352 THE ANATOMY OF THE HORSE. 



foramen; its posterior edge, much more extensive than the anterior, is 

 thin, ill-defined, and united to the coccygeal origin of the semimem- 

 branosus. Its outer surface is crossed by the great sacro-sciatic nerve, 

 and is covered by the biceps femoris and semitendinous muscles, which 

 in part arise from it. Its inner surface is lined anteriorly by peritoneum, 

 and is related posteriorly to the compressor coccygis and retractor ani 

 muscles, some of whose fibres take origin from it. The internal pudic 

 nerve and vessels cross this surface, or they may be partly embedded in 

 the texture of the ligament. 



The Great Sacro-sciatic Foramen is an elliptical opening in the lateral 

 wall of the pelvis, its anterior boundary being formed by the ischiatic 

 edge of the ilium, and its posterior by the sacro-sciatic ligament. It 

 transmits the gluteal nerves and vessels and the great sciatic nerve. 



The Small Sacro-sciatic Foramen is an interval in the lower and 

 posterior part of the lateral wall of the pelvis. Its upper edge is formed 

 by the sacro-sciatic ligament ; its lower by the smooth and rounded 

 external border of the ischium, between the tuber and the superior 

 ischiatic spine. By this opening the common tendon of the obturator 

 internus and pyriformis emerge from the pelvis, and the nerves to these 

 muscles pass in. 



The Superior Ilio-sacral Ligament (Fig. 53) is cord-like, and passes 

 between the internal angle of the ilium (the angle of the croup) and the 

 summits of the sacral spines. 



The Inferior Ilio-sacral Ligament (Fig. 53) is membranous and 

 triangular in form. Its anterior edge is fixed to the upper part of the 

 ischiatic border of the ilium; its lower edge is attached to the lateral 

 lip of the sacrum ; its posterior or upper edge is ill-defined, being con- 

 tinuous with the fascia investing the muscles of the tail. 



THE CAVITY OF THE PELVIS. 



Directions. — Fix the pelvis on a table, with the inlet looking upwards. 

 Sponge out the cavity and distend the bladder with air or some preserv- 

 ative fluid, tying the urethra to prevent its escape. 



The pelvis is not distinct from the abdominal cavity, but is merely a 

 backward continuation of it. It is, in fact, that portion of the general 

 cavity of the belly which is posterior to the bony circle formed by the 

 sacrum, pubes, and ilio-pectineal lines. The plane of separation between 

 the abdominal cavity proper and the pelvic cavity is termed the inlet of 

 the pelvis ; the posterior extremity of the pelvic cavity is termed its 

 outlet. 



The inlet or brim of the pelvis is circumscribed by the promontory of 

 the sacrum above, by the anterior margin of the pubic bones below, and 

 by the ilio-pectineal line on each side. It looks downwards and forwards 



