THE PUBIS—THE OBTURATOR FORAMEN 109 
THE PUBIS 
The pubis (Os pubis) is the smallest of the three parts of the os coxe. 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 
muscular 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 pubicum. 
The medial border joins the opposite bone at the symphysis 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 cox ossifies from one chief center. 
The center for the ilium first appears near the acetabulum, followed quickly by one 
for the ischium, and a little later by the pubic center. Secondary centers appear 
for the crest and tuber coxe 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 usually united with each other before birth, 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 oblique convergent lines or ridges, to which the 
obturator internus muscle is attached. Not rarely small eminences may be present along the 
symphysis. 
