56 



VERTEBRATE SKELETON 



rod, T-shaped, cruciform, rhomboidal, etc., in outline. It is sup- 

 posed to have arisen from structures hke the ventral scales of 

 Stegocephals and has been homologised with the entoplastron of 

 turtles. 



AVES. — The avian sternum (fig. 63) is usually more or less quad- 

 rangular or rhomboid in outline and is connected with more ribs 

 than in reptiles. In the flightless Ratites (fig. 63, A) its vaulted 

 lower surface is smooth and its margins are entire or with a pair of 

 short horns. In all other living birds (Carinates, B — E) a strong keel 

 (carino) is developed on its ventral side, increasing the surface for 



Fig. 63. — Sterna of birds (Selenka, '91, and Shufeldt, '82, '09). A, Struthio; 

 B, Circus; C, Cathartes aura; D, Eremophila alpestris; E, Centrocercus young, showing 

 developing parts and processes, c, coracoid; /, lophosteon; p, pleurosteon; m, met- 

 osteon; s, scapula. 



attachment of the muscles of flight. Many have an entire sternum, 

 but frequently there are fenestra?, or these may extend to the margin, 

 causing embayments bordered by horns of varying length, features 

 of little morphological importance. 



The sternum arises, so far as known, by a pair of procartilages 

 between the ribs, these soon fusing in the median fine. Ossification 

 proceeds from two to five centres, and in the adult practically the 

 whole is ossified. There is nothing to show that the horns separating 

 the gaps in the postero-lateral border are xiphisternal in character 

 as is often assumed. 



MAMMALIA. — In this class the sternum is usually elongate and 

 much narrower than in reptiles or birds, and generally the number 

 of ribs which reach it is greater than in those classes. The develop- 

 ment has been outlined above (p. 52). It has a weak carina in 

 bats and some fossorial forms and only in Monotremes (fig. 274) and 

 tapirs does the presternum persist as a distinct structure, although 

 when the manubrium extends in front of the first pair of ribs this 



