riiK I'KLris. » 



This arrangement is particularly remarkable in the Mare, on which it 

 confers great strength and solidity. It is not present in the Cow ; con- 

 sequently, that animal is liable to a kind of incomplete luxation, which 

 may at times become an obstacle in parturition. 



We have already alluded to the salient angle formed by the union of 

 the last lumbar vertebra with the sacrum [sacro-vertebral aiujle), and 

 which looks downward into the abdominal cavity.^ 



2, 3. Sacro-iliac Articulations. 



The sacro-iliac articulation of each side establishes the union of the 

 posterior limbs with the spine, and is formed by the sacrum and ossa 

 ilii ; it belongs to tiie arthrodial class of joints. The two surfaces which 

 come into apposition have been already described, and it only now re- 

 mains to point out that the sacrum is fixed between the antero-superior 

 extremities of the ossa ilii like a horizontal wedge or the keystone of an 

 arch inverted ; the transverse diameter is greater below than above — 

 the pressure it has to resist being from below. The oblong roughened 

 surfaces on the sacrum and ilium have a layer of cartilage between 

 them to diminish shock and facilitate movement, which is further pro- 

 moted by each articulation being provided with a synovial membrane, 

 though the amount of synovia secreted is very trifling. The union of 

 the bones at this part is strengthened by three powerful ligaments : the 

 sacro-iliac — superior and inferior, and the sacro-iliac inopcr. There is 

 also the sacra-sciatic or sacro-iscJiiatic to be noticed hereafter. Though 

 the movements of this articulation are very limited, but still useful in 

 locomotion and parturition, yet it rarely, if ever, becomes consolidated. 

 The diarthrodial union between the bones appears to be chiefly, if not 

 exclusively, intended to obviate the fractures which must occur had 

 they been united in a more solid manner ; while the two articulations 

 being the centre towards which all the impulsive efforts of the posterior 

 extremities converge, a great degree of mobility would not be compatible 

 with their solidity. 



4. Ischio-puhic Synipliysis. 



The syviphrjsis pubis, as it is sometimes termed, is the amphiarthrosis 

 formed by the union, inferiorly, of the two ossa pubis and ischia. The 

 articulation is consolidated by means of a layer of fibro-cartilage between 

 the margin of these bones, which becomes ossified more or less com- 

 pletely and rapidly according to species ; and by a layer of white liga- 

 mentous fibres — short and compact — which pass across above and 

 below, the latter being the strongest. The movements of this articula- 

 tion are very limited, and depend solely upon the elasticity of the 

 interosseous cartilage ; they are abolished when ossification occurs. 

 This happens in the majority of horses before adult age ; though some- 

 times the posterior portion is cartilaginous after this period. 



5. Sacro-coccygeal Articulations. 



These resemble those of the vertebra} in general, there being a thick 

 disc of fibro-cartilage placed between each tail-bone, the first of which 



' This angle ia much more marked in wom.an, and 'n named the promontory ; it i« 

 immediately at the entrance to the pelvis, and for these reasons it is frequently a cause of 

 difficult parturition in her; while, from its less development and distance from the 

 pslvis, it offers no obstacle in animals. 



