THE PEL VIS. 



13 



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 promoted 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 strength- 

 ened by three powerful ligaments : the sacro-iliac — superior and inferior, 

 and the sacro-iliac proper. There is also the sacro-sciatic or sacro-ischiatic 

 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 impul- 

 sive efforts of the posterior extremities converge, a great degree of 

 mobility would not be compatible with their solidity. 



4. Ischio-pubic Symphysis. 



The symphysis 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 be- 

 tween the margin of these bones, and which becomes ossified more or 

 less completely and rapidly according to species ; and by a layer of 

 white ligamentou fibres — short and compact — which pass across above 



b 



Fig. 5- 



Ligaments of the Lumbar Vertebr/E, Sacrum and Pelvis, seen from below. 



A, Intertransverse Ligament of the Lumbar Vertebrae ; B, Capsular Ligament of the Spinous 

 Process of the fifth and sixth Lumbar Vertebrae ; C, Capsular Ligament of the Sacrum ; D, 

 Inferior Sacro-iliac Ligament ; E, Obturator Ligament ; F, Transverse Ligament of the 

 Ischio-pubic Symphysis. 



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

 ticulation 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. 



