The deep fascia is a thin, but dense and strong, bluish membrane that lies under the superficial fascia and is attached to it by fibrous strands. The deep fascia clothes the muscles, investing them so closely that it forms a tight sheath around the limb and preserves the contours of the limb. From its deep surface it sends in wide sheets that form partitions or septa among the muscles. In that way the deep fascia provides fascial sheaths for many of the muscles, and for vessels and nerves that lie among the muscles; and parts of some muscles are attached to the investing fascia and to the septa. Some of the septa are attached to the bones and to the ligaments of joints that deeply among the muscles. The investing fascia is attached to the ligaments of joints and to the parts of bones that come to the surface between muscles; and in certain places it is thickened to form strong, restraining bands, called retinacula, that hold tendons or sinews in position, and serve also as pulleys on which the sinews move – for example, at the wrist and ankle.