A longitudinal groove is visible grossly on the lateral surface of the main trunk, except at the proximal end; distally a second groove is visible posterior to the first one (fig. 6). The long anterior groove indicates the boundary between the peroneal and tibial nerves; this groove may disappear distally, although the posterior groove is always visible distally. The posterior groove, which is continuous with the division of the sciatic nerve into anterior and posterior branches, represents the boundary between two divisions of the tibial nerve. (This is discussed in detail below.) In the middle of the thigh the peroneal and tibial nerves are enclosed in separate connective tissue sheaths, although the two sheaths are fused together; the point of fusion is marked by the anterior groove. If the two sheaths are slit open, the two nerves can be removed and can be seen to be entirely separate. In the proximal part of the main trunk the peroneal and tibial components are enclosed in a single sheath and appear as an undivided trunk; but if the sheath is removed, the two components can be pulled apart rather easily, although there may be some intermingling of a few fibers. This separation can be extended to a point proximal to the origin of all the branches of the sciatic nerve; thus it can be determined which branches arise from the peroneal component and which from the tibial. (These branches arise from the sciatic nerve as, or immediately before, the nerve passes through the ilio-ischiatic fenestra; since this level of the intact nerve could not be adequately observed, it was necessary to cut the nerve inside the pelvis and pull the intrapelvic part of the nerve out through the ilio-ischiatic fenestra. In doing this, care had to be taken to avoid damaging the most proximal branches.) Three main branches arise from the peroneal component (apart from the main trunk) and two from the tibial. Including the peroneal and tibial components of the main trunk, the sciatic nerve can be divided into seven major divisions--anterior peroneal, middle peroneal, dorsal peroneal, posterior or main peroneal (contributes to main trunk), anterior or main tibial (contributes to main trunk), middle tibial, and posterior tibial. Farther distally, the posterior peroneal division becomes the peroneal nerve and the anterior tibial division becomes the tibial nerve. For descriptive purposes, the term peroneal (or tibial) _nerve_ will be applied only where the nerve is enclosed in its own sheath, but regardless of whether or not the sheath is fused with another; proximal to this, where the separation may not be precise, the terms peroneal (or tibial) _division_ or _component_ will be used.