96 THE SKELETON OF THE HORSE 



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 external or acetabular angle joins the ilium and ischium at the acetabu- 

 lum. 



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 two l)ranches which 

 meet at a right angle; these are termed the transverse or acetabular branch 

 (Ramus acetabularis) and the longitudinal or symphyseal branch (Ramus sym- 

 physeos). 



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

 faces downward and outward, and consists of an articular and a non-articular part. 

 The articular part (Facies lunata) is crescentic, and is cut into internally by the 

 non-articular part, Avhich lies at a lower level, and is termed the acetabular fossa 

 (Fossa acetabuli). The inner 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 pubo-femoral and round 

 ligaments to the head of the femur. 



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- 

 externally for the obturator nerve and 

 vessels. 



Development. — Each division of the os 

 coxae ossifies from one chief center. The 

 center for the ilium appears first 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 external angle of the 

 (^,___^ 'K_>' ilium, the tuber and posterior border of 



the ischium, and the acetabular part of 



Fig. 62. — Ossa Coxarum of Stallion, Dorsal , , . rrii i • i • i • 



View. (After Ellenberger-Baum. Anat. the publS. The publS and ISChmUl are 



fiir Kiin.stier.) United at birth or soon after, but are 



not fused with the ilium until the second 

 year. The epiphyseal parts fuse with the main mass at four and a half to five 

 years of age. 



The acetabular part of the pubis ossifies from a separate center. It is most distinct in the 

 embryo at three montlis, and is often called the os acetabuli. Martin says that the ilium has a 

 center for the acetabular part, one for the shaft and wing, and a third foi' the crest. He also 

 states that there is a special center for the acetabular part of the ischium, and a transitory nucleus 

 in the symphyseal part of the pubis, 



THE PELVIS 



The bony pelvis is composed of the ossa coxarum, the sacrum, and the first 

 three coccygeal vertebrte. The dorsal wall or roof is formed by the sacrum and 

 first three coccygeal vertebra?, and tlie ventral wall or floor by the pubic and ischial 

 bones. The lateral walls are formed by tlie ilia and the acetabular part of the 

 ischia. The defect in the skeleton here is supplied in the fresh state by the sacro- 

 sciatic ligaments and semimem]3ranosus muscles. 



The anterior aperture or inlet (Apertura pelvis cranialis) is bounded by the 

 terminal line (Linea terminalis) or brim, composed of the base of the sacrum dor- 

 sally, the ilio-pectineal lines laterally, and the anterior border of the pubis ventrally. 



