58 PROFESSOR OWEN ON THE 



The fii-st sacral shows the transversely extended and concave articular surface of the 

 centrum (PI. XIX. fig. 1, c) ; the subcircular pit (ib. i)) for the head of the rib is behind 

 the middle of the side of the centrum, at its upper part ; the inferior surface is ridged 

 lengthwise ; and a transverse low but sharp ridge defines the posterior boundary, the 

 depressions in front of which indicate the hindmost origins of the subve'rtebral muscle 

 (longus colli ■?). The anterior outlet of the neural canal (ib. n) is subcu'cular in one spe- 

 cimen, vertically elliptic m others, and 3 lines or less m transverse diameter. From the 

 sides of the neurapophyses stretch out the strong buttresses of bone which blend with 

 the under part of the ilia, giving off' from the fore part of theii- base the prsezygapophyses 

 (ib. 2), and from the back part of their apex the surface (ib. d), or part of it, for the 

 tubercle of the last moveable rib, the ilium in the latter variety affording tlie rest of 

 that surface. The fore part of the strong neural spine (ib. ns) is roughened by a syn- 

 desmotic surface ; it rises to a height of 14 lines, curving forward, and is confiuent at its 

 summit vath. the approximated anterior margins of the ilia. A continuous track of bone, 

 formmg a smootlily obtuse longitudinal ridge, represents tlie summits of the succeeding 

 sacral spines (ib. fig. 2, ns) to the hmdmost vertebra of the series, without any trace of 

 their primitive division ; but this track rises, posteriorly, above the shallow channel on 

 each side, in which are the foramma (ib. 0), indicating most of the constituent vertebras. 



The second sacral vertebra abuts against the ilium by a pleurapophysis (ib. fig. 1, 

 j^il -2 ), as well as a diapophysis (ib. d-2); but the former is a slender, straight filament, or 

 narrow plate of bone, confluent at both ends. 



In the next two vertebrae the pleurapophysis (ib. ^j/3&4) assumes more breadth and 

 robustness, but is short and straight, abutting against the inner surface of the ilium an 

 inch in advance of the acetabulum. The first of these rib-buttresses inclines forward, 

 and is completely confluent with the ilium ; the thicker one (ib. pi i) has retained part 

 of its primitive ligamentous attachment to the ilium : the proportions of both are sub- 

 ject to some variety. 



These are succeeded by three or four vertebrae in which the pleurapophysis is not de- 

 veloped, the attachment to the ilia being by diapophyses only (ib. d d), which are short 

 slender lamellae, directed upward and backward; below and between them are the double 

 orifices for the separate motory and sensory roots of the sacro-spinal nerves. In the 

 next vertebra the pleurapophysis (ib. jpl s) reappears, longer but more slender than in 

 the fourth sacral, extending obliquely backward, and expanding at its extremity to abut 

 against a prominence on the underside of the ilium, opposite the hind part of the 

 acetabulum, with which prominence the rib has completely coalesced by an expanded 

 end. The under part of all these vertebrae is traversed by a sharp median longitudmal 

 ridge, which is more feebly and interruptedly continued to near the end of the sacral 

 series. 



Eight vertebrae, abutting by diapophyses only (ib. d d) against the ilia, succeed the 

 one last described ; their coalesced bodies are less than half the breadth of those of the 



