THE PECTORAL LIMB 



merely believed to be the most efficient ones by means of which their best 

 speed is attained. 



This outhne of the known factors responsible for fore limb conforma- 

 tion and action in highly specialized aquatic mammals has been given 

 first in order that the reader may have a better understanding of what 

 follows. The details to be discussed in the present chapter have such 

 great interdependence that it is difficult to arrange the subjects properly 

 and to avoid some repetition. 



Clavicular conditions vary greatly among all sorts of mammals and it 

 is not easy to distinguish just the critical factor that determines the 

 presence or absence of this bone. In general it may be said to be lack- 

 ing in those mammals which use the pectoral limb for support only, 

 and present in those which are in the habit of using the hands for grasp- 

 ing; but there are many exceptions, to the latter statement especially. The 

 clavicle should be considered not in the nature of a strengthening mem- 

 ber, but rather as a strut to prevent an undesirable degree of adduction of 

 the shoulder. In heavy mammals that bound about, landing solidly upon 

 the fore legs, it would be in danger of breakage, and is accordingly ab- 

 sent; nor does there seem to be much need for it in the case of the more 

 narrow-chested mammals. 



The clavicle is lacking from all the more highly specialized aquatic 

 mammals — pinnipeds, sirenians and cetaceans — it is functionally ab- 

 sent in the carnivores, usually present in rodents but absent in a number 

 of heterogeneous sorts (including the capybara) , and present in all in- 

 sectivores except Potomogale, and presumably Limnogale. The latter 

 is really the only significant fact. For all we know the terrestrial an- 

 cestors of pinnipeds, sirenians and cetaceans may have lacked a clavicle. 

 It seems reasonable to suppose, however, that the way in which whales 

 use the flippers would introduce a stimulus for the elimination of the 

 clavicle were one present. On the other hand, one would surmise that 

 the otariids might find a clavicle of advantage to the way in which they 

 constantly adduct the flippers during swimming. 



It is perhaps unsafe to attempt to analyze the muscular factors under- 

 lying the attachment of the shoulder to the body of aquatic mammals, 

 but existing conditions may be mentioned and a few possibilities ad- 

 vanced. 



A trapezius is lacking in whales but is present in both pinnipeds and 



sirenians, and it seems fully as likely that this muscle has always been 



absent from the cetacean stock as that it has been eliminated by aquatic 



habits. It is not known what scapular motions a whale finds of advan- 



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