160 THE ABTICCLATIONS. 



tlie ischiatic border and the internal angle of tlie ilium, in becoming 

 confounded with the preceding ligament. Its inferior margin is inserted 

 into the rugged lip which borders the sacrum laterally. Its posterior border 

 is united to the aponeurosis covering the coccygeal muscles, and its external 

 face is in contact with the principal gluteal and the long vastus muscles ; 

 while the internal corresponds to the lateral sacro-cocoygeal muscle. 



d. Sacro-sciaiic or isdiiatic liijament (Fig. 90, 2).— This is a vast mem- 

 branous expansion situated on the side of the pelvis, between the sacrum 

 and the coxa, and serves more as a means for inclosing this portion of the 

 pelvic cavity than to assure the solidity of the sacro-iliac articulation. Its 

 form is irregularly quadrilateral, and permits its circumference to be divided 

 into four borders : a superior, attached to the rugged lateral ridge of the 

 sacrum ; an inferior, fixed to the supercotyloid ridge, as well as the ischial 

 tuberosity, and forming by the portion comprised between these two in- 

 sertions, with the small ischiatic notch, the opening by which the internal 

 obturator and pyramidal muscles leave the pelvis ; an anterior, imperfectly 

 limited, along with the great ischiatic notch, circumscribes the opening 

 through which the gluteal vessels and nerves, and the sciatic nerves pass ; a 

 posterior, doubled in the form of two laminse which embrace the semi- 

 membranosus muscle, and is confounded superiorly with the aponeurosis 

 enveloping the coccygeal muscles. The external face of this ligament is 

 traversed by the sciatic nerves, and is covered by the long vastus and the 

 semitendinosus muscles, which derive numerous insertions from it. Its 

 internal face is covered, in front, by the peritoneum, and posteriorly is in 

 contact with the ischio-coccygeal and ischio-anal muscles, to which it gives 

 attachment. 



Synovial membrane. — This lines the sacro-iliac ligament, but only 

 furnishes a small quantity of synovia. 



Movements. — The two sacro-iliac articulations being the centres towards 

 which all the impulsive efforts communicated to the trunk by the posterior 

 limbs converge, they do not offer much mobility, as that would oppose 

 the integral transmission of the quantity of movement. So that they 

 permit only a very restricted gliding of the articular surfaces ; and the 

 union of the sacrum and coxa by diarthrosis appears to be exclusively 

 designed to prevent the fractures to which these bones would be incessantly 

 exposed if they were fixed together in a more intimate manner. 



B. Aktioulation op the Two Cox^, or Ischio-pubic Svmphtsis. — The 

 two coxae are united to each other throughout the whole extent of the inner 

 border of ike pubis and the ischial bones. In youth, this is a veritable 

 amphiarthrosis, fixed by an interosseous cartilage and bundles of peripheral 

 fibres. 



The cartilage is solidly fixed to the small rugged eminences which cover 

 the adjacent articular surfaces, and becomes ossified, like the sutural 

 cartilages, as the animal advances in age. In adult Solipeds the coxae are 

 always fused with each other. 



The peripheral fibrous fasciculi extend tranversely from one bone to 

 the other, above and beloM^ the symphysis ; those on the inferior face are 

 incomparably stronger and more abundant than the others. 



The movements of this articulation are most restricted, and depend 

 solely upon the elasticity of the interosseous cartilage. They cease after its 

 ossification. 



The fusion of the two cox» proceeds very slowly in the /.emale of the Cat, Dog, Pig, 

 Ox, Sheep, and Goat epeoiea, 



