912 



STERNUM 



generally of importance, and it may be pointed or truncated or bifid. 

 Some of the more important modifications can be formulated thus : — 



A s'pina interna is present, although very small, in PterocUdee, 

 Columbx and Trochili. 



Both a spina interna and a spina externa occur, fusing to form a 

 spina communis, which in most cases contains a cavity into which the 

 inner corners of the Coracoids fit, in Gallinse, Meropidse, Bucerotidse, 

 Upupidx and Cijpselidae. 



A spina externa only, which is (1) very short, slightly U-shaped 

 at its free end in Colymbi, Podicipedes, Tubinares, Ciconiidse, Plataleime 

 (partly), Palamedea3 and Accipitres ; (2) Y-shaped in Steganopodes 

 (mostly), Phoenicopterus, Coraciidm, Alcedinidae, Todidse, Troganidx, 

 Galbulidx, Pici, Menura and Passeres generally ; or (3) X-shaped in 

 Sphenisci, Ardeidse, Scopus, Plataleinse (partly), Anseres, some 

 Cucnlidx, Eurylxmidge and Rhamphastidse. 



No spina externa in some Steganopodes, Crypturi, Turniz, some 

 Piallidx, Otididse, Colwnbse (partly) and Caprimulgidse. It would, 

 howcA'-er, 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 afi'ects 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, Grus, Psophia, Cathartes and Vulttir, while they attain a 

 great size in Dromseus, Apteryx, Aptenodytes, Botaurus, Rallus, Crypturi, 

 Gallinse, Cuculidse, Todus, Merops, Upupa, Buceros, Colius, Pici, 

 Atrichornis, Menura and the Passeres. As a rule these processes 

 seem to be smallest in the Bix'ds 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, and 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 



