142 GENERAL ORNITHOLOGY. 



trate the tail, though the neural arches of all the coccyt^oals he still perviuiis. All may he 

 freely movahle, as in the American Ostrich (Rhea) ; but iu almost all l)irds only the anterior 

 ones are distinct and vertebra-like, the rest, to a variable number, being aljortive, and melted 

 into that extraordinary aftair called the "ploughshare" or pi/gosti/le (Gi'. nv-yrj, puge, the 

 ruuip ; (TTvKos, a post), which may consist of no fewerthan ten such metamori)hosed tail-bones. 

 It has usually a shape suggesting the share of a ])lough (see fig. .56, py), but is too variable to 

 be concisely described. The pygostyle supports the tail-feathers ; and as these are morphologi- 

 cally one pair to each rectrix-bearing vertebra, the number of tail-feathers may be primarily 

 equal to the number of vertebra which fuse in the pygostyle. Thus the swan is said to have 

 ten vertebra? in this mass; our wild swan {Oijgnus columhianuK) has twenty tail-feathers. In 

 this view, six should be the usual compositinu of the share-bone. A bird's tail is really more 

 extensive and lizard-like than commonly supposed; thus the swaii, besides its ten iu the 

 pygostyle, has seven free caudals, and ten uro-sacrals — twenty-seven post-sacral vertebra^ iu 

 all (Huxley). In the i-aven, the free caudals are si.\-, exclusive of the pygostyle. These all 

 have large fiaring transverse processes and moderate spinous processes, and the latter ones are 

 also provided with hypapophyses, some of which are bifurcate. The pygostyle in many birds 

 expands below into a hirge cii'cular cir polygonal disc. 



2. THE THORAX: RIBH AND STERNUM. 



The Thorax (Gr. 6apa^, a coat of mail; in anat., the chest; adj. thoracic; see fig. 56) is 

 the bony box formed by the ribs on each side, the breast-bone below, and the back-bone above. 

 In birds, it is very extensive, including most or all of the abdominal as M-ell as the thoracic 

 viscera, and its cavity is not paititioned oft' from that of the belly by a completed diaphragm, 

 though a rudimentary structure of that kind i.s found in the class. The thorax is usually sol- 

 dered liehind to the pelvis by union of one or more pairs of ribs with the ilia; in front it al- 

 ways and entirely bears the pectoral arch (see p. 145). The thorax is very movable in birds, 

 by reason of the great length and jointedness of the ribs. 



The Ribs (Lat. casta, a rib; pi. costce; adj. costal; sec tig. 56, c, t', R, cr, sr, »),as said 

 above, are the pleurapophysial elements of vertebrfe, which remain small and anchylosed, or 

 become long and free. In the latter state only are they " ribs" iu ordinary language. The 

 one or more cer\acal ribs, hoAvever elongated, and the aboilive lumbar and uro-sacral ribs, arc 

 to be excluded from the present description, and have been already considered. I'nie ribs are 

 those which belong to the dorsal vertebrsB proper, and are jointed iu themselves; that is, have 

 articulated hcemapophyses (see p. 137), by which they may or do articulate with the sternum. 

 Such true ribs arc fixed, when they reach from back-bone to breast-bone; floating, when either 

 or neither of these connections is made. Usually the last rib, tliough bearing a perfect hrem- 

 apophysis, does not reach the sternum; in the loon, for example, the last rib floats at Imth 

 ends, having connection neither with vertebra nor sternum ; and the two ne.vt. ribs float at 

 their sternal ends. The perfected ribs are few, — five or six is a usual number, thousjh uiue 

 are hajmapophysis-bearing in the loon. The la.st rib at least is usually "sacral;" «. e., be- 

 longs to a dorsal vertebra which is anchylosed with the "sacral" nuiss ; and two or even, as iu 

 the loon, three ribs may likewise issue out from under cover of the ilia. These "sacral ribs" 

 are furthermore distinguished by being devoid of the cpipleural or uncinate pi-ocesses (Lat. 

 uncus, a hook ; fig. 56, m) with which other true ribs are furnished, forming a series of splint- 

 bones proceeding obliquely from one rib to shingle over the uext succeeding one, and thus 

 increase the stability of the thoracic side-walls. Such splints uuiy be either articulated or an- 

 chylosed with their respective ribs; they have independent (Jssitic centres. The upper (pleura- 

 pophysial) part of a rib, or "vertebral rib," when perfected, articulates with the side of the 



