DISSECTION OF THE PELVIS. 363 



Sacral Nerves. On the upper aspect of the sacrum the superior 

 primary branches of the sacral nerves will be found at their points of 

 emergence from the spinal canal, the first four issuing by the superior 

 sacral foramina, and the last by the foramen between the sacrum and 

 the first bone of the coccyx. These nerves are much smaller than the 

 corresponding inferior primary branches ; and after giving twigs to the 

 muscles on the side of the spine, they pass upwai'ds to the skin of the 

 croup. Slender branches of the lateral sacral artery issue from the 

 spinal canal in company with them. 



JOINTS AND LIGAMENTS OP THE SACRUM AND COCCYX. 



The sacral portion of the spine in the adult animal does not present 

 any joints between its constituent pieces, which are fused by anchylosis. 

 The lumbar supraspinous ligament is prolonged on the summits of the 

 sacral spines. This region, howevei-, furnishes the important joint 

 between the vertebral column and the skeleton of the hind limb — the 

 sacro-iliac articulation. 



The Sacro-iliac Articulation. The bony surfaces that concur to 

 form this are the auricular facet on the lateral aspect of the sacrum, and 

 the corresponding fecet on the pelvic surface of the ilium. The move- 

 vients permitted in the joint are scarcely appreciable, as the student 

 may prove by grasjiing the sacrum and the part of the ilium left in 

 connection with it. Since this joint is the bond of connection between 

 the skeleton of the trunk and that of the hind limb, in which, in loco- 

 motion, the main pi'opulsive eftbrts are originated, it is necessary that 

 but slight movement should be permitted, as otherwise these effoi-ts 

 would not be transmitted with precision to the trunk. The stability of 

 the joint is effected mainly by one ligament — the sacro-iliac, and to a 

 less degree by the superior and inferior ilio-sacral ligaments and the 

 sacro-sciatic ligament already described (page 339). 



The Sacro-iliac Ligament. — This ligament is composed of strong fibres 

 passing between the sacrum and ilium, in close relation to the joint. 

 It consists of an upper and a lower half, corresponding I'espectively to 

 the anterior and posterior sacro-iliac ligaments of human anatomy. The 

 former is much the stronger of the two; and the necessity for its strength 

 is apparent when one reflects that whatever weight is placed on the 

 back and loins of the horse, tends to drive the sacrum downwards from 

 its connection with the iliac bones, and that this tendency is rather 

 favoured than otherwise by the form of the articular surfaces, which 

 offer an aiTangement comparable to an inverted arch. 



The bones should be disarticulated to show the articular surfaces. 

 The joint is provided with a rudimentary synovial membrane. 



Sacro-coccygeal and Intbr-coccygeal Articulations. Ordinarily 

 these are movable joints, the articular surfaces being the opposed 



