AQUATIC MAMMALS 



likened to a pontoon bridge, for the body is supported in the water 

 throughout its entire length, and in theory each individual vertebra bears 

 its proportionate share of the load, although muscle stress introduces a 

 disturbing element. The curve of the vertebral column is always rather 

 gradual, but there must be some one point upon the arc from which the 

 column curves in either direction. In the anterior thorax as well as the 

 posterior neck the spinous processes of otariids are moderately high, 

 which is chiefly for supplying an efficient lever arm for the muscles rais- 

 ing the head and neck, but spinous height rapidly decreases toward the 

 posterior thorax and in the lumbar region, indicating that the lumbar 

 spinal musculature need not be very strong for such actions as the ani- 

 mal finds necessary. In phocids, on the other hand, the thoracic spines 

 are very low, with no better definition than the lateral processes. Their 

 height is slightly greater in the lumbar series, however, and here the cen- 

 tra are also more massive. 



As in all aquatic mammals the articulations of the pinniped vertebrae 

 are reduced, for as stresses are applied while swimming to a large ex- 

 tent throughout the columnar length rather than at particular points there 

 is little need for local strength, while there is increased need for flexibility 

 of the column. This is attained (especially in the Otariidae) by virtual 

 abandonment of an interlocking type of zygapophyses, and at the same 

 time reducing all other processes which might offer mechanical restric- 

 tion to limberness. Anapophyses are absent as such in the lumbar series. 

 In otariids (at least in Zalophus) the zygapophyses of one side are very 

 close to those of the other and in the lumbar region the articulations are 

 such as apparently to prevent any marked concavity in the outline of the 

 dorsal surface, but permitting an unusual amount of convexity. In a 

 seal (Phoca), relative to size, the zygapophyses of opposite sides are about 

 three times as far apart, and in the lumbar region the articulations are 

 such as to allow very definite concavity in the outline of the dorsal sur- 

 face. Especially is the latter the case between the lumbar and sacral ser- 

 ies, to the degree where the sacrum may be elevated to a quite surprising 

 extent. And although the cleaned skeleton does not show it there is pos- 

 sible a very remarkable amount of concavity in the posterior thoracic 

 and anterior lumbar regions of at least some phocids, for photographs of 

 Mirounga show that this animal can. bend the column to quite a right 

 angle at this point. Convexity of the lumbar series cannot be determined 

 from osteological examination, for this would depend mainly upon the 

 amount of play allowed by the zygapophyseal articulations. Presumably 

 the possible amount of convexity is not very great and somewhat less than 

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