SEC. IV ANATOMY OF BIRDS 203 



line, there is often a hypapophysis (Gr. wro, hwpo, under ; Fig. 55, hy). 

 These several processes, with some others not necessary to mention 

 here, make with the centrum a vertebra in strictness ; that is, when 

 existing at all, they are completely consolidated with one another 

 and with the centrum into one bone. But certain important ele- 

 ments of a vertebra, developed from independent ossific centres, 

 may or may not ankylose therewith, in different regions of the 

 same spinal column. These are the pleurapophyses (Gr. TrXevpov, 

 pleuron, a rib; Fig. 54, pi; Fig. 55, r). Any rih is in fact the pleura- 

 pophysial element of a vertebra ; it may be, and in most regions of 

 the spinal column it is, quite small when existing at all, and 

 ankylosed with the vertebra to which it belongs, as an integral 

 portion thereof. But in the lower region of the neck, and through- 

 out the thoracic region, such pleurapophyses elongate, and are 

 movably articulated with their respective vertebrse ; they then 

 become the " ribs " of ordinary language. Moreover, the true 

 thoracic ribs of birds are jointed near the middle, each thus con- 

 sisting of two pieces ; the upper piece is pleurapophysis proper : 

 the lower is called a hcemapophysis (Fig. 54, h ; Fig. 65, cr); it corre- 

 sponds to a " costal cartilage " of human anatomy. Once again : 

 since the sternum, (breast-bone) is theoretically, and doubtless 

 archetypically, a solidified set of those parts of the vertebral 

 segments which complete the hsemal arches below, each segment of 

 a sternum to which a hsemapophysis 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, 

 haemapophyses, and hsemal spines, any "vertebra" of ordinary 

 language is the compound bone which consists of centrum and 

 neui-, di-, pre-, and post-zyg-, pleur-, hyp-, and other -apophyses, 

 if any, and neural spine ; the latter often called " spinous 

 process." 



The Vertebrse 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 these 

 articular processes are developed. Zygapophyses bear upon their 

 free ends smooth articular facets, the faces of which are mostly 

 horizontal ; those of the prezygapophyses looking downward, and 

 overriding the reversed faces of the postzygapophyses. The mode of 

 jointing of the centra of such vertebrae as are freely movable upon 

 each other is highly characteristic of birds, in so far as the shapes of 

 the articular ends of the vertebral centra are concerned. In anatomy 

 at large, a vertebral centrum which is cupped or hollowed at both 

 ends is of course biconcave. Such a vertebra is called amphicodms 

 (Gr. a/i<^i, amphi, on both sides ; koiAos, hoilos, hollowed) ; this is 

 the rule in fishes, and obtained in some extinct Cretaceous birds, as 



