THE ANATOMY OF BIRDS.— OSTEOLOGY. 137 
bones which in some respects correspond with the centrums of vertebrae. The entire chain or 
series of vertebree 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 vertebre. The skull-bones have certainly 
the position and relations of parts of vertebrae; to a certain extent they resemble vertebre, 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 vertebre 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 flexibility of the chain of vertebre ; 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 newrapophyses (fig. 54, n, 1) ; 
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. 61a, 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, 2); there are two on each side ; 
one anterior, on the front border of an arch, a pre-zygapophysis ; one posterior, on the hind 
border, a post-zygapophysis. From the under-side of a centrum, in the middle line, there is often 
a hypapophysis (Gr. im6, hupo, under: fig. 55, hy). These several processes, with some others 
not necessary to mention here, make with the centrum «@ vertebra in 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 different regions of the same spinal column. These are the 
pleurapophyses (Gr. mdevpdv, plewron, a rib; fig. 54 pl; fig. 55,7). Any rib is in fact the 
pleurapophysial element of a vertebra ; it may be, and in most regions of the spinal column it is, 
quite 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 thoracie 
region, such pleurapophyses elongate, and are movably articulated with their respective verte- 
bre ; 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- 
pophysis proper: the lower is called a hemapophysis (fig. 54, h; fig. 55, cr) ; it corresponds 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 
which complete the heemal arches below, each segment of a sternum to which a hemapophysis 
is articulated is called a hemal spine, being compared to a neural spine above. Aside from any 
consideration of the ribs proper and sternum, or free pleurapophyses, heemapophyses, and 
hemal spines, any ‘‘ vertebra” of ordinary language is the compound bone which consists of 
centrum and neur-, di-, pre- and post-zyg-, pleur-, hyp- and other -apophyses, if any, and 
neural spine ; the latter being often called the ‘‘ spinous process.” 
The Vertebrz join one another, forming a continuous chain. Their centra are placed 
end to end, one after another; their neural 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 
