AQUATIC MAMMALS 



ossification centers, or, more likely, the two centers of the manubrium 

 were first separated by an antero-medial indentation, then becoming atro- 

 phied, and the first sterneber finally became hypertrophied to take the 

 place of the true manubrium. This point should be more thoroughly 

 investigated when adequate material is available. 



In mysticetes the sternum is still further reduced to a single bone, evi- 

 dently the manubrium, to which is attached but the first pair of ribs. It 

 is frequently stated that this bone is of some particular shape in one spe- 

 cies and of another shape in a second, but as a matter of fact the shape 

 appears to be almost as variable individually as specifically. It is prob- 

 ably always more or less roughly triangular or heart-shaped in the ba- 

 laenid whales, while in the others it may be cither U-shaped, T-shaped, 

 cross-shaped or rather irregular. Usually, however, there is a postero- 

 medial projection forming with the lateral process a concave border into 

 which fits the end of the first rib. Certainly in these whales the sternum 

 has suffered as much reduction as is possible without a change in the at- 

 tachments of (chiefly) the smaller, more fleshy pectoral (when there are 

 two divisions) and of the sterno-mastoid. 



Winge (1921) stated that when there is a tendency for the costal car- 

 tilages to disappear the sternum loses an essential stimulus and becomes 

 reduced. This is self-evident, whether it be considered that the sternal 

 elements are morphologically derived from the costae, or (according to 

 a later thesis) , that the derivation of this complex was originally from 

 the medioventral part of the shoulder girdle. Schulte (1916) ascribed 

 to J. C. Vaughan the verbal opinion that in descending to great depths 

 the pressure of the water on the abdomen would force rostrad the dia- 

 phragm, which would in turn force the ribs out, accounting for the re- 

 duction of rib connections and of the sternum. Muller (1898) believed 

 that the reason the mysticcte sternum is more reduced than in odontocetes 

 is that the diaphragm of the former is less muscular, indicating a greater 

 use of the thorax in breathing. 



The sternum as a whole can hardly suflFer reduction in the number of 

 its elements after the fashion characteristic in Cetacea without corres- 

 ponding reduction of the costal cartilages. That the latter is not neces- 

 sarily dependent upon decrease in strength of the costal-vertebral arti- 

 culations is suggested by the fact that in the manati these articulations 

 arc strengthened, while the sternal ribs are considerably reduced. While 

 not absolutely dependent one on the other it is but logical to infer that 

 increase in the elasticity of the dorsal thorax has usually gone hand in 

 hand with the same adaptation ventrad. Then there is the matter of the 

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