SKELETON 855 
vertebrae, their definition being rather practical, and applicable to 
particular skeletons, tlian of general morphological value. 
It has been already pointed out (p. 849) that dorsal and thoracic 
vertebr are not necessarily identical, and in like manner the most 
anterior preesacrals may bear complete ribs and thus become thoracic 
also, or they may bear movable ribs which, though possessing dorsal 
and ventral portions, do not reach the sternum, and are therefore 
floating or false ribs, or again ribs reduced to short dorsal pieces 
which may or may not fuse with the superimposed iliac bones. As 
a rule the centrum and the spinous process of all sacral vertebrae 
are ossified into one continuous mass. 
The most important features of the Sacrum are best seen on a 
ventral view and may be thus grouped :— 
(1) The anterior or crural portion composed of vertebrae connected 
with the ilium by strong dorsilateral and ventrilateral processes. 
The first of these vertebrae often bears a complete thoracic rib, and 
is followed by others bearing aborted ribs having a tendency to lose 
their “head” and “neck,” while the shaft fuses with the ventral 
surface of the iliac expansions. Between the transverse processes 
of the successive vertebre are foramina through which pass the 
spinal nerves forming the crural plexus. 
(2) The second or ischiadic portion, composed of vertebrae which 
have neither ribs nor ventrilateral processes, but only dorsilateral, 
and these last reduced to thin transverse blades extending obliquely, 
or sometimes almost vertically, upwards, and ultimately reaching the 
dorsal median rim of the iliac bones. The safest guide to the number 
of vertebrae composing this portion is afforded by the number of fora- 
mina through which pass the nerves forming the ischiadice plexus. 
In most Birds the number is from 3 to 5. Owing to the absence of 
ribs and ventrilateral processes, the space between the fused centra 
of the vertebre and the right and left iliac bones constitutes a large 
hollow or fovea wherein is imbedded part of the KIDNEYS. 
(3) The third portion is connected with the dorsimedian rim of 
each ilium by transverse dorsiventral and ventrilateral bony bridges. 
The first two vertebre of this portion are the primitive or true 
Sacrals before mentioned, and they lie just behind a line which 
might be drawn from one acetabulum to the other. Their lateri- 
ventral buttresses are not outgrowths of the centra, but are ribs, 
though their true nature is only revealed in embryos or very young 
birds. 
(4) The postsacral portion consists of vertebrae which in many 
birds, Pavo, for example, behave partly as do the primitive sacrals, 
and partly come by degrees to resemble the caudals. Dorsilateral 
and ventrilateral processes are always present, and, fusing with each 
other, abolish the transverse foramen, while they abut upon the 
dorsal rim of the postacetabular part of the ilia. The first post- 
