The curved lines separate the gluteal planes. The upper or posterior hne commences just in front of the posterior inferior spine and reaches the crest about 2 1/2 inches in front of the upper spine. Gluteus maximus arises behind it, and can therefore extend directly on to the great sciatic ligament, which is fastened to the lower spine and the margin of the notch just in front of this (see Fig. 107), and can thus reach the sacrum. The middle line commences near the deepest part of the greater notch and ends about 2 inches behind the anterior superior spine : observe (Fig. 107) that the line does not actually reach the outer lip of the crest. Gluteus medius arises between this line and the upper one, and some of its deeper posterior fibres may be separated from it as an additional origin of Pyriformis from the top of the notch : just in front of this the margin of the notch may be slightly grooved by the Gluteal art ery.
Fig. 106.-To illustrate the formation of the ligamentum teres. In its early stage the ilium (il) and ischium (is) are alone concerned in the articulation, the synovial lining passing off them on to the capsule which is attached round their surfaces. The pubis is covered by these fibres and has no articular area. In the next stage the covering fibres are destroyed and the pubis has acquired an articular surface (p). This extends, and the front part of the original ischial capsule is caught, so to speak, between the extending surface and the ischium ; these fibres persist and remain attached to the ischial region, but on their surface the synovial cavity has extended, as shown in the last diagram, and has joined the older cavity below as well, passing between the femur and the lower portion of the attached capsule. Thus a synovial funnel is formed, wider below where it includes the attachment of the fibres and narrowed at its femoral end, where it is fastened to the fovea.
The lower curved line is variable as to its commencement : it is directed toward the upper part of the anterior inferior spine, or toward the notch above this.
But the arrangement of the lines is not quite so simple as this description implies and by comparing several bones the following account can be verified.
The Gluteus maximus is in the same curved plane as the Tensor fasciae femoris, which arises from the bone just below the crest for about 3 inches behind the anterior superior spine. The two muscles are connected by a strong aponeurotic sheet, the Gluteal aponeurosis, which splits behind to enclose G. maximus, and in front to enclose the Tensor : between these two it covers the medius and is attached to the outer edge of the crest. On the bone the line of this aponeurosis is easily followed, and the layer that passes deep to maximus is seen to form the superior curved line and so reach the great sciatic ligament. Again, in front, the line on the bone is seen to divide to enclose the Tensor, and the middle curved line runs into the lower of the two divisions : thus the middle line never really reaches the crest, but the very front fibres of medius, at their origin, lie just under cover of the posterior fibres of the Tensor, which otherwise only covers the minimus. The area of Tensor fasciae turns down along the outer side of the notch below the superior spine, with its enclosing lines, here lying to the outer side of and behind Sartorius,
The lines join again below the origin of the muscle, indicating the junction of the two layers of its covering aponeurosis, and this complete layer turns back above and outside the Rectus to form the inferior curved line, the deepest of the gluteal markings.
We thus have an aponeurotic sheet covering the Glutei and turning deeply round the front margin of Gluteus minimus, forming a sort of septum that divides this group from those in front : this will be placed in contact with the outer aspect of the tendon of Rectus, and is pierced by the ascending branch of the external circumflex artery coming from under that muscle to run among the Glutei. The front fibres of the gluteal muscles are attached to this " septum," which is in fact made from their degenerated fibres, and it thus passes with them to the trochanter of the femur : the aponeurotic band that covers the front of minimus is usually strong, and is sometimes termed the ilio-trochanteric band ; this can therefore be placed on the Ilium at the lower front part of the Tensor area. The " septum " being continuous with the aponeurotic layer on the deep aspect of minimus, comes into contact with the capsule and is attached to it : this can sometimes be followed on the bone when the Gluteus minimus is large.
Below minimus the back of the acetabulum is covered by Pyriformis, and has the great sciatic nerve resting on it. This is soon separated from bone by Obturator internus and gemelli, but the small nerve to Quadratus femoris still lies with its accompanying vessels on bone deep to these, finally passing to the deep aspect of its muscle, superficial to the tendon of Obturator externus (Fig. 108). The tendon of the internal Obturator turns round the lesser sciatic notch, which is lined by cartilage and a bursa : upper Gemellus has a rounded tendinous origin on the base of the spine above, while lower Gemellus has a linear origin along the top of the tuberosity below, therefore is covered somewhat by the tendon leaving the notch. The spine of the Ischium is crossed by the nerve to Obturator internus, and at its tip by the internal pudic vessels, these separating it from the coccygeal branch of the sciatic artery running Ao pierce the great sciatic ligament. The small sacro-sciatic ligament is attached to'the tip of the spine.