THE TRUNK 



capitulum and centrum, a process representing the former undoubtedly 

 indicates that there is strong hgamentous connection. 



Accompanying the increased elasticity of the dorsal thorax is a com- 

 parable development of the ventral thorax brought about by a reduction in 

 the costal attachments to the sternum. This is attained partially by an 

 increase in the number of the so-called floating ribs through the elimina- 

 tion of the cartilages which more usually are attached to the series of 

 costae often but unfortunately termed false ribs, and by a reduction in 

 number of the true sternal or cartilaginous ribs, which in odontocetes 

 seem invariably to become calcified with age. Reduction of the sternal 

 ribs is inevitably accompanied by a corresponding reduction in the length 

 of the sternum. Atrophy of the latter complex does not follow any one 

 rule but may be attained by elimination of any of the posterior elements. 

 The xiphoid and posterior sternebers may disappear, but whales ap- 

 parently never show the condition encountered in Halicore, in which the 

 sternebral series has become so reduced that this has no individual cen- 

 ters of ossification (apparently), several costal cartilages thus having at- 

 tachment to the xiphomanubrial interval, although this later experiences 

 general calcification. 



That a reduction of the cetacean sternum is now in progress is indicated 

 by the statement of Beddard (1900) that in Phocaeua it shrinks progres- 

 sively from the young to the adult state. Among Odontoceti there may 

 be seven pairs of ribs with sternal attachment (Betaidius), six pairs 

 ( Monodon), or the sternum may be much more reduced, the elements 

 finally fusing in the adult, and resulting in a single bone that may be 

 broader than long (as Neomeris). Even when the sternum consists of 

 several elements complete ossification of the whole often occurs in old 

 age. In all porpoises without exception, I believe, the manubrium is 

 broader than any other sternal element, is indented antero-medially, and 

 produced in a process antero-laterally (chiefly for the sternohyoid mus- 

 cle) , with a lateral expansion for the pectoraHs minor attachment. The 

 sternum of Physeter is of peculiar form and Flower (1876) considered 

 that it consists of a manubrium and two sternebrae ; but Owen stated that 

 it has four elements. In all odontocetes at least the ossification of the 

 twe centers of each sterneber is delayed. Occasionally (as in Monodon, 

 figure 31) two pairs of ribs have attachments well forward upon the first 

 sternal element and there thus seems to be some question regarding the 

 homology of the latter. Either all or most of the ribs have shifted for- 

 ward, thus placing the second pair upon the manubrium, the latter and 

 first sternebra have experienced first crowding and then fusion of their 



[173] 



