THE ANATOMY OF BIRDS. — OSTEOLOGY. 145 



stylets or riblets, completely anchylosed with the neural arches in adult life, and lying parallel 

 with the long axes of the bones. The anchylosis of pleuropophyses distinguishes most cervical 

 vcrtebriE in another way : for from it results, on each side of the neural arch, a foramen 

 (Lat. foramen, a hole, pi. foramina), through which blood-vessels (vertebral artery and vein) 

 pass to and from the skull. The series of these foramina is called the vertebrarterial canal ; 

 none such exist in those posterior cervical vertebrae which bear free ribs ; thus, in the raven the 

 canal begins abruptly at the fourth fi-om the last cervical. But, as in Bhea for instance (and 

 doubtless in many other cases), the vertebrarterial canal shades visibly into the series of 

 foramina formed by the spaces between the head and shoulder of any rib and the side of the 

 vertebra to which it is attached ; such being, as I suppose, the true morphology of the canal. 

 The cervical is the most flexible region of a bird's spine ; the articular ends of the vertebral bodies 

 are the most completely saddle-shaped (heterocoelous) ; the zygapophyses are large and flaring, 

 overriding each other extensively ; the largest processes are at the fore ends of the bones ; the ap- 

 positions of the central and zygapophysial articular surfaces are collectively such, that the column 

 tends to bond in an S-shape or sigmoid curve. The vertebral bodies are more or less contracted 

 in the middle, or somewhat hour-glass-shaped ; on several lower cervicals, hypapophyses are 

 lilci'ly to be well developed; as are neural spines toward both the beginning and end of the 

 scries. The vertebrae on the whole are large ; their neural canal is also of ample calibre. The 

 first two cervicals are so peculiarly modified for the articulation of the skull as to have received 

 special names. The first one, fig. 56, at, the atlas (so called because it bears up the head, as 

 the giant Atlas was fabled to support the firmament), is a simple ring, apparently without a 

 centrum. The lower part of the ring is deeply cupped to receive the condyle of the occiput 

 into ball-and-socket joint. The second cervical is the axis, ax, which subserves rotary move- 

 ments of the skull. It has a peculiar tootli-like odontoid (Gr. 68ovs, 686vtos, odous, odontos, 

 tooth ; (Ihos, eidos, form) process, borne upon the anterior end of its body, fitting into the lower 

 ])art of the atlantal ring; about which pivot the atlas, bearing the head, revolves like a wheel 

 ujxm an eccentric axis. The cervicals of birds vary greatly in number; according to Huxley 

 there are never fewer than eight, and there may be as many as twenty-three ; Stejneger gives 

 twenty-four for some of the swans. Twelve to fourteen may be about an average number. 



Thoracic or Dorsal Vertebrae (fig. ,56, dv) extend from the cervical to or into tlie 

 pelvic region of the spine. In most animals, and in ordinary anatomical language, a "dorsal" 

 is one which bears a distinct free rib, and is therefore truly thoracic, since '' ribs" are the side- 

 walls of the chest. But in birds, as we have seen, certain cervicals have distinct elongate 

 ril>s ; and, as will be seen soon, long jointed pleurapophyses are usually found in that region 

 commonly called "sacral." The first dorsal, in birds, is arbitrarily considered to be that one 

 which bears the first rib which is jointed, and which reaches the sternum by its lower (luema- 

 l.ophysial) half. Five or six vertebra? of birds commonly answer this description ; though the 

 last one which bears a long free jointed rib (which may or may not reach the sternum) is com- 

 monly anchylosed with the sacrum, as .<?r. So few as only three haemapophysis-bearing ribs may 

 reach the sternum. There may also be a long free-jointed rib which " floats " at both ends ; 

 i. e., is articulated neither with the sternum nor with the vertebra to which it belongs as in the 

 loon, for example. As the dorsal series thus shades insensibly behind into an»)ther series, the 

 lumbar (wliich lias no free, nor any di.stinct rihs, — ribs that one would not hesitate to call 

 such), it is best to consider as dorsal or thoracic all those vertebrae, succeeding the last 

 cervical (which is to be determined as explained ill tlic hist paragraph), which have distinct 

 jointed ribs, whatever the connection or disconnection of such pleurapophyses at either end. 

 On this understanding, one, sometimes two or even three "dorsal" vertebrae anchylose with 

 the pelvic region of the sjiine. Fixity of the dorsal region being of advantage to flight, these 

 vertebrae ai-f very li^^htly locked together; not only by the close apposition or even 



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