THE TRUNK 



resting, and wriggling along the surface, so there is at such times actually 

 as much force pressing upward against the strenum as downward. Pre- 

 sumably these factors have been of importance in shaping the thoracic 

 cavity. There are other elements, however, which further modify the 

 shape of the trunk. Because the spinal musculature has become greatly 

 broadened for the purpose of lateral movements of the hinder end and 

 because there is no need for much movement in the sagittal plane, the 

 spinous processes of the anterior thorax are very low and the musculature 

 concerned very thin. This further reduces the sagittal dimension of the 

 trunk. The transverse dimension is increased by the fact that the lateral 

 muscles of both trunk and neck need as great a lever arm as possible 

 more effectively to accomplish the lateral movements used in swimming. 

 The muscles not only are very robust but they are overlain by a thick layer 

 of fat, and between them and in all interstices there is much connective 

 tissue surrounding networks of blood vessels. The broad thorax is of 

 distinct advantage to the animal in swimming. Whether flattening in 

 the sagittal plane actually increases the swimming ability of the seal or 

 whether, not of advantage, it is yet of insufficient disadvantage to be of 

 consequence, is unknown. 



In comparison with a typical terrestrial carnivore the chief character- 

 istics of the vertebral column of the Pinnipedia are the looseness of the 

 articulations, the elasticity of the intervertebral disks, the latter being 

 difficult to investigate after death, and the fact that the spinous processes 

 exhibit no definite change in slope, or anticline. The latter is a character 

 shared by all essentially aquatic mammals. The usual quadruped has a 

 definite center of motion in the vertebral column, which it would be more 

 logical to call the center of suspension for the reason that the back-bone 

 of a quadruped may with propriety be compared to a double-pier, canti- 

 lever bridge. From the above center the anterior spinal and some of the 

 shoulder muscles operate in one direction and the posterior spinal and 

 some of the pelvic muscles in the other, resulting in a backward slope 

 of the spines in the anterior thorax and their forward inclination in the 

 extreme posterior thorax and lumbar region. And there is usually a 

 rather abrupt alteration in the character of the spines where the slope 

 changes. The position of this center depends upon the stress encoun- 

 tered, as pointed out by D'Arcy Thompson (1917) . If the chief weight 

 is borne by the fore legs the center will be farther forward, and if the 

 hind limbs bear all the burden, as in the kangaroo, then the center will 

 be shifted far to the rear. In a completely aquatic mammal the skeleton 

 has no resemblance to a double-pier, cantilever bridge, but may better be 



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