82 OSTEOLOGY. 
The posterior surface is rough and irregular. Convex from above downwards it 
displays mesially a crest (crista sacralis media) whereon are seen four elongated 
tubercles—the spines of the upper four sacral vertebre. External to these the 
bone forms a groove—the sacral groove—the floor of which is made up of the con- 
fluent laminze of the corresponding vertebree. In line with the intervals between 
the spines, and wider apart above than below, another series of tubercles is to be 
seen. These are due to the fusion of the articular processes of the sacral vertebra, 
and together they form faint irregular ridges on the bone (cristz sacrales articulares). 
Normally, the spine of the lowest sacral segment is absent, and the laminz do not 
coalesce mesially, thus leaving a gap in which the spinal canal is exposed (hiatus 
sacralis); whilst inferiorly the tubercles corresponding to the inferior articular 
Superior articular processes Transverse process of first sacral vertebra 
Iliacus 
Ala 
Lateral 
Mass 
Pyriformis 
{Anterior sacral 
foramen 
Inferior lateral angle 
——Coccygeus 
Groove for fifth sacral nerve 
Coccygeal articular surface 
Fic. 62.—THE SacRUM (anterior view). 
processes of the last sacral vertebra form little down-projecting processes—the 
sacral cornua (cornua sacralia)—by means of which the sacrum is in part united to 
the coceyx. Just wide of the articular tubercles are the posterior sacral foramina 
(foramina sacralia posteriora), for the transmission of the posterior divisions of the 
sacral nerves. These are in correspondence with the anterior foramina, so that a 
probe can be passed directly through both openings; but it is to be noted that the 
posterior are much smaller, and their margins much sharper, than is the case with 
the anterior. The surface of the lateral mass external to the posterior sacral 
foramina is rough and irregular, owing to the presence of four more or less elevated 
tubercles, which constitute the lateral ridges on either side of the bone (criste 
sacrales laterales), and which are serially homologous with the true transverse 
processes of the lumbar vertebree. 
The posterior surface of the bone furnishes an extensive surface for the origin 
of the erector spine muscles, whilst the edge of the bone external to the third and 
fourth foramen gives attachment to the gluteus maximus. 
The base of the bone displays features more in accordance with a typical 
