282 ARTICULATIONS. 



substance. At an early period of life, occasionally in the adult, and in the 

 female during pregnancy, they are smooth, and lined by a delicate synovial 

 membrane. The ligaments connecting these surfaces are the anterior and pos- 

 terior sacro-iliac. 



The Anterior Sacro-iliac Ligament (Fig. 194) consists of numerous thin liga- 

 mentous bands, which connect the anterior surfaces of the sacrum and ilium. 



The Posterior Sacro-iliac (Fig. 195) is a strong interosseous ligament, situated 

 in the deep depression between the sacrum and ilium behind, and forming the 

 chief bond of connection between those bones. It consists of numerous strong 

 fasciculi, which pass between the bones in various directions. Three of these 

 are of large size ; the lwo>superior, nearly horizontal in direction, arise from 

 the first and second transverse tubercles on the posterior surface of the sacrum, 

 and are inserted into the rough uneven surface at the posterior part of the inner 

 surface of the ilium. The third fasciculus, oblique in direction, is attached by 

 one extremity to the third or fourth transverse tubercle on the posterior surface 

 of the sacrum, and by the other to the posterior superior spine of the ilium ; it 

 is sometimes called the oblique sacro-iliac ligament. 



2. LIGAMENTS PASSING BETWEEN THE SACRUM AND ISCHIUM. (Fig. 195.) 



The Great Sacro-sciatic (Posterior). 

 The Lesser Sacro-sciatic (Anterior). 



The Great or Posterior Sacro-sciatic Ligament is situated at the lower and 

 back part of the pelvis. It is thin, flat, and triangular in form ; narrower in 

 the middle than at the extremities ; attached by its broad base to the posterior 

 inferior spine of the ilium, to the third and fourth transverse tubercles on the 

 sacrum, and to the lower part of the lateral margin of that bone and the coccyx; 

 passing obliquely downwards, outwards, and forwards, it becomes narrow and 

 thick ; and at its insertion into the inner margin of the tuberosity of the ischium, 

 it increases in breadth, and is prolonged forwards along the inner margin of the 

 ramus, forming what is known as the falciform ligament. The free concave 

 edge of this ligament has attached to it the obturator fascia, with which it 

 forms a kind of groove, protecting the internal pudic vessels and nerve. 

 One of its surfaces is turned towards the perineum, the other towards the 

 Obturator Internus muscle. 



The posterior surface of this ligament gives origin, by its whole extent, to 

 fibres of the Gluteus Maximus. Its anterior surface is united to the lesser sacro- 

 sciatic ligament. Its superior border forms the lower boundary of the lesser 

 sacro-sciatic foramen. Its lower border forms part of the boundary of the peri- 

 naeum. It is pierced by the coccygeal branch of the sciatic artery. 



The Lesser or Anterior Sacro-sciatic Ligament, much shorter and smaller than 

 the preceding, is thin, triangular in form, attached by its apex to the spine of 

 the ischium, and internally, by its broad base, to the lateral margin of the 

 sacrum and coccyx, anterior to the attachment of the great sacro-sciatic liga- 

 ment, with which its fibres are intermingled. 



It is in relation, anteriorly, with the Coccygeus muscle ; posteriorly, it is cov- 

 ered by the posterior ligament, and crossed by the pudic vessels and nerve. 

 Its superior border forms the lower boundary of the great sacro-sciatic foramen; 

 its inferior border, part of the lesser sacro-sciatic foramen. 



These two ligaments convert the sacro-sciatic notches into foramina. The 

 superior or great sacro-sciatic foramen is bounded, in front and above, by the 

 posterior border of the os innominatum ; behind, by the great sacro-sciatic 

 ligament ; and below, by the lesser ligament. It is partially filled up, in the 

 recent state, by the Pyriformis muscle. Above this muscle, the gluteal vessels 

 and superior gluteal nerve emerge from the pelvis; and below it, the ischiatic 

 vessels and nerves, the internal pudic vessels and nerve, and the nerve to the 

 Obturator internus. The inferior or lesser sacro-sciatic foramen is bounded, in 



