gi2 STERNUM 
generally of importance, and it may be pointed or truncated or bifid. 
Some of the more important modifications can be formulated thus :— 
A spina interna is present, although very small, in Pteroclidx, 
Columb and Trochili. 
Both a spina interna and a spina eaterna occur, fusing to form a 
spina communis, which in most cases contains a cavity into which the 
inner corners of the Coracoids fit, in Gallinx, Meropidx, Bucerotide, 
Upupidx and Cypselidx. 
A spina externa only, which is (1) very short, slightly U-shaped 
at its free end in Colymbi, Podicipedes, Tubinares, Cicontidx, Plataleine 
(partly), Palamedex and Accipitres; (2) Y-shaped in Steganopodes 
(mostly), Phenicopterus, Coracudx, Alcedinide, Todidx, Trogonidz, 
Galbulidx, Pici, Menura and Passeres generally ; or (3) L-shaped in 
Sphenisci, Ardeidx, Scopus, Plataleine (partly), -Anseres, some 
Cuculidx, Eurylemide and Lhamphastide. 
No spina externa in some Steganopodes, Crypturi, Turniz, some 
Rallidx, Otidide, Columbx (partly) and Caprimulgide. It would, 
however, be a great mistake to suppose that the presence, absence 
or form of the spina sterni is enough to determine the systematic 
position of a Bird. The Breastbone taken as a whole, regard being 
paid more to the anterior than to the posterior portion, no doubt 
affords excellent taxonomic help ; but it is obvious that its numerous 
processes and ridges are only the result of special requirements. 
Thus the number of Ribs naturally affects the length of the side 
of the Sternum, while the development of the muscles is inti- 
mately connected with the various irregularities of its surface. 
It is easy to make sweeping generalizations based upon a few 
evident facts, but such generalizations become hard when put to 
the crucial test of extended research. As an instance may be cited 
the anterior lateral processes which are scarcely developed in 
Dinornis, Gus, Psophia, Cathartes and Vultur, while they attain a 
great size in Dromxus, Apteryx, Aptenodytes, Botaurus, Rallus, Cryptura, 
Galline, Cuculidx, Todus, Merops, Upupa, Buceros, Colius, Pict, 
Atrichornis, Menura and the Passeres. As a rule these processes 
seem to be smallest in the Birds which are capable of long enduring 
flight, and largest in those not remarkable for that power. They 
are obviously most intimately correlated with the development of 
the sternocoracoid muscles (cf. p. 605) which arise from them 
and are inserted on the basal portion of the Coracoids, acting 
as levatores of the Ribs, aind therefore aiding respiration ; but what 
really determines the numerous modifications of these muscles 
we do not know. Birds which fly well may in a general way 
be said to have the Breastbone more consolidated than those which 
fly badly. A good instance of this is shewn by the Tubinares with 
their generally enormous wing-area, and above all by Fregata, which 
have a very short Sternum, while this is much longer in proportion 
2 (and oe <2- 
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