Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/737

] great majority of the vertebrae of Crocodilia, Dinosauria, Dicynodontia, and Pterosauria. [Tho triangular facets for the tubercular processes are scarcely scooped ; those for the capitular are neat, round, shallow cups.] The dis crimination of the proper lumbar, sacral, and anterior caudal vertebrae, in the ankylosed mass which constitutes the so- called &quot;sacrum&quot; of the Bird, is a matter of considerable difficulty. The general arrange ment is as follows : The most anterior lumbar vertebra has a broad transverse process, which corresponds in form and position with the tubercular transverse process of the last dorsal. In the succeeding lumbar vertebras the process extends downwards ; and in the hindermost [the third] it is continued from the centrum, as well as from the arch of the vertebra, and forms a broad mass which abuts against the ilium. This process might well be taken for a sacral rib, and its vertebra for a proper sacral vertebra. But, in the first place, I find no distinct ossification iu it [there are Jive of these lumbar vertebra in the Emeu, two more than in the Fowl, and they all have distinct ribs ; and the ribless vertebrae are five in the Fowl and four in the Emeu] ; and, secondly, the nerves which issue from the intervertebral foramina in front of and behind the vertebra enter into the lumbar pleXUS, Which gives Origin to the crural and obturator nerves, and not into the sacral plexus, which is the product of the nerves which issue from the intervertebral foramina of the proper sacral vertebrae in other Vertebrata. Behind the last lumbar vertebra follow, at most, five vertebra which have no ribs ; but their arches give off horizontal, lamellar processes, which unite with the ilia. [In the Emeu these four vertebrae show not the least trace of ribs, and are flat bricks of bone, below, jammed together like the cervical centra of a Cetacean.] The nerves which issue from the intervertebral foramina of these vertebne unite to form the sacral plexus, whence the great sciatic nerve is given off; and I [Professor Huxley] take them to be the homologues of the sacral vertebras of the Eeptilia. The deep fossae between the centra of these vertebras, 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 vertebrae possess ribs, which, like the proper sacral ribs of Reptiles, are suturally united, or ankylosed proximally, with both the neural arches and the centra of these vertebrae ; while, distally, they expand and abut against the ilium. The ankylosed caudal vertebrae may be distinguished as uro-sacral. Fic - 29.-Thc &quot;sacrum&quot; of a .. . young Fowl; natural size, s*en from beloiv. d.l., dorso-lumbar, sncra! c caudal vcrt(bra - We now give a table showing the number of bones in the so-called sacrum of Birds so many vertebrae as are covered by the ilia and ankylosed together. Here the distinction between dorsal and lumbar is, that the former possess elongated ribs ; and the table will show forms of extreme length and of extreme shortness, for a Bird ; and also, as in the Fowl, of a medium type. Most of the in stances are derived from the sacral bones of young Birds.

Dromccus novcc-hollandue ... 1 Cygnus olor 2 Colymliis glacialis aiid ) C. septenlrionalis Gallus domcsticus 1 Alcedo ispida 1 Upupa epops 1 Cypsdus apus 2 u.s. Total. 11 = 21 10 = 21 7 = 17 C = 15 C = 13 5 = 11 5 = 11 In both the Hoopoe and the Swift the first of these sacrals has an outstanding rib-process. In the Swift the rib on the second of the enclosed dorsals is very long, and its flanking rib nearly reaches the sternum. The next or third vertebra, the lumbar, has below it, neither reaching to it above, nor by its sternal piece to the sternum below, another rib ; it is two-thirds the size of its predecessor, and only occurs on the rigid side. This will show how, by grada tions the most gentle, the vertebrae and their ribs are specialized in each particular type, and also how very arbi trary is our nomenclature. The Swan has eight free vertebrae behind the uro-sacral, and as the last of these is in these types composed of ten vertebrae originally, there are primarily twenty-seven verte brae in. the Swan s tail. The caudal vertebrae which suc ceed the uro-sacral may be numerous and all distinct from one another, as in Archceopteryx, or few and distinct, as in Ehea ; but more generally, only the anterior caudal vertebras are distinct and movable, the rest being ankylosed into a ploughshare-shaped bone or pygostyle, which sup ports the tail feathers and the uropygial gland, and some times, as in the AVoodpecker and some other Birds expands below into a broad polygonal disk. The centra of the movable presacral vertebrae of Birds I 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 disk, with free an terior 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 communicate by the aperture of the meniscus. Some times 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 of the meniscus, and in the Duck contains the intervertebral portion of the notochord. As Jager has shown, it is the homologue of the odontoid ligament in the cranio-spinal articulation, and of the pulpy central part of the inter vertebral fibro-cartilages in Mammalia. All the verte bral ribs in the dorsal region, except, perhaps, the very last free ribs, have widely separated capitula and tubercula. More or fewer have well-ossified unciuate processes attached to their posterior margins, as in the Crocodilia [and Hat- teria. These are separate, both as cartilage and as bone, at first; we have only failed to find them in the Crested Screamer (Chauna chavaria). Among the Ratitce they are veiy small and few in number ; in the Emeu and in the Aptcryx they are large ; they evidently correspond with the un- severed rib-flaps of the little Ant-eater (Parker s Shoulder- 