THE ISCHIUM 



107 



part of the ischiatic spine (Spina ischiadica), which is roughened laterally, smooth 

 medially. 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 wing or wide part of the bone by a constricted part, termed the shaft (Corpus 

 ossis ilium). The latter is of three-sided, prismatic form. Its lateral 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 nerve. Its ventral 

 surface is crossed by vascular grooves, below which there is a rough area, which is 

 bounded medially by the psoas tubercle. 



Tuber Gluteal 



coxce surface 



of ilium 



Ischiatic spine 



Acetabulum 

 Obturator foramen 



Ischial arch 



Fig. 86. — Osba Coxarum of Horse; Dorsal View. 

 A, Wing; A', shaft of ilium: B, acetabular, B', symphyseal branch of pubis; C, body, C\ acetabular branch (or 

 shaft), C", symphyseal branch, of ischium; 1, gluteal line; 2, grooves for obturator nerve and vessels; 3, symphysis 

 pelvis; 4, greater sciatic notch; 5, iho-pectineal eminence ; 6, pubic tubercle. Dotted lines indicate primitive separa- 

 tion of three bones. 



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. The body of the ischium (Corpus ossis 

 ischii) is irregularly quadrilateral, and may be described as having two surfaces, 

 four borders, and four angles. 



The pelvic surface (Facies pelvina) is smooth and slightly concave from side 

 to side. 



The ventral surface (Facies ventralis) 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. 



