THE YERTEBRJE 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- 

 tebrae of Heptilia. The deep fossa; between the centra of these 

 vertebras, their transverse processes, and the ilia, are occujDied 

 by the middle lobes of the kidneys. 



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

 successors are aijtterior caudal. They have expanded upper 

 transverse processes, like the proper sacral vertebras ; but, in 

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

 possess ribs which, like the proper sacral ribs of reptiles, are 

 suturally united, or anchylosed, proximally, with both the 

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

 they expand and abut against the ilium. The anchylosed 

 caudal vertebrse may be distinguished as urosacral. The 

 caudal vertebrae which succeed these may be numerous and all 

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

 more generall}-, only the anterior caudal vertebrae are distinct 

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

 shaped bone, or pygostyle^ which supjDorts 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 vertebras 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 

 cartilaofe. 



A ligament traverses the centre of the aperture in the 

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

 tion of the notochord. As Jitger * 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 3fa77imaUa, 



*"Da9 "Wirbclkorpergelenk der Vogel." SitzungsLericlite der Wiener 

 Akademie, 1858. 



