_D_, Ilium; _Sch._, pubis; _S_, ischium; _a_, anterior border (crest) of ilium; _b_, internal angle of ilium; _c′_, external angle of ilium; _d_, great sciatic notch; _e_, external border of ilium; _f_, iliac surface; _g_, linea arcuata; _h_, rough ligamentous area; _i_, auricular surface; _k_, ilio-pectineal line; _l_, psoas tubercle; _m_, transverse branch, and _n_, symphyseal branch of pubis; _o_, _o′_, symphysis pelvis; _p_, anterior border of pubis (pecten); _q_, tuberculum pubicum; _r_, ilio-pectineal eminence; _s_, tuber ischii; _t_, ischial arch; _u_, lesser sciatic notch; _v_, acetabular branch, and _w_, symphyseal branch of ischium; _x_, obturator foramen; _y_, articular surface of acetabulum; _z_, fossa acetabuli; _1_, groove for ilio-lumbar artery; _2_, groove for iliaco-femoral artery; _3_, subpubic groove; _4_, depression for inner tendon of origin of rectus femoris; _5_, rough area for attachment of adductor muscles. (Ellenberger-Baum, Anat. d. Haustiere.) ] The =internal= or =sacral angle= (Tuber sacrale) curves upward and a little backward close to the first sacral spine, and forms here the highest point of the skeleton. It is somewhat thickened and rough. The =external= or =coxal angle= (Tuber coxæ) forms the basis of the point of the hip. It is a large quadrangular mass, narrow in its middle, and enlarged at either end, where it bears a pair of tuberosities. It is roughened for muscular attachment. The =posterior= or =acetabular angle= meets the other two bones at the acetabulum, of which it forms about two-fifths. Its prominent upper border forms part of the =superior ischiatic spine=, which is roughened externally, smooth internally. Two depressions above and in front of the acetabulum give attachment to the 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 horizontal 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 roughened for the attachment of the adductor muscles. The =anterior border= forms the posterior margin of the obturator foramen.