LUMBAR VERTEBRA, 331 
and look inwards ; the posterior are convex, nearer together, and look 
outwards. 
PECULIARITIES OF THE LUMBAR VERTEBRA, 
THE FOURTH LUMBAR VERTEBRA possesses two surfaces of articulation 
on the posterior border of its transverse process. 
Tue FIFTH (or the sixth, if there is one) assumes somewhat the charac- 
ters of the first sacral bone. The transverse processes, which are longest in 
the middle of this region, gradually shorten towards the sacrum. On the 
fifth there is posteriorly an articular surface of an oval form, and on the 
sixth there is a similar one on each surface, the posterior being slightly 
concave, and fitting to a corresponding facette on the sacrum. 
THE SACRUM. 
TuIs BONE, in form irregularly prismatic, is made up from the conjune- 
tion by ossification of five vertebra, in which latter condition it is found 
in the embryo. It is united in front with the last lumbar vertebra by 
three articulatory surfaces (4,6, 6, Fig. 19) posteriorly with the first bone 
Fic. 19.—TuE Sackum 
1. Fore view of the sacrum. 6..6. Surfaces articulating with transverse pro- 
2. 2. 2. &e. Spinous processes. cesses of last lumbar vertebre. 
8. Lateral free surface. 7. 7. 7. &c. Foramina for the passage of the 
4. Head articulating with the concavity on the inferior sacral nerves. 
head of the last lumbar vertebra, 8. 8. Surfaces articulating with the ilium, 
5. 5. Oblique processes. 
of the coccyx, and on each side with the ilium or hip bone, by a large 
rough surface (s, 8, Fig. 19). The whole bone is slightly curved, with its 
concavity downwards. This inferior surface is broad and smooth, forming 
the superior boundary of the pelvic basin. On it are plainly traced four 
transverse lines, marking the situation of the original divisions into separate 
bones. Here are also four large holes on each side for the passage of the 
inferior sacral nerves. The superior surface is furnished with five spines 
(2, 2,2, 2, Fig. 19), corresponding to the spinous processes of the lumbar 
vertebra, They unite at their bases, but are distinct above, terminating 
