142 
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. wvyr], puge, the 
rump; cttvXos, a post), which may consist of no fewer than ten such metamorphosed tail-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 tail-feathers may be primarily 
equal to the number of vertebrae which fuse in the pygostyle. Thus the swan is said to have 
ten vertebrae in this mass; our wild swan (Cygnus columhianus) 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 
all (Huxley). In the raven, the free caudals are six, exclusive of the pygostyle. These all 
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: RIBS AND STERNUM. 
The Thorax (Gr. 6u)pa^, 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 well as the thoracic 
viscera, and its cavity is not partitioned off fi*om that of the belly by a completed diaphragm j. 
though a rudimentary structure of that kind is found in the class. The thorax is usually sol- 
dered behind 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 inbirds^ 
by reason of the great length and jointedness of the ribs. 
The Ribs (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 vertebrae, 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. 137), by which they may or do articulate with the sternum. 
Such true ribs aje 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;" 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. 
uncus, a hook ; fig. 56, u) 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 
