Fascia is a term applied to masses of connective tissue large enough to be visible to the unaided eye. Its structure is highly variable but, in general, collagen fibres in fascia tend to be interwoven and seldom show the compact, parallel orientation seen in tendons and aponeuroses.
Fascia that is organized into condensations on the surfaces of muscles and other tissues is termed investing fascia, but this may not be its sole function. Between muscles that move extensively, it takes the form of loose areolar connective tissue and provides a degree of mechanical isolation. It constitutes the loose packing of connective tissue around peripheral nerves, blood and lymph vessels as they pass between other structures and often links them together as neurovascular bundles. It forms a dense connective tissue layer investing some large vessels, e.g. the common carotid and femoral arteries, and its presence here may be functionally significant, aiding venous return by approximating large veins to pulsating arteries.


Superficial fascia is a layer of loose connective tissue of variable thickness that merges with the deep aspect of the dermis. It is often adipose, particularly between muscle and skin. It allows increased mobility of skin, and the adipose component contributes to thermal insulation and constitutes a store of energy for metabolic use. Subcutaneous nerves, vessels and lymphatics travel in the superficial fascia; their main trunks lie in its deepest layer, where adipose tissue is sparse. In the head and neck, superficial fascia also contains a group of striated muscles (collectively termed the muscles of facial expression), which are a remnant of more extensive sheets of skin-associated musculature found in other mammals.
The quantity and distribution of subcutaneous fat differs in the sexes. It is generally more abundant and widely distributed in females. In males, it diminishes from the trunk to the extremities; this distribution becomes more obvious in middle age, when the total amount increases in both sexes. There is an association with climate (rather than race), and superficial fat is more abundant in colder geographical regions. Superficial fascia is most distinct on the lower anterior abdominal wall, where it contains much elastic tissue and appears many-layered as it passes through the inguinal regions into the thighs. It is well differentiated in the limbs and the perineum, but is thin where it passes over the dorsal aspects of the hands and feet, the sides of the neck and face, around the anus and over the penis and scrotum, and is almost absent from the external ears. Superficial fascia is particularly dense in the scalp, palms and soles, where it is permeated by numerous strong connective tissue bands that bind the superficial fascia and skin to underlying structures.


Deep fascia is also composed mainly of collagenous fibres, but these are compacted and in many cases arranged so regularly that the deep fascia may be indistinguishable from aponeurotic tissue. In limbs, where deep fascia is well developed, the collagen fibres are longitudinal or transverse, and condense into tough, inelastic sheaths around the musculature.