The Vertebral Column
vertebra ribs sacral processes dorsal ankylosed lumbar cervical transverse anterior
THE VERTEBRAL COLUMN, RIBS, AND SrEtt-st-m.3 The spinal column of birds contains numerous and well-ossified vertebrae, a considerable number of which (more than six) are ankylosed together to form a sacrum. Of the vertebra; which enter into the composition of this complex bone, however, not more than from three to five can be regarded as the homologues of the sacral vertebrme of a Crocodilian or Lacertilian Reptile. The rest are borrowed, in front, from the lumbar and dorsal regions ; behind, from the tail. The cervical region of the spine is always long ; and its vertebrae, which are never fewer than eight, and may be as many as twenty-three, are, for the most part, large in proportion to those of the rest of the body.
The atlas is a relatively small, ring-like bone ; and the transverse ligament may become ossified and divide its aperture into two - an upper for the spinal cord, and a lower for the odontoid process of the axis vertebra. The os odontoideum is always ankylosed with the second vertebra, and constitutes a peg-like odontoid process.
The spines of the succeeding cervical vertebra are often obsolete, and are never very prominent in the middle region of the neck. The anterior faces of their elongated vertebral centra, are convex 11012L above downwards, and concave from side to side ; whilst the posterior faces are cylindrical, slightly excavated from above dowimards, and convex from side to side. (The contrary of this is stated in Professor Huxley's Vold). Anim., p. 276, where the author, by a lapses memorice, puts it vice versa.) Hence, in vertical section the centra appear opisthoccelous ; in horizontal section, proccelous, and not the contrary, as is stated by our author ; and the structure is exceedingly characteristic of birds. The under surfaces of the centra frequently give off median inferior processes. In the Ratitce it is obvious that the cervical vertebra have short transverse processes and ribs, disposed very much as in the Crocodi/ia. For, in young birds, the anterior end of the lateral face of each vertebra bears two small processes, an upper and a lower ; and this expanded head of a styliform rib is articulated with these by two facets, which represent the capitulum and the tuberculum (Huxley, op. cit., p. 276). In the chicken of the Emeu (Dronmens norce-hollandice) the writer, in 1843, carefully worked out and figured these parts. Of the twenty cervical vertebra only the atlas and axis were d3void of distinct ribs ; this individual was six weeks old. These riblets were bony wedges, with a sharp point ; but that was free, and the thick upper end was jammed in between upper and lower transverse processes (diapophysis and parapo-physis). The last but one of the ribs became suddenly larger, and the last was two-thirds the size of its successor - the first dorsal. Then followed six large ribs on each side, the last two floating. The vertebra bearing the last of these, and twenty more, are closely embraced by the fore-and-aft growth of the ilium, and form the so-called sacrum. Of the twenty vertebra between the first overlapped bone with a floating rib and the nine rildess caudals, there are five with free ribs, small, and hatchet-shaped, quite like those in the neck of the Crocodile. These, from being attached to a parapophysial cup near the fore-end of the centrum, get more forward, and wedge in between their own vertebra and the one in front. The next four vertebra, which give exit to the sacral plexus (or at least to most of it), have no ribs, and are very broad and short. They develop lamellar upper transverse processes, but their spines are aborted. Then come eleven vertebra, in front of the free caudal, that have short ribs ; the first two pairs are ankylosed already, then four pairs are distinct, and the remaining five have their ribs ankylosed, and then becoming shorter and more pedate externally, get further backwards on the centrum. Thus, in a Bird as old a3 six weeks after hatching, there are eighteen pairs of cervical, and nine pairs of so-called sacral ribs still distinct. Moreover, the ribs are quite aborted on the first and second cervical, on the four true sacral, - perchance, the next after this is also sacral, - and on all the caudal vertebra which have only papilliform transverse processes. There are fifty-five vertebra in all in the Emeu, thus : - cervical, twenty ; dorsal, five ; dorso-lumbar (the first with a large rib and really the sixth dorsal), six ; sacral (proper), four ; urosacral, eleven ; caudal, nine. We shall return to these data in describing the sacrum of the Fowl.
With age the cervical ribs (of the Ratitce) may become completely ankylosed. In Apteryx australis one, below, remains free ; in Struthio canuleu, two ; and in Dromcms novce-hollandice, three ; and then they appear like transverse processes, perforated at the base by a canal, which, as in the Crocodilia, contains the vertebral artery and vein, and the main trunk of the sympathetic nerve. The cervical ribs and transverse processes are similarly disposed in very young Carinake; but in these birds their form frequently becomes much modified in the adult, and they develop prolongations which extend downwards and inwards, and protect the carotid artery or arteries. The neural arches have well-developed pre- and post-zygnpophyses. The ribs of one or two of the posterior cervical vertebra become elongated and freely movable in the Cannata', as in the Rat it ce.
The first dorsal vertebra is defined as such by the union of the ribs with the sternum by means of a sternal rib, which not only, as in the Crocoditia, becomes articulated with the vertebral rib, but is converted into complete bone, and is connected by a true articulation with the margin of the sternum. The number of the dorsal vertebrae. (reckoning under that head all the vertelirre, after the first dorsal, which possess distinct ribs, whether they be fixed or free) varies. The centra of the dorsal vertebrae either possess cylindroidal articular faces, like those of the neck, as is usually the case ; or more or fewer of them may have their faces spheroidal, as in the Penguins [Plovers (and their kin Vancllus crixtatus, Taunus fuscus, &c.), Gulls, Cormorants, and Parrots]. In this case the convex face is anterior, the concave, posterior. They may, or may not, develop .inferior median processes [which may be simple, as in the Cormorant, where they exist on several lower cervical, en all the dorsal, and in five sacro-lumbar ; or they may bifurcate into two broad, bony leaves, as in Co/pains], They usually possess well-marked spinous processes [which begin in the two or three lower cervicals]. Sometimes they are slightly movable upon one another [bound strongly, in many cases, by ossified tendons of great strength and elasticity]; sometimes they become ankylosed together into a solid mass. [When this takes place the last cervical is ankylosed to the three first dorsal, as in the fowl, the fourth remaining free, and the fifth coalescing with the lumbar; or, as many as five may ankylose together, leaving one free, and the last ankylosed to the lumbar, as in Falco cesalon. But this number often differs with age, as may be seen in different individuals of Psophia crepitans, and other, more typical, Cranes.] It is characteristic of the dorsal vertebra of Birds that the posterior, no less than the anterior, vertebra present a facet or small process on the body, or lc-gar part of the arch, of the vertebra for the capitulum of the rib, while the upper part of the neural arch gives off a more elongated process for the tuberculum. Thus, the transverse processes of all the dorsal vertebra of a Bird resemble those of the two anterior dorsals of a Crocodile, and no part of the vertebral column of a Bird presents transverse processes with a step for the head of the rib, like those of the great majority of the vertebra of Crocodilia, Dinosauria, Dicynodontia, and Pterosauria. [The triangular facets for the tubercular processes are scarcely scooped ; those for the capitular are neat, round, shallow cups.] The discrimination of the proper lumbar, sacral, and anterior caudal vertebra, in the ankylosed mass which constitutes the so-called "sacrum" of the Bird, is a matter of considerable difficulty. The general arrangement 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 vertebra 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.' fhis 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 in it [there are ,flee of these lumbar vertebra in the Emeu, two more than in the Fowl, and they all have distinct ribs ; and the ribless vertebra 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 vertebra in other rerlebrata. 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 vertebra show not the least trace of ribs, and are flat bricks of bone, below, jammed together like the cervical centre of a Cetacean.] The nerves which issue froth the intervertebral foramina of these vertebra 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 vertebra of the Reptilia. The deep fossm between the centra of these vertebra, their transverse processes, and the ilia, are occupied by the middle lobes of the kidneys. If these be the true sacral vertebra, 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 vertebra 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 vertebra may be distinguished as uro-sacral.
We now give a table showing the number of bones in the so-called sacrum of Birds - so many vertebra 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. .lost of the instances are derived from the sacral bones of young Birds.
d. . s. u.s. Total.
Colynants glucialis and C. scptentrionalis 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 right side. This will show how, by gradations the most gentle, the vertebrae and their ribs are specialized iu each particular type, and also how very arbitrary is our nomenclature.
The Swan has eight free vertebra behind the uro-sacral, and as the last of these is in these types composed of ten vertebra originally, there are primarily twenty-seven vertebra in. the Swan's tail. The caudal vertebra which succeed the uro-sacral may be numerous and all distinct from one another, as in Archceopteryx, or few and distinct, as in Rhea ; but more generally, only the anterior caudal vertebra are distinct and movable, 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 Woodpecker and some other Birds expands below into a broad polygonal disk.
The centra, of the movable presacral vertebra of Birds are connected together by fibro-cartilaginous rings, which extend front the circumference of one to that of the next. Each ring is continued inwards into a disk, with 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 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 of the meniscus, and in the Duck contains the intervertebral portion of the notochord. As Jager3 has shown, it is the homologue of the odontoid ligament in the cranio-spinal articulation, and of the pulpy central part of the intervertebral fibro-cartilages in Mammalia. All the vertebral ribs in the dorsal region, except, perhaps, the very last free ribs, have widely separated capitula and tubercula. More or fewer have well-ossified uncinate processes attached to their posterior margins, as in the Crocodilia [and Iliateria. 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 very small and few in number ; in the Emeu and in the A pteryx they are large ; they evidently correspond with the unsevered rib-flaps of the little Ant-eater (Parker's Shouldergirdle and Sternum, plate 22, figs. 19, 20).] The vertebral ribs are completely ossified up to their junction with the sternal ribs.