THE ISCHIUM THE PUBIS 95 



tendons of origin of the rectus femoris muscle. This angle is connected with the 

 ala or wide part of the bone by a constricted part, often termed the shaft. The 

 latter is of three-sided prismatic form. Its external surface is convex and rough, 

 and gives attachment to the deep gluteus muscle. Its pelvic surface is smooth and 

 is grooved for the obturator vessels and nerves. Its ventral surface is crossed by 

 vascular grooves, below which is a rough area, bounded internally by the psoas 

 tubercle. 



THE ISCHIUM 



The ischium (Os ischii) forms the posterior part of the ventral wall or floor of 

 the bony pelvis. It slopes a little downward and inward, but is practically hori- 

 zontal in the longitudinal direction. It is irregularly quadrilateral, and may be 

 described as having two surfaces, four borders, and four angles. 



The superior or pelvic surface (Facies pelvina) is smooth and slightly concave 

 from side to side. 



The inferior surface (Facies externa) is nearly flat, and is in great part rough- 

 ened for the attachment of the adductor muscles. 



The anterior border forms the posterior margin of the obturator foramen. 



The posterior border is thick and rough. It slopes obliquely inward and for- 

 ward to meet the border of the other side, forming with it the ischial arch (Arcus 

 ischiadicus). 



The internal border meets the opposite bone at the symphysis. 



The external border is thick and rounded, but concave in its length; it forms 

 the lesser sciatic notch, the lower boundary of the lesser sciatic foramen. 



The antero-internal angle or symphyseal branch (Ramus symphyseos) meets 

 the pubis, with which it forms the inner boundary of the obturator foramen. 



The antero-extemal angle or acetabular branch joins the other two bones at 

 the acetabulum, of which it forms more than half. Superiorly it bears part of the 

 superior ischiatic spine (Spina ischiadica), and internally it is grooved for the ob- 

 turator vessels. 



The postero-extemal angle is a thick three-sided mass, the tuber ischii (Tuber 

 ischiadicum) ; its lower border is the inferior ischiatic spine, to which the biceps 

 femoris and semitendinosus muscles are attached. 



THE PUBIS 



The pubis (Os pubis) is the smallest of the three parts of the os coxse. It 

 forms the anterior part of the pelvic floor, and may be described as having two 

 surfaces, three borders, and three angles. 



The superior or pelvic surface (Facies pelvina) is convex in the young subject 

 and the stallion, concave and smooth in the mare and usually in the gelding also. 

 The urinary bladder rests on it. 



The inferior or ventral surface (Facies externa) is convex, and in great part 

 rough for muscular attachment. Near the anterior border it is crossed by the 

 subpubic groove, the inner part of which is occupied by a large vein, the outer part 

 by the pubo-femoral ligament. 



The anterior border is thin in its inner part (except in the young subject and 

 the stallion), forming the pecten ossis pubis. Externally it bears the rough ilio- 

 pectineal eminence (Eminentia iliopectinea), beyond which it is continuous with 

 the ilio-pectineal line. 



The internal border joins the opposite bone at the symphysis pubis. 



The posterior border forms the anterior margin of the obturator foramen, and 

 is marked externally by the obturator groove. 



The internal angle meets its fellow at the anterior end of the symphysis. This 



