THE ANATOMY OF BIRDS. — OSTEOLOGY. 143 



boues which iu some respects correspond with the centrums of vertebrae. The entire chain or 

 series of vertebrae composes the back-bone or spinal column; with its connections (thorax and 

 pelvis) and anterior continuation (skull) it is the axial skeleton. The skull is considered by 

 some competent anatomists to consist of modified vertebrae. The skuU-bones ha\-e certainly 

 the position and relations of parts of vertebrae ; to a certain extent they resemble vertebrae, as 

 in being divisible into several segments, like as many vertebral segments ; they are also direct- 

 ly in the axis of the body, enclosing a part of the cerebro-spinal nervous system above, and 

 portions of the visceral systems below. But supposed strict morphological correspondence of 

 cranial bones with vertebrae is not supported by their mode of development, and is now gen- 

 erally denied, the relation being considered rather analogical and physiological than homological 

 and morphological. 



1. THE SPINAL COLUMN. 



A Vertebra (so called from the tlexibility of the chain of vertebrae ; Lat. verto, I turn) 

 consists of a solid body or centrum, and more or fewer processes or apophyses, some of which 

 have separate ossific centres. . Plate-like processes which arch upward from either side of a 

 centrum to enclose the neural canal are the neural arches or neur apophyses (fig. 54, n, n) ; 

 at their union in the middle line above they commonly send up a process called the neural spine 

 (ns). Transverse processes from the sides of the neural arch are diapophyses (Gr. bia, dia, across) 

 (figs. 54, 55, d, d). Oblique processes from the sides of the same arches, serving to lock them 

 together, are zygapophyses (Gr. ^vyov, zugon, a yoke ; fig. 55, z) ; there are two on' each side ; 

 one anterior, on tlie front border of an arch, a pre-zygapophysis ; one posterior, on the hind 

 border, & post-zygapophysis. From the under-side of a centrum, in the middle line, there is often 

 a hypapophysis (Gr. utto, hupo, under: fig. 55, hy). These several processes, with some others 

 not necessary to mention here, make with the centrum a vertebra iu strictness ; that is, when exist- 

 ing at all, they are completely consolidated with one another and with the centrum into one bone. 

 But certain important elements of a vertebra, developed from independent ossific centres, may or 

 may not anchylose therewith, in difierent regions of the same spinal column. These are the 

 pleurapophyses (Gr. irktvpov, pleuron, a rib; fig. 54 pi; fig. 55, r). Any rib is in fact the 

 pleurapophysial element of a vertebra ; it may be, and in most regions of the spinal column it is, 

 ([uite small when existing at all, and anchylosed with the vertebra to which it belongs, as an 

 integral portion thereof. Only in the lower region of the neck, and throughout the thoracic 

 region, such pleurapophyses elongate, and are movably articulated with their respective verte- 

 brae ; they then become the "ribs" of ordinary language. Moreover, the true thoracic ribs of 

 birds are jointed near the middle, each thus consisting of two pieces; the upper piece is pleura- 

 l>ophysis proper: the lower is called a hcemapophysis (fig. 54, h; fig. 55, cr) ; it coiTcsponds to 

 a " costal cartilage " of human anatomy. Once again : since the sternum (breast-bone) is theo- 

 retically, and doubtless archetypically, a solidified set of those parts of the vertebral segments 

 whicli complete the haemal arches below, each segment of a sternum to which a haemapophysis 

 is articulated is called a hcemal spine, being compared to a neural spine above. Aside from any 

 consideration of the ribs proper and sternum, or free pleurapophyses, htemapophyses, and 

 h;emal spines, any "vertebra" of ordinary language is the compound bone which consists of 

 centrum and neur-, di-, pre- and post-zyg-, jdeur-, hyp- and other -apophyses, if any, and 

 neural spine ; the latter being often called the " spinous process." 



The Vertebrae join one another, forming a continuous chain. Their centra are placed 

 end to end, one after another ; their neunJ arches are also locked together by the zygapophyses, 

 when such articular processes are developed. Zygapophyses bear upon their free ends smooth 

 articular facets, the faces of which are mostly horizontal ; those of the pre-zygapophyses looking 

 downward, and overriding the reversed faces of the post- zygapophyses. The mode of jointing 



