AQUATIC MAMMALS 



huge, occupying practically the whole of the supraoccipital region, while 

 the insertion of the posterior rectus capitis was reduced and shifted 

 largely lateral to the condyle so as to have almost no function in sagittal 

 movements. In Neonieris I found the posterior rectus to be much larger, 

 the attachment covering an area perhaps one-third of that occupied by 

 the correspondingly reduced semispinalis, which was double; while in 

 Monodon the rectus was enormous and the double attachment of the long 

 back musculature very thin; and Murie (1873) showed that the same 

 condition obtains in Globiocephala. Obviously the rectus aids more in 

 static than in active strength, while the longer muscles to the occiput 

 are of chief use in active movements. The former does not need an oc- 

 cipital with a marked slope, while for proper efficiency in the case of 

 a mammal holding its head parallel with the body axis, the latter does. 

 This will be discussed further. 



It will thus be seen that there may have been, and doubtless were, more 

 than one stimulus for a given development. These can be discussed, 

 but we cannot tell which were the stronger or, indeed, if some have not 

 been counteracted by antagonistic influences. 



We have no means of knowing the static or tensile strength of the 

 cetacean occipital musculature. The term strength is but relative and 

 indefinite at best. Certainly in this order the unbroken body contour 

 in the cervical region means that the underlying muscles are very robust, 

 but in Neomeris, for instance, although the complexus was extremely 

 heavy at the base of the neck, it was very thin indeed at its broad attach- 

 ment to the skull. Of course a porpoise with skull of moderate size may 

 toss the head for a few inches with some show of force, and it is likely 

 that relatively prodigious power would be needed in order to twist the 

 creatures head far to the side in opposition to its wish, but nevertheless 

 cetaceans with large heads cannot even lift these from the ground if they 

 become stranded. The cetacean's head can be but little heavier than the 

 water it displaces, so levitation is almost entirely by flotation. Further- 

 more it is inconceivable that a whale while swimming does not adjust 

 the position of the head so that it is in equilibrium, balanced so that 

 there is no aquatic force tending to twist the head below or to one side. 

 Hence, leaving out of consideration the fact that the side muscles are 

 employed only occasionally while the dorsal ones are in use almost con- 

 stantly, the supraoccipital muscles need not be any stronger than those 

 of the side of the neck, unless complications be introduced by the flatten- 

 ing of the whole head. 



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