SKELETON 855 



vertebrae, their definition being rather practical, and applicable to 

 particular skeletons, than of general morphological value. 



It has been already pointed out (p. 849) that dorsal and thoracic 

 vertebrae are not necessarily identical, and in like manner the most 

 anterior prsesacrals 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 pmiion 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 beai'ing 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 vertebrae 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 ischiadic 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 vertebrae 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 vertebrae 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- 



