The lower or true pelvic surface (Fig. in) is enlarged by the presence of the obturator membrane in the recent state, and the Obturator internus has an extensive origin (Xo. 2 in Fig. in) from this membrane and from the bone as far up as the lower sacro-iliac hgaments. From this origin the fibres converge on the lesser sciatic notch, where they form a group of tendons that play round the notch on a cartilage-covered surface lined by a bursa. It is therefore evident that the area of bone (A) between the muscle origin and the notch is covered by the muscle that plays over it.
In front of the muscle and above it a surface of bone (B) is left exposed. This is covered by peritoneum, but under the peritoneum it is crossed by the obliterated hypo gastric artery (hyp.), the vas deferens or round ligament (v.d.), and further back may have some relation to the external iliac vein (V.). Below the vein, the obturator nerve (O.N.) runs to the top of the foramen and is joined by the vessels.
To understand the remaining relations and attachments it is necessary to follow the disposition of the pelvic fasciae on the bone to some extent.
The Obturator internus is covered by the parietal fascia of the pelvis which is attached all round its pelvic surface : in No. 2, it is represented by the fine black hne pp, which is seen practically to map out the upper and front margins of the muscle origin. At x it is continuous with the iliac fascia with some shght attachment to the bone, and at y it runs into the ligaments. Along the front border of the great sciatic notch it is continuous with the fascia covering Pyriformis with some shght attachment to the bone : this is shown at xx in this drawing and in No. 3, where the fasciae are represented in situ. The two fasciae separate at the ischial spine : pp is continued along the front of the base of the spine and is carried from its lower border across the notch and tendon to the top of the tuberosity by attachment along the lower part of the lesser sciatic ligament and from this to the front border of the greater ligament. It is now attached to the falciform extension of the greater ligament, and is thus carried to the pubic ramus, where it again closely follows the muscle origin, reaching with it the upper border (m) of the obturator membrane.
When this parietal fascia is in position the bone is left uncovered in the area B, while the spine of the ischium projects behind and between the fascia and that covering Pyriformis. The muscular floor of the pelvis is attached to these two bony surfaces -that is, to the lower and front part of the bone above and in front of the Obturator and to the spine of the ischium, and between these points it arises from the parietal fascia in a line connecting them.
The floor is formed mainly by Levator ani, which has therefore a marking on the back of the pubic body, but the back portion of the sheet is the Coccygeus : this, then, arises from the ischial spine in addition to the hinder fibres of the Levator, and as its back and upper fibres are converted into the lesser hgament. this structure is attached to the tip of the spine and is in the plane of the pelvic floor.
The floor, cut away near its origin, is seen in No. 3 (Fig. 111) with its covering fascial sheath : its upper surface supports the viscera, so the fascia on this aspect is the visceral layer, that on the lower surface being the anal fascia. The two layers are continuous in front round the margin of the pubic origin (as seen in No. 2) and run into the lesser ligament behind.
It is evident that the structures on the bone below the level of a line drawn from the pubis to the spine of ischium must be below, the origin of the pelvic floor, and therefore are concerned in forming the outer wall of the ischio-rectal fossa. These are : the lower part of the obturator internus covered by its fascia as far as the line pp and the falciform edge, and the inner aspect of the bone below this line.
The extent of the ischio-rectal fossa is seen in No. 1 in the figure. The muscular floor is drawn down into a sort of cone round the anal canal, so that each half is a muscular sheet forming a curved plane, but looking as a whole downwards and outwards : the fossa is seen to extend forward between the Levator and the ramus as far as the front end of the muscle, while it passes behind into the narrow cleft between the two sciatic hgaments, where it ends as these come into apposition and partial fusion. A finger placed in the fossa would have the ramus and tuberosity with their attached structures to its outer side.
The front part of the fossa as seen in No. 1 is covered in (from below) by the perineal shelf of triangular ligament, etc., thrown across from ramus to ramus. The deeper layer of the ligament runs into the anal fascia, where this turns round the free edge of the Levator in front of the central point of the perineum to join the visceral layer. Thus the front part of the fossa ends in a pyramidal cul-de-sac floored by the deep layer of the triangular ligament, and limited in front as well as internally by the junction of this layer with the fasciae on the Levator. This cul-de-sac is seen opened up in No. 3 and on transverse section in No. 4, and it is clear that the Obturator internus and its fascia alone form the inner wall here, because the ramus below this muscle is altogether taken up by the structures that make the transverse shelf. Thus the only part of the lower margin of the pelvic aspect of the bone that is really directly concerned in forming the outer wall of the fossa is that part of the ischium that lies below the falciform ridge (D in Nos. 2 and 3), for the ramus in front of this is for the attachment of the structures lying between the deep layer of the triangular ligament above and Colles' fascia below.
We can now place the various perineal structures on the bone. It is well to have several male bones on which the markings can be followed, for different bones vary considerably in the extent and nature of the secondary lines that are apparent on them, and it is very exceptional to find a bone that has its markings complete here.
First look at the everted surface. This carries the crus with its muscle, so that its upper and inner border must be for the superficial layer of the triangular ligament and its lower border for Colles' fascia. Trace the falciform ridge forward and it leads to the upper border, thus showing that the falciform process of the great sciatic ligament is in plane with the triangular ligament. Occasionally a line can be traced from the falciform ridge to the lower border of the everted surface : this shows the attachment of Colles' fascia to the bone.
The deep layer of the triangular ligament is attached to the bone in the line of the lower and front attachment of the parietal fascia, and this can as a rule be found on the bone without difficulty : the Compressor urethrae can now be placed on the bone, and behind this the two layers of the triangular ligament are seen to fuse, so that they form a single layer at their bony attachment, when they are joined by Colles' fascia.
The various lines of fasciae are shown in No. 2 in Fig. til, where the superficial layer and deep are placed as blue fines and Colles' fascia green : observe that the deep layer is continuous on the bone with the fascia covering the Levator (black).
The crus is firmly fastened by fibrous tissue to the everted surface : behind and internal to it is the origin of Ischio-cavernosus and behind this Transversus perinei, and all these must be between the blue and green lines.
The extent to which these different areas can be traced backwards varies in individual bones, and they are of course badly marked and much smaller in female bones, but the plan of arrangement remains the same. The everted surface ends below in a thick edge to the inner hp of which Colles' fascia is attached, while the outer lip is for the fascia of the inner side of the thigh.
No. 4, Fig. in, is a plan of the areas just described on the inner aspect of the pubic ramus as shown by section : (a) surface of origin of obturator, (b) surface for compressor, (c) everted surface.
Now examine the area above the falciform ridge. The lower margin of the obturator internus comes down to the ridge, for the muscle is a fairly thick one and makes a mass that fills the slight concavity and has a convex lower margin. The parietal fascia covers this part and is attached below it to the falciform process, making a sheath (Alcock's canal) for the internal pudic vessels and nerve, which are thus carried along the top of the falciform process and brought by it to the back of the triangular ligament. They reach the sheath by crossing the inner or ischio-rectal aspect of the obturator tendon. In No. 3 the vessels are shown piercing * the lining fascia above the level of the ischial spine (S) ; thus, when they come in again below the spine they are deep to the fascia and below the level of the pelvic floor. The figure also shows the course of the canal.
The extent of origin of the Levator from the pubis is variable : it may be as shown in No. 2, Fig. in, or its level may be at the dotted line la in No. 3, or even higher. There may be tendinous fibres causing roughnesses, but as a rule the rough area near this region is for the Obturator, as in No. 2. The covering fasciae also reach the bone, and thickenings in the visceral layer form the anterior pubo-prostatic ligaments, which may apparently be sometimes indicated.