148 GENERAL ORNITHOLOGY. 



trate the tail, though the neural arches of all the coccygeals be still pervious. All may be 

 freely movable, as in the American Ostrich {Rhea) ; but in almost all birds only the anterior 

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

 into that extraordinary affair called the "ploughshare" or pygostyle (Gr. nvyr]^ puge, the 

 rump ; arvKos, a post), which may consist of no fewer than ten such metamorphosed taU-bones. 

 It has usually a shape suggesting the share of a plough (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 taU-feathers may be primarily 

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

 ten vertebrae in this mass; our wild swan (Cygmis columbianus) has twenty tail-feathers. In 

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

 extensive and lizard-like than commonly supposed; thus the swan, besides its ten in the 

 pygostyle, has seven free caudals, and ten uro-sacrals — twenty-seven post-sacral vertebrae in 

 aU (Huxley). In the raven, the free caudals are six, exclusive of the pygostyle. These aU 

 have large flaring 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 large circular or polygonal disc. 



2. THE THORAX: lilBS AND STERNUM. 



The Thorax (Gr. 6copa^, a coat of maU; 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 well as the thoracic 

 viscera, and its cavity is not partitioned off from that of the belly by a completed diaphragm, 

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

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

 ways and entirely bears the pectoral arch (see p. 151). The thorax is very movable in birds, 

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



The Kibs (Lat. costa, a rib; pi. costce; adj. costal; see fig. 56, c, c', R, cr, sr, u),as said 

 above, are the pleurapophysial elements of A^ertebrae, which remain small and anchylosed, or 

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

 one or more cervical ribs, however elongated, and the abortive lumbar and uro-sacral ribs, are 

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

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

 articulated hcEmapophyses (see -p. 143), by which they may or do articulate with the sternum. 

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

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

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

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

 their sternal ends. The perfected ribs are few, ■ — five or six is a usual number, though nine 

 are haemapophysis-bearing in the loon. The last rib at least is usually " sacral ;" i. e. , be- 

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

 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 epipleural or uncinate processes (Lat. 

 tmcus, 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 next succeeding one, and thus 

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

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

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



