THE VEETEBEiE IN BIRDS. 279 



unite to form tlie sacral plexus, whence the great sciatic 

 nerve is given oS ; and I take them to be the homologues 

 of the saci'al vertebrae of Beptilia. The deep fossae between 

 the centra of these vertebrae, their transverse processes, and 

 the ilia, are occupied by the middle lobes of the kidneys. 



If these be the true sacral vertebrae, it follows that their 

 successors are anterior caudal. They have expanded upper 

 transverse processes, like the proper sacral vertebrae ; but, 

 in addition, three or four of the most anterior of these ver- 

 tebrae possess ribs which, like the proper sacral ribs of rep- 

 tiles, are suturally united, or ankylosed, proximally, with 

 both the neural arches and the centra of their vertebrae, 

 while, distally, they expand and abut against the ilium. The 

 ankylosed caudal vertebrae may be distinguished as uro- 

 sacral. The caudal vertebrae which succeed these may be 

 numerous and all distinct from one another, as in Archceop- 

 teryx and Bhea ; but, more generally, only the anterior caudal 

 vertebrae are distinct and moveable, the rest being ankylosed 

 into a ploughshare-shaped bone, or pygostyle, which supports 

 the tail feathers and the uropygial gland, and sometimes, as 

 in the Woodpeckers and many other bii-ds, expands below 

 into a broad polygonal disc. 



The centra of the moveable presacral vertebrae of Bii-ds 

 are connected together by fibro-cartilaginous rings, which 

 extend from the circumference of one to that of the next. 

 Each ring is continued inwards into a disc vrith free 

 anterior and posterior faces — the meniscus. The meniscus 

 thins towards its centre, which is always perforated. The 

 synovial space between any two centra is, therefore, divided 

 by the meniscus, into two very narrow chambers, which com- 

 municate by the aperture of the meniscus. Sometimes the 

 meniscus is reduced to a rudiment ; while, in other cases, it 

 may be united, more or less extensively, with the faces 

 of the centra of the vei-tebrae. In the caudal region, the 

 union is complete and the meniscus altogether resembles an 

 ordinary intervertebral cartilage. 



A ligament traverses the centre of the aperture in the 

 meniscus ; and, in the chick, contains the intervertebral por- 



