RESPIRATION 



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consequence of the dorsal position of the lungs. Particularly in Mysticetes, 

 the diaphragm is far more horizontally placed than in terrestrial mammals, 

 and the same is also true of sea-cows. All these factors cause the surface 

 of the lung facing the diaphragm to be much larger than it is in terrestrial 

 mammals, and the diaphragm to play a more important part in breathing 

 than the ribs. This is also borne out by the fact that the diaphragm is 

 relatively large and consists almost exclusively of muscle fibre, a central 

 tendon being absent or rare. 



The exceptional development of the diaphragm, its predominant role 

 in breathing, and the dorsal position of the lungs have undoubtedly had an 

 important effect on the structure of the ribs. In Cetaceans, the number 

 of true ribs and false ribs, i.e. those attached to the sternum, is small, 

 while the number of floating ribs is comparatively large. In dolphins, 

 the number of true ribs is 8 to 1 1 out of a total of 1 1 to 1 5, and the last or 

 the last two ribs are frequently not even attached to the vertebrae. 

 Moreover, in dolphins, as in Archaeocetes, the costal cartilages are 

 ossified, but in Sperm Whales and Ziphiids they are cartilaginous and are, 

 moreover, very short, amounting to between 3 and 5 only. In Mysticetes, 

 they are completely absent, all ribs being floating ribs, with the exception 

 of the first which is attached by ligaments to a much-reduced sternum 

 (Fig. 21). In terrestrial mammals, the ribs usually articulate with the 

 vertebrae by capitular and tubercular attachments (heads), and while 

 10 of the 13 ribs of the oldest Archaeocetes were two-headed, only 7 are 

 so in their youngest representatives. The oldest fossil Mysticetes still had 

 8 two-headed ribs, but extant species have no more than 3, and the Little 



Figure 81. Thoracic vertebra of a Bottlenose Dolphin with {left) a one-headed and (right) 



a two-headed rib. 



