148 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



tion as to how far the sternum of the amphibia is homologous 

 with the similarly named structure in the amniotes. 



In the amphibia the sternum arises as a pair of longitudinal 

 cartilaginous rods in the connectiAe tissue on the ventral sur- 

 face of the bod)-. These 

 rods soon unite, and form an 

 unpaired plate in the median 

 line between the origin of 

 the fore limbs. In the uro- 

 deles the sternum remains 

 as a small plate just behind 

 the ventral portion of the 

 shoulder girdle, but in the 

 anura it extends farther for- 

 ward. Its median portion is 

 caught between the epicora- 

 coids, and is reduced to a 

 ver)- slender thread ; but in 

 front of the girdle it expands 

 again in a plate, the so-called 

 omostemum. In the uro- 

 deles the sternum is cartilaginous ; but in the anura portions of 

 the omosternum, as well as of the posterior portion (termed 

 xiphisternum, a term adopted from human anatomy), become 

 ossified. The sternum is lacking in the footless amphibia. 



In the amniotes the sternum arises from the ventral ends of 

 the ribs. The distal ends of these become separated from the 

 rest, and unite to form a pair of ventral rods, which then unite 

 to form the unpaired structure, which in many forms shows 

 evidences of its origin from a series of elements, — stemebrae. 

 The sternum is lacking in snakes and turtles. In the dinosaur 

 Amphiccelias, it is said to have been paired in the adult, the two 

 halves possibly having been united by cartilage. In the lizards 

 it is usually a broad rhomboidal plate. In the birds but few 

 (at most eight) ribs contribute to the sternum, which is a broad 

 plate, and in the ordinary birds bears a strong keel or carina 

 upon its ventral surface. In the flightless birds the keel is 

 absent, and the presence or absence of keel was formerly em- 



FiG. 156. Sternum and ventral portion 

 of the shoulder girdle of Rana, after W'ieders- 

 heim. cl, claflcle ; co, coracoid ; ec, epicora- 

 coid ; g^ glenoid fossa ; os, omostemum ; s, 

 ventral part of scapula; st, sternum; x, 

 xiphisternum. 



