Sacrum The number of sacral vertebrae varies slightly. For instance, the S1S5 vertebrae of a horse will fuse, the S1S3 of a dog will fuse, and four pelvic vertebrae of a rat will fuse between the lumbar and the caudal vertebrae of its tail. The "Stegosaurus" dinosaur had a greatly enlarged neural canal in the sacrum, characterized as a "posterior brain case”. The sacrum is a complex structure providing support for the spine and accommodation for the spinal nerves. It also articulates with the hip bones. The sacrum has a base, an apex, and three surfaces – a pelvic, dorsal and
Body of sphenoid bone orbital plate of the frontal bone. The inferior surface presents, in the middle line, a triangular spine, the sphenoidal rostrum, which is continuous with the sphenoidal crest on the anterior surface, and is received in a deep fissure between the alæ of the vomer. On either side of the rostrum is a projecting lamina, the vaginal process, directed medialward from the base of the medial pterygoid plate, with which it will be described. Body of sphenoid bone The body of the sphenoid bone, more or less cubical in shape, is hollowed out in its interior to form two large cavities,