THE ANATOMY OF BIRDS.— OSTEOLOGY. 143 
body of a vertebra by its head or capitulum (Lat. dimin. of caput, head), and also with the 
lateral process of the same vertebra by its shoulder or tuberculum (Lat. dimin. of tuber, a 
swelling). In well-marked cases, the head and shoulder are quite far apart, the rib seeming 
prolonged above; either of these vertebral connections may be disestablished, the other re- 
maining, or both may be lost. The lower (hemapophysial) part of a rib, or ‘sternal rib,” 
articulates with the side of the sternum by a simple enlargement; the ends of those sternal ribs 
which thus join the sternum tend to cluster closely together at a part of the breast-bone called 
its costal process (fig. 58) ; those which do not make the sternal connection are simply bundled 
together. Commonly five or six, sometimes four, rarely only three ribs reach the sternum. 
The ribs are ordinarily as slender and strict as those shown in fig. 56; but in Apteryx, for 
example, their pleurapophysial parts are expansive and plate-like. They lengthen rapidly 
from before backward, both in their vertebral and their sternal moieties; these parts meet at 
angles of decreasing acuteness from before backward; but these angles, as those of the ribs 
both with vertebrae and sternum, incessantly increase and diminish in the respiratory move- 
ments of the chest; all being in expiration more acute, and more obtuse in inspiration. 
The Avian Sternum (Gr. otépvov, sternon, the breast; fig. 56, S) is highly specialized ; 
its extensive development is peculiar to the class of Birds, and its modifications are of more 
importance in classification than those of any other single bone. Thereupon it becomes an 
interesting object. Theoretically it is a collection of hemal spines of vertebrae. Though 
such morphological character is appreciable in those animals which have a long jointed ster- 
num, the segments of which, answering to pairs of ribs, develop from separate centres, there 
is little or nothing in the development or physical characters of the avian sternum to favor 
this view. The great bone floors the chest and more or less of the belly, and furnishes the 
main point @appui of both the bony and museular apparatus of flight, receiving important bones 
of the scapular arch and giving origin to the immense pectoral muscles. (See also fig. 58.) 
Birds offer two leading types of sternal structure, the ratite and the carinate, or the “ raft- 
like” and the ‘‘boat-like”, according as the bone is flat or keeled (Lat. ratis, a raft; adj. 
ratite ; in an arbitrary nom. pl., Ratite, a name of one of the leading divisions of birds: Lat. car- 
ina, a keel; adj. carinate: nom. pl. Carinate, name of another such division). 1. In all stru- 
thious birds, comprehending the ostrich and its allies (and also in the Cretaceous Hesperornis), 
the sternum is a flattish, or rather concavo-convex, buckler-like bone, of somewhat squarish 
or rhomboidal shape, developed from a single pair of lateral centres of ossification, —a “flat 
boat, ” without any keel, built with reference to an important modification of the shoulder-gir- 
dle, and a reduced or rudimentary condition of the wings, which are unfit for flight. 2. In all 
flying birds, and some which from other than any fault of the sternum do not fly, —comprising 
all remaining recent birds, or Carinat@, and also the Cretaceous Ichthyornis, —the sternum 
is keeled and develops from a median ceutre of ossification as well as from lateral paired cen- 
tres; usually two of these, making five in all. In a few Carmate the keel is rudimentary, as 
the flightless ground parrot of New Zealand, Stringops habroptilus ; or otherwise anomalous, 
as in the extraordinary Opisthocomus cristatus, where it is cut away in front, and in the rail- 
like Notornis, where the sternum is extremely like a lizard’s. In general, the development of 
the keel is an index of wing-power, whether for flying or swimming, or both; the effectiveness 
of the pectoral muscles being rather in proportion to depth of keel than to extent of the sides 
of the ‘‘ boat-bone;” thus, the keel is enormous in swifts (Cypselid@) and humming-birds 
(Trochilide). 
The carinate sternum normally develops from five centres, having consequently as many 
separate pieces in early life. Two of these are lateral and in pairs; the third is median and 
single. The median ossification, which includes the keel, is the lophosteon(Gr. Addos, lophos, 
a crest; doreov, osteon, a bone). The anterior lateral piece, that with which the ribs, or some 
