THE ANATOMY OF BIRDS. — OSTEOLOGY. 143 



body of a vertebra by its head or capitulum (Lat. diiniu. of caput, head), and also with the 

 lateral process of the same vertebra by its shoulder or tuhercnhun (Lat. diniin. <.f 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 (hfemapophysial) 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 Apteryj.; for 

 example, their pleurapophysial parts are expansive and plate-Uke. They length«ni 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 respiratcjry move- 

 ments of the chest ; all being in expiration more acute, and more obtuse in inspiration. 



The Avian Sternum (Gr. arepvov, sternon, the breast; fig. 56, S) is liighly 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 c)f heemal 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 fi-om 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 furaishes the 

 main point (Tappui of both the bony and muscular apparatus of flight, receiving important bones 

 of the scapular arch and gi^ang oi-igin to the immense pectoral muscles. (See also fig. 58.) 



Birds ofier tivo leading types of stenial structure, the ratite and the carinate, or the ' ' raft- 

 like" and the "boat-like", according as the bone is flat or keeled (Lat. rails, a raft; adj. 

 ratite ; in an arbitrary nom. pL, Ratitce, a name of one of the leading divisions of birds: Lat. car- 

 ina, a keel; adj. carinate: nom. pi. Carinatce, name of another such division). 1. In all stru- 

 thious birds, comprehending the ostrich and its allies (and also in the Cretaceous He^perornis), 

 the sternum is a flattish, or rather concavo-convex, buckler-like bone, of somewhat sciuarish 

 or rhomboidal shape, developed from a single pair of lateral centres of ossification, — a "fiat 

 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 CarinatcB, and also the Cretaceous Ichthyornis, — ^the sternum 

 is keeled and develops from a median centre of ossification as well as from lateral paired cen- 

 tres ; usually two of these, making five in all. In a few Carinatce the keel is rudimentary, as 

 the flightless ground parrot of New Zealand, Stringops liabroptilus ; or otherwise anomalous, 

 as in the extraordinary Opisthoconius cristatus, where it is cut a\A-ay in front, and in tlie rail- 

 like Notornis, where the sternum is extremely like a lizard's. In general, the development of 

 the Tceel is an index of wing-power, whether for flying or swimming, or both ; the eflectiveness 

 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 (^Cypselicke) and humming-birds 

 {Trochilidce) . 



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 mediiui and 

 single. The median ossification, which includes the keel, is the lophosteon(Gr . 'K6(f)os, /o////o.s-, 

 a crest; oareov, osteon, a bone). The anterior lateral piece, that with which the ribs, or some 



