86 THE SURGICAL ANATOMY OF THE HORSE 



cavity. It cannot be said to take any great part in maintaining the 

 solidity of the sacro-iliac joint, and its utiUty seems to be more in the 

 direction of enclosing the cavity of the pelvis. 



In form it is somewhat quadrilateral, and has therefore four borders. 

 These may be termed respectively superior, inferior, anterior, and 

 posterior. Its superior edge is attached to the roughened lateral border 

 of the sacrum. Inferiorly it is attached to the superior ischiatic spine 

 and to the tuber ischii. Its anterior border forms the posterior 

 boundary of the greater sacro-sciatic foramen, an important opening 

 to be shortly described. Its posterior border is related to the semi- 

 membranosus muscle, and is confounded superiorly with the aponeurotic 

 covering of the coccygeal muscles. 



The outer surface of this big ligament is crossed by the great 

 sciatic and external popliteal nerves, and is clothed by the gluteal, 

 biceps femoris, and semitendinosus muscles, many of the fibres of which 

 arise from the ligament. The deep face of this ligament is covered in 

 front by the peritoneum, and posteriorly it is related to the coccygeal 

 muscles, and also to those of the anus, whilst anteriorly it is related to 

 the rectum and in the female to the vagina. 



Near the upper border of the ligament, when the gluteal muscles 

 have been removed, the ischiatic artery, which is one of the terminal 

 divisions of the lateral sacral, will be seen. 



The Greater Sacro-Sciatic Foramen. — This is an elliptical opening 

 which is disposed with its long axis directed downwards and backwards. 

 It is bounded posteriorly, as already stated, by the anterior edge of the 

 great sacro-sciatic ligament, whilst its anterior boundary is the ischiatic 

 border of the ilium. 



This is the opening through which the great sciatic nerve and the 

 gluteal nerves and vessels gain exit from the pelvic cavity. 



The Lesser Sacro-Sciatic Foramen. — As its name implies, this opening 

 is much less than the one just described. It is also elliptical in outline, 

 the long axis of the ellipse in this case being directed horizontally. It 



