298 
DR. MANTELL ON THE OSTEOLOGY OF 
the floor of the large intervertebral foramen. The preceding foramina intervertebralia, 
instead therefore of being situated more or less over the union of two contiguous 
bodies, are thrown forwards in the same ratio as the neural laminae, encroach on the 
centrum in front, and generally occur over the junction of its posterior and two an- 
terior thirds. In the first sacral the body has a subquadrangular section ; the lateral 
aspects are impressed by a fossa beneath the root of the neuropome, and meet the 
inferior surface nearly at right angles, which are rounded off; the lower aspect is 
but slightly convex transversely ; sometimes it presents a median ridge separating two 
very shallow concavities, perforated by vascular foramina. The anterior oblique 
processes project considerably beyond the margin of the neural arch, and are nearly 
horizontal, and separated from each other by a wide notch. The bodies of the three 
succeeding vertebrae, as already mentioned, are narrow, constricted in the centre, 
compressed laterally in the lower moiety, and rounded transversely below, with a 
more or less distinct longitudinal mesial ridge, sometimes replaced by a groove in 
the third. In other cases they appear to be flattened inferiorly, without our being 
able to regard them as distinct, so that there appears to be a considerable range of 
variation attributable to age and sex, &c. Each lateral surface presents a small digital 
fossa (Plate XXX. fig. 16) towards the middle of its height and nearer its posterior 
extremity, as if the centrum had been pinched up between the thumb and fore-finger. 
Above the fossa the centrum expands, the anterior angle (Plate XXX. fig. 14*) of the 
expanded portion being, as it were, removed and flattened out by the base of the 
neural lamina, and also, perhaps chiefly, by the sacral rib, which is wedged deeply 
into the intervertebral space ; the posterior angle (Plate XXX. fig. 14§), like that of 
the first sacral, is removed, but to a much less extent than the anterior ; between them 
is the smooth, semilunar, oblique notch (Plate XXX. figs. 14, 15, 16, ^), impressing the 
slightly elevated parapet which bounds externally the wide, deeply concave floor of 
the spinal canal. In the two posterior sacral vertebrse the bases of the neural laminae 
begin to expand inwards, so as to cover the upper surface of the centrum, in the 
last sacral meeting in the centre and leaving only a small triangular tract in front 
and behind exposed ; thus the calibre of the canal is diminished. The bodies of the two 
last vertebrae expand to nearly the same diameter as the first, but the lateral surfaces 
converge more or less rapidly to an inferior mesial convexity, varying in breadth 
and prominence. By the relative size of the two extremities of the spinal canal in 
the sacrum, we are enabled most readily and certainly to determine its position. The 
bases of the strong sacral ribs (Plate XXVI. h, h, h, Plate XXX. figs. 15, 16 A) are 
compressed in the antero-posterior diameter and impacted in the intervertebral 
spaces, descending nearly to the inferior surface of the centre, and rising high upon 
the neural laminae, which are also excavated to give an additional surface of attach- 
ment. The neural laminae also send out vertical processes {plagiajjophyses) which are 
superimposed upon the sacral ribs, and early coalesce with them to form the thick 
partitions, which extend outwards, gradually increasing in length to the last ; and 
