132 Anatomy of Skeleton 



The floor, cut away near its origin, is seen in No. 3 (Fig. in) 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. i 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 ligaments, 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. I 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 inter- 

 nally by the junction of this layer with the fascia; 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 Colics' fascia below. 



FIG. in. i. Pelvis seen from below, with the Levator ani in position : on each side, the ischio-rectal fossa 

 is between the muscle and the bone, which is covered in part by Obturator internus. When the perineal 

 shelf, composed of triangular ligament, etc., is thrown across the anterior portion of the opening of the 

 pelvis, it makes a floor for the front part of the fossa on each side ; this can be seen in No. 3. 2. Inner 

 aspect of pelvic part of os innominatum. Right side. Account is given in the text. A; area covered 

 by Obturator internus, but not giving origin to it ; hence smooth and concave. B, area covered by 

 sub-peritoneal tissue, therefore smooth, and crossed under this by obliterated hypogastric artery (hyp) 

 and vas deferens (vd) ; sometimes the Levator ani arises from part of this area (la in No. 3) and makes 

 a slight roughness on the bone. V, situation of external iliac vein. D, inner slope of lower part of 

 ischial tuberosity, covered by fatty tissue continuous with that in ischio-rectal fossa (see Fig. 107). 

 3. Left side, showing in diagrammatic form the structures in position. B and D as in No. 2. Above 

 and in front of D is the side wall of the ischio-rectal fossa, extending forward to the meeting of the 

 triangular ligament and fasciae. C, Coccygeus and lesser sciatic ligament. S, spine of ischium. P, 

 Pyriformis. The figure is for comparison with No. 2. 4. Scheme of a section through the front part 

 of the ischio-pubic ramus, to show the various surfaces in relation with the structures represented in 

 the other figures : a, surface for origin of Obturator internus ; b, surface for Compressor urethra^ ; 

 c, everted surface for crus. 



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 Colics' fascia. Trace the falciform ridge forward and it leads 

 to the upper border, thus showing that the falciform process of the great sciatic liga- 

 ment is in plane with the triangular ligament. Occasionally a line can be traced from 



