406 THE AKATOMY OF THE HORSE 



surface, which it separates, so that its superior face presents in front a con- 

 cavity to receive the tubercle on the zygomatic process of the temporal bone, 

 and a convexity behind, which is lodged in its glenoid cavity. As to its 

 inferior face, it is indented by an oblong furrow, in which the condyle of the 

 inferior maxiDary bone is imbedded. 



These hones are united hy a capsular ligament, covering a synovial capsule, 

 and two lateral ligaments, one external and the other internal. 



A Jibrous cover, a true capsular ligament, surrounds this articulation, and 

 is attached by its edges to the articular surfaces which it unites, as well as 

 to the borders of the inter-articular fibro-cartilage ; thus forming two distinct 

 capsules, namely, one supei'iorly, and one inferiorly, which are lined inter- 

 nally by synovial membranes. The larger of the two, after lining the upper 

 surface of the disc, is reflected upward to the glenoid cavity of the temporal 

 bone. The inferior sjmovial membrane is interposed between the inferior 

 surface of the cartilage and the condyle of the lower jaw ; and thus a double 

 joint is constituted. 



The external lateral ligament is a short fasciculus of fibres, attached 

 superiorly to a tubercular prominence, situate on the supero-external part 

 of the squamous temporal bone, and inferiorly to the external surface of 

 the condyle, and to the postero-external surface of the neck of the lower 

 jaw, just below the condyle ; its fibres take a backwai-d and downward 

 course. 



The internal lateral ligament is looser and more elongated than the external. 

 It extends from the inner surface of the squamous temporal bone to the 

 cartilage and inner surface of the condyle of the superior maxillary bone, 

 reaching down to the inner part of its angle. 



Movement. — The temporo-maxillary az'ticulation is the centre of every 

 movement of the lower jaw. These are — elevation, lowering, lateral 

 movement, and horizontal sliding, which motions together accomplish the 

 grinding action necessary to triturate the hard grain upon which the 

 horse feeds. 



THE SACRO-ILIAC ARTICULATION 



This joint establishes the union of the posterior members with the spine, 

 and is formed by the sacrum and os innominatum. It belongs to the arthro- 

 dial order of joints. On each of these two bones is a large and irregular 

 articular surface, lined with a thick layer of cartilage, which is firmly united 

 to them. The joint thus formed is strengthened by four ligaments, namely 

 — 1, the sacro-iliac ; 2, the superior ilio-sacral; 3, the inferior ilio-sacral ; 

 and 4, the sacro-sciatic. 



The sacro-iliac ligament is composed of large ligamentous fibres, which 

 everywhere envelop the articulation, by firmly attaching themselves at their 

 extremities to the impressions around the articular surfaces, situated on the 

 sacrum and internal border of the iUum. The inferior half of this ligament 

 is covered by the psoas muscles. Its posterior half, much stronger than the 

 former, is hidden by the ilium. 



The superior ilio-sacral ligament is a large, strong, short ligament, which, 

 arising from the internal part of the ilium, is carried backwards and fixes 



