THE TRUNK 



As the above is the case one might expect to find a somewhat similar 

 situation in the dorsal muscles, and this is so to a modified degree. In 

 the seal the spinal musculature is the chief agent in lateral movements 

 of the posterior end. This has phenomenal breadth, as well as consid- 

 erable thickness, in the lumbar region. The sublumbar musculature is 

 well, although not remarkably, developed, but the iliocostalis and longis- 

 simus have expanded and where the latter is attached to the ilium this 

 bone has turned laterad to provide greater accommodation for the mus- 

 cular connection. As the iliocostal is the most lateral division of the 

 erector spinae it naturally is of most consequence in the lateral move- 

 ments employed for swimming. It might very well be expected to main- 

 tain its robustness as far as the occiput, but this it does not do. As it con- 

 tinues onto the thorax from the lumbar region it gradually thins and vir- 

 tually disappears over the anterior thorax. The function of the iliocos- 

 talis is therefore almost exclusively for operating the hinder end in swim- 

 ming. The forward end of the animal must consequently be controlled 

 by a different group of dorsal muscles. As with the more ventral an- 

 terior muscles concerned in swimming, this is accomplished partly by 

 muscles extending from the arm to the head or neck. The splenius, 

 humerotrapezoid, cephalohumeral and both atlantoscapulars are better 

 situated in the seal than the sea-lion for purely lateral motion of the 

 head, and for antagonism the spinotrapezoid projects farther back in the 

 former, while the latissimus is more extensive and is double. The con- 

 formation of the latter indicates that it might be of distinct aid in side- 

 wise curving of the hinder end, but it seems probable that the chief 

 stimulus for its development was to act in antagonism to prebrachial mus- 

 culature. 



By the above statements it is not meant to imply that only those muscles 

 mentioned are used by the seal in swimming. There are present ex- 

 tremely heavy subvertebral cervical muscles which I suspect are used in 

 antagonism to translate into lateral movement the action of the semi- 

 spinalis capitis, which otherwise would largely result in raising the head. 

 Similarly almost every muscle of the body and neck should have some use 

 in swimming, but the actions of many of them in this connection are ob- 

 scure. 



The sirenian skeleton is remarkably heavy and dense, especially in the 

 manati. This is popularly believed to be for the purpose of enabling 

 these animals more readily to sink from the surface to their pastures upon 

 the bottom. This may be the proper explanation but it should not be ac- 

 cepted without considerable reservation. Sirenians descend to very mod- 



[165] 



