflammatory conditions involve the fat itself, and depth of invasion into and through the layers above it is one of the things a pathologist reports on a sample. Otherwise it appears on the diagram, as it does throughout this site, chiefly to make clear where the layers that matter stop.
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