THE OS INXOMINATUM. 175 



ischium ; it is smooth and concave, and forms a little more than two-fifths of 

 the acetabular cavity ; its outer margin is bounded by a prominent rim or lip, 

 the external border, to which the cotyloid fibro-cartilage is attached. 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 the lateral boundary of the true pelvic cavity. This surface 

 is perforated by two or three large, vascular foramina, and affords attachment 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 

 continuous with that on the external surface, for the tendon of the Obturator 

 externus muscle. The lower edge of this groove is formed by the tuberosity of the 

 ischium, and affords attachment to the Gemellus inferior muscle. This surface is 

 limited, externally, by the margin of the acetabulum ; behind, by the posterior bor- 

 der ; 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 cotyloid fibro-cartilage. The internal border is 

 thin, and forms the outer circumference of the obturator foramen. The posterior 

 border of the body of the ischium presents, a little below the centre, a thin and 

 pointed, triangular eminence, the spine of the ischium, more or less elongated in 

 different subjects; its external surface gives attachment to the Gemellus superior, 

 its internal surface to the Coccygeus and Levator ani ; whilst to the pointed 

 extremity is connected the lesser sacro-sciatic ligament. Above the spine is a 

 notch of a large size, the great sacro-sciatic, converted into a foramen by the 

 lesser sacro-sciatic ligament; 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 Quadratus 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 sacro-sciatic ; it is smooth, 

 coated in the recent state with cartilage, the surface of which presents two or 

 three ridges corresponding to the subdivisions of the tendon of the Obturator 

 internus, which winds over it. It is converted into a foramen by the sacro-sciatic 

 ligaments, and transmits the tendon of the Obturator internus, the nerve which 

 supplies that muscle, and the internal pudic vessels and nerve. 



The tuberosity presents for examination three surfaces : external, internal, 

 and posterior. The external surface is quadrilateral 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 obtu- 

 rator 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 attachment to the Quadratus femoris, 

 and anterior to this to 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 prolongation of the great 

 sacro-sciatic 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 ante- 

 riorly, has attached the Transversus perinsei and Erector penis muscles. The 

 posterior surface is divided into two portions a lower rough, somewhat triangular 

 part, and an upper smooth, quadrilateral portion. The anterior portion is subdivided 

 by a prominent vertical ridge, passing from base to apex, into two parts: the 

 outer one gives attachment to the Adductor magnus; the inner to the great sacro- 

 sciatic ligament. The upper portion is subdivided into two facets by an oblique 

 ridge which runs downward and outward ; from the upper and outer facet arises 



