THE PUBIS THE OBTURA.TOR FORAMEN 109 



The Pubis 



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

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

 surfaces, three borders, and three angles. 



The 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 ventral surface (Facies ventralis) is convex, and in great part rough for 

 niuscular attachment. Near the anterior border it is crossed by the pubic groove 

 (Sulcus pubis), the medial part of which is occupied by a large vein, the lateral part 

 by the accessory ligament; the groove leads to the acetabular notch. 



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

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

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

 the ilio-pectineal line. Near the symphysis is a variable prominence, the tuber- 

 culum pubiciun. 



The medial border joins the opposite bone at the symphy.sis pubis. 



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

 is marked laterally by the obturator groove. 



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

 part is very thick in the young subject and the stallion, but in the mare, and usually 

 in the gelding also, it becomes thin with advancing age. 



The acetabular angle joins the ilium and ischium at the acetabulum. 



The posterior angle joins the ischium, with which it forms the inner boundary 

 of the obturator foramen. 



The pubis may conveniently be regarded as consisting of a body (Corpus ossis 

 pubis) and two branches; the latter are termed the acetabular branch (Ramus 

 acetabularis) and the "symphyseal branch (Ramus symphyseos). 



The Acetabulum 



The acetabulum is a cotyloid cavity which lodges the head of the femur. It 

 faces ventro-laterally, and consists of an articular and a non-articular part. The 

 articular part (Facies lunata) is crescentic, and is cut into internally by a rough 

 non-articular depression, the acetabular fossa (Fossa acetabuli). The medial 

 part of the rim is correspondingly cut into by the acetabular notch (Incisura 

 acetabuli), which is converted into a foramen by the transverse ligament in the 

 fresh state, and transmits the accessory and round ligaments to the head of the 

 femur. 



The Obturator Foramen 



The obturator foramen (Foramen obturatum) is situated between the pubis 

 and ischium. It is oval in outline, the longer axis being directed forward and 

 outward. Its margin is grooved antero-laterally for the obturator nerve and ves- 

 sels. 



Development. — Each division of the os coxae ossifies from one chief center. 

 The center for the ilium first appears near the acetabulum, followed quickh' by one 

 for the ischium, and a little later by the pubic center. Secondary centers appear 

 for the crest and tuber coxse of the ilium, the tuber and posterior border of the 

 ischium, and the acetabular part of the pubis. The symphyseal branches of the 

 pubis and ischium are united with each other at birth or soon after, but the three 



1 The pelvic surface of the pubis is quite variable. In the mare and in geldings which have 

 been castrated early the two pubic bones form a central depression of variable depth and curvature. 

 This depression is bounded posteriorly by two obhque convergent lines or ridges, to which the 

 obturator internus muscle is attached. Not rarely small eminences may be present along the 

 symphysis. 



