AQUATIC MAMMALS 



abrupt transition in the thickness of the centra between the thoracic and 

 cervical series. The proper interpretation of these facts seems to be not 

 merely that the cetacean neck has become markedly shortened, but that all 

 vertebrae anterior to the thoracic pivot of motion have experienced strong 

 stimuli for shortening, to which they have responded to various degrees 

 according to their capabilities, this shortening having been necessary in 

 order that the animal might assume the fusiform shape best fitted for 

 swimming. 



Winge (1921) has stated that the abandonment by the fore limbs of 

 the function of supporting the body results in the reduction of the height 

 of the anterior thoracic spines. This character, however, is only secondar- 

 ily dependent upon function of the anterior limb. If in quadrupeds the 

 fore limb be of greater importance during locomotion than the hind, then 

 not only must the spinal musculature have corresponding strength in this 

 region but the spines must be higher to give better leverage for muscles 

 that elevate a heavier head and neck (as in Bison) (Thompson, 1917). 

 When the fore limb is no longer used for support the anterior part of the 

 erector spinae usually becomes weaker (phocids, bats, etc.) , or it may un- 

 dergo an independent modification for increased strength, resulting in 

 spinous processes that are phenomenally developed, as in most Cetacea. 

 The height, and to a lesser extent the character, of these varies consider- 

 ably, however, reflecting corresponding variation in the spinal muscula- 

 ture. In Aiesoplodon the height of the spines of the entire column is quite 

 phenomenal. In mounted skeletons of Delphinus and Lagenorhynchus 

 the spines are relatively broad sagittally and so close together that it is 

 difficult to understand how the requisite amount of movement would be 

 possible; but perhaps in life the intervertebral disks are unusually thick. 

 In Globwcephala, on the other hand, there is sufficient distance between 

 the spines for great mobility of the column. Winge (1921) has said 

 that "bending of the column in the vertical plane — is reduced or aban- 

 doned." This is an extraordinary statement in view of the fact that all 

 the modifications of the vertebrae with the possible exception of spinous 

 height are for increase of mobility and all locomotor movements in whales 

 are instigated by motion of the column in this plane. 



In some respects the lumbar region of Cetacea really constitutes a part 

 of the tail, from a functional standpoint, but certain of its characteristics 

 may be discussed in the present chapter. Flower and Lydekker (1891) 

 have stated that Neobalaena and Inia have but three lumbar vertebrae. At 

 the other extreme Grampus may have 21, Lagenorhynchus, 23, and Del- 

 phinus, 24 (21 in another skeleton). This of course, is figuring on the 



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