376 THE ANATOMY OF THE HORSE. 



while the other runs on the upper aspect of the tail between the 

 erector and curvator coccygis. These cords are expended in branches 

 to the muscles and skin of the tail. 



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 upwards 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 OF 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, however, 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 facet on the pelvic surface of the ilium. The move- 

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

 may prove by grasping 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 propulsive efforts are originated, it is necessary that 

 but slight movement should be permitted, as otherwise these efforts 

 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 351). 



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 respectively 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 arrangement comparable to an inverted arch. 



