THE VERTEBRA IN BIRDS. 239 



form the sacral plexus, •whence the great sciatic nerve is given 

 off; and I take them to be the homologues of the sacral ver- 

 tebra of Heptilia. The deep fossaa 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 

 Buocessors are anterior caudal. They have expanded upper 

 transverse processes, like the proper sacral vertebr£e ; but, in 

 addition, three or four of the most anterior of these vertebrae 

 possess ribs vi^hich, like the proper sacral ribs of reptiles, are 

 suturally united, or anchylosed, proximally, with both the 

 neural arches and the centra of their vertebrae, while, distally, 

 they expand and abut against the ilium. The anchylosed 

 caudal vertebrae may be distinguished as urosacral. The 

 caudal vertebrae which succeed these may be numerous and all 

 distinct from one another, as in Archceopteryx and Rhea ; but, 

 more generally, only the anterior caudal verlebriE are distinct 

 and movable, the rest being anchylosed into a plough-share 

 shaped bone, or pygostyle, which supports the tail-feathers 

 and the uropygial gland, and sometimes, as in the Wood- 

 peckers and many other birds, expands below into a broad 

 polygonal disk. 



The centra of the movable presacral vertebrae of Birds 

 are connected together by fibro-cartilaginous rings, which 

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

 Each ring is continued inward into a disk with free anterior 

 and posterior faces — the meniscus. The meniscus thins tow- 

 ard 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 communicate 

 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 

 vertebrae. 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- 

 tion of the notochord. As JSger * has shown, it is the homo- 

 logue of the odontoid ligament in the cranio-spinal articula- 

 tion ; and of the pulpy central part of the intervertebral fibro- 

 cartilages in Mammalia. 



*"Da3 Wirbeliorpergelenk der Vogel." Sitzungsteriohte der WieEel 

 Akademle, 1858. 



