THE OS INNOMINA TUM 211 



Below the acetabulum, between it and the tuberosity, is a deep groove, along 

 which the tendon of the Obturator externus muscle runs as it passes outward 

 to be inserted into the digital fossa of the femur. The internal surface is smooth, 

 concave, and enters into the formation of ',ne lateral boundary of the true pelvic 

 cavity. This surface is perforated by two or three large, vascular foramina, and 

 affords origin to part of the Obturator internus muscle. The posterior surface 

 is quadrilateral in form, broad and smooth. Below, where it joins the tuberosity, 

 it presents a groove, the obturator groove (sulcus obturatorius), continuous with 

 that on the external surface; in this groove is received the posterior fleshy part of 

 the Obturator externus muscle when the thigh is flexed. The lower edge of this 

 groove is formed by the tuberosity of the ischium, and affords origin to the Gemellus 

 inferior muscle. The posterior surface is limited, externally, by the margin of the 

 acetabulum; behind, by the posterior border; it supports the Pyriformis, the two 

 Gemelli, and the Obturator internus muscles in their passage outward to the great 

 trochanter. The external border forms the prominent rim of the acetabulum, 

 and separates the posterior from the external surface. To it is attached the coty- 

 loid fibrocartilage. The internal border is thin, and forms the outer circumference 

 of the obturator foramen. The posterior border of the body of the ischium is 

 continuous with the posterior border of the ilium; it presents, a little below the 

 centre, a thin and pointed, triangular eminence, the spine of the ischium (spina 

 ischiadica), more or less elongated in different subjects; its external surface gives 

 origin to the Gemellus superior, its internal surface to the Coccygeus and Levator 

 ani; while to the pointed extremity is connected the lesser sacrosciatic ligament. 

 Above the spine is a notch of large size, the great sacrosciatic notch, converted into 

 a foramen, the great sacrosciatic foramen (foramen ischiadicum majus), by the 

 lesser and greater sacrosciatic ligaments; it transmits the Pyriformis muscle, 

 the gluteal vessels, and superior and inferior gluteal nerves; the sciatic vessels, 

 the greater and lesser sciatic nerves, the internal pudic vessels and nerve, and the 

 nerves to the Obturator internus and Quadratics femoris. Of these, the gluteal 

 vessels and superior gluteal nerve pass out above the Pyriformis muscle, the other 

 structures, below it. Below the spine is a smaller notch, the lesser sacrosciatic 

 notch (incisura ischiadica minor); it is smooth, coated in the recent state with 

 cartilage. It is converted into a foramen, the lesser sacrosciatic foramen (foramen 

 ischiadicum minus), by the sacrosciatic ligaments, and transmits the tendon of 

 the Obturator internus, the nerve which supplies that muscle, and the internal 

 pudic vessels and nerve. 



ihe tuberosity of the ischium (tuber ischiadicum) is the portion of bone between 

 the body and the ascending ramus. The tuberosity presents for examination 

 three surfaces external, internal, and posterior. The external surface is quadri- 

 lateral in shape, and rough for the attachment of muscles. It is bounded above 

 by the groove for the tendon of the Obturator externus; in front it is limited by 

 the posterior margin of the obturator foramen, and below it is continuous with the 

 ramus of the bone; behind, it is bounded by a prominent margin which separates 

 it from the posterior surface. In front of this margin the surface gives origin 

 to the Quadratus femoris, and anterior to this some of the fibres of origin of the 

 Obturator externus. The lower part of the surface gives origin to part of the 

 Adductor magnus. The internal surface forms part of the bony wall of the true 

 pelvis. In front it is limited by the posterior margin of the obturator foramen; 

 behind, it is bounded by a sharp ridge, for the attachment of a falciform pro- 

 longation of the great sacrosciatic ligament; it sometimes presents a groove on 

 the inner side of this ridge for the lodgement of the internal pudic vessels and nerve; 

 and, more anteriorly, has attached the Transversus perinaei and Erector penis vel 

 clitoridis muscles. The posterior surface is divided into two portions a lower 

 rough, somewhat triangular part, and an upper smooth, quadrilateral portion. 



