206 GENERAL ORNITHOLOGY PART ii 



Tlie_/ire< 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 a 

 ball-and-socket joint. The second cervical is the axis, ax, which 

 subserves rotary movements of the skull. It has a peculiar tooth- 

 like odontoid (Gr. oSous, dSoyros, odous, odontos, tooth ; efSo?, eidos, 

 form) process, borne upon the anterior end of its body, fitting into 

 the lower part of the atlantal ring ; about which pivot the atlas, 

 bearing the head, revolves like a wheel upon 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. 



Thoraeie or Dorsal Vertebrae (Fig. 56, dv) extend from the 

 cervical to or into the 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 ribs ; 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 (hsemapophysial) half. Five or 

 six vertebrae 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 commonly ankylosed with the sacrum, as 

 sr. So few as only three haemapophysis-bearing ribs may reach the 

 sternum. There may also be a long free-jointed rib which " floats " 

 at hoth 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 another series, 

 the lumbar (which has no free, nor any distinct ribs, — 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 in the last paragraph), which have 

 distinct jointed ribs, whatever the connection or disconnection of such 

 pleurapophyses at either end. On this understanding, one, some- 

 times two or even three " dorsal " vertebrae ankylose with the pelvic 

 region of the spine. Fixity of the dorsal region being of advantage 

 to flight, these vertebrae are very tightly locked together ; not only 

 'by the close apposition or even ankylosis of their bodies and pro- 

 cesses, but also, in many cases, by ossifications of the tendons of 

 muscles of the back, and coossifications of these with the vertebrae, 

 like a set of splints, till the consolidation of the thoracic is only 



