AQUATIC MAMMALS 



function of the ventral thorax as a scaffold for muscular attachments. 

 The costal cartilages cannot be reduced unless conditions permit of al- 

 teration in the costal attachments of the transversalis abdominis, and in 

 the more ventral of the intercostal muscles. The sternum cannot be re- 

 duced without alteration in certain aspects of the rectus abdominis at- 

 tachments and reduction in origin of the pectoralis. Chief of these are 

 the latter two. The major sternal attachment of the rectus may easily 

 shift forward anyway, and an extensive origin of the ectal pectoral may 

 not greatly involve the sternum proper but only the linea alba. Where 

 there is a heavy anterior pectoral mass attached broadly to the bone, 

 which is the usual character of the minor division when this is present, 

 or of the major if this be greatly reduced, this affects the sternum mainly 

 in a broadening of the manubrial part. Hence it appears likely that mus- 

 cular adjustments permitting the shortening of the sternum are easily 

 made. In this connection there should also be mentioned the possibility 

 that the extraordinarily developed gular musculature of mysticetes may 

 well have been instrumental in reducing the sternum in this group. 



In fine, all we can logically infer regarding the reduction of the ster- 

 num in the Cetacea may be summed up in a single paragraph. There is 

 lacking the brachial and muscular stimulus for a sternum of moderate 

 size. Vaughan's opinion that water pressure at great depths would force 

 the diaphragm forward hardly seems well taken. Such pressure is na- 

 turally the same over all parts of the animal at any given depth and we 

 know that at great depths external and internal pressure must be equal- 

 ized, for no thorax could otherwise withstand a pressure in excess of a 

 ton to the square inch. It therefore seems logical to infer that the in- 

 creased elasticity dorsad and the reduction of the sternum in this order is 

 to allow for the amount of compressibility of the thorax needed to pre- 

 vent water pressure from cracking the ribs. 



The vertebral column of the Cetacea is noteworthy for the very marked 

 reduction (least pronounced in Platanista), which really amounts to en- 

 tire abandonment, of interlocking or articulation of the vertebrae, and 

 increase in size of the intervertebral disks, both of which are modifica- 

 tions to increase the uniformity with which the column may be curved 

 without tendency to bend at one or more particular points. It is per- 

 mitted by the fact that the body has been supported by flotation only for 

 a very long period of time and, incidentally, is a step in retroversion to- 

 ward the primitive chordate condition. As support is by flotation acting 

 upon all parts of the body it might be expected that the static position as- 

 sumed by the vertebral column would be a straight line. The column, 



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