3 2 OSTEOLOGY. 



In a groove at their base, opposite the interspinous spaces, are 

 the four superior sacral foramina, which give exit to the superior 

 branches of the sacral nerves. An irregularly-elevated ridge, 

 external to the grooves, marks the division between the superior 

 and inferior surface of the bone. The inferior or pelvic surface 

 is smooth and rather concave ; at its anterior end is a trans- 

 verse elevation, the sacral promontory, not very pronounced in the 

 horse, and crossing the bone transversely are four slightly-elevated 

 lines, which show the connections between the original segments. 

 Between these lines, and close to the borders on each side are 

 the four large foramina through which pass the inferior branches 

 of the sacral nerves, and at the anterior end two notches, which 

 correspond with similar notches in the last lumbar vertebra, and 

 form the foramina through which the first sacral nerves find 

 their exit. 



The anterior extremity or base is almost entirely articular ; 

 the central part, oval and convex, articulates with the last lumbar 

 vertebra, and above it is the spinal canal surmounted by the 

 articular processes. The lateral parts are the thick expanded 

 transverse processes which terminate in sharpened extremities, 

 and contain two broad facets for articulation with those on the 

 last lumbar transverse processes. The posterior extremity or 

 apex presents the diminished spinal canal in its centre, with the 

 last spinous process above it, and below the flat surface which 

 articulates with the first bone of the coccyx, while the notches and 

 the sacral cornua are on its sides. 



The lateral borders, anteriorly, are roughened for attachment 

 to the iliac bones under which they lie, and with which they 

 form the sacro-iliac joint ; posteriorly, they are roughened for 

 the attachment of the sacro-sciatic ligaments, and terminate in 

 small tranverse processes the sacral cornua. 



The sacrum has five articular surfaces on its base, one on each 

 side, and one on its apex. Its spinal canal is somewhat triangular 

 in shape, and decreases much in size posteriorly. 



THE COCCYX. 



(PL. I. I.) 



The coccygeal or caudal vertebras are false vertebrae, varying 

 in number from thirteen to twenty. In form, they are the 

 most rudimentary of all the vertebral segments ; their centra are 



