THE PECTORAL LIMB 



eral and latissimus are also of major import) are directly concerned with 

 flipper movement, while others have the function of adjusting the po- 

 sition of the shoulder itself. It is certain that these latter adjustments 

 are extremely frequent and extensive. The part which they play in actual 

 swimming is unknown, but during terrestrial activity the scapula slides 

 about beneath the skin in a quite surprizing fashion. The vertebral bor- 

 ders of the scapulae may appear to meet well above the back bone, or 

 they may be slid far ventrad. The pectoralis is of course largely re- 

 sponsible for the latter act, and the humerotrapezius is peculiarly fitted 

 for elevation of the scapula, inserting upon almost the entire length of 

 the humerus and with only incidental attachment to the spine of the 

 scapula. 



It is only from the pinnipeds, sirenians and cetaceans that we can hope 

 to learn anything regarding scapular tendencies in aquatic mammals. 

 Beyond any question this bone has undergone some degree of broaden- 

 ing in both sea-lions and whales. There may be considered to have been 

 some tendency in this direction in the manati, and in some seals, while 

 in the dugong and the majority of seals the scapula assumes a more falci- 

 form shape. It may safely be assumed that the variation in the general 

 shape of the scapula, including the position of the spine, is due entirely 

 to muscle stress. In Cetacea this bone is usually quite broadly fan- 

 shaped, more so in some and less in Physeter, in which the flipper may 

 be presumed to be less efficient as a rudder because of the unwieldly size 

 of the head. Invariably the infraspinous space occupies practically the 

 entire lateral aspect of the bone, while the bony area of the supraspinous 

 fossa is insignificant and perhaps but one-hundredth as large, or occa- 

 sionally it actually does not exist, as in Platanista (figure 36) and Megap- 

 tera. The supraspinous muscle is, then, of decreased importance. But 

 the inf raspinatus is not relatively larger so as to fill the infrapinous space. 

 On the contrary this muscle covers but a half or two thirds of the lat- 

 ter; but its details are variable. We must therefore seek other muscles 

 that have had need of a greater angle of leverage and have accordingly 

 stimulated the broadening of the scapula. Judging by conditions in 

 Monodon and Neomeris it is likely that in toothed whales this stimulus 

 has been supplied either by the subscapularis, which covers the entire 

 medial surface of the bone, or (and) the deltoid, which in the latter 

 genus arises from the entire, and in the former, almost the whole, verte- 

 bral border. Contributing to the situation may also have been a stimu- 

 lus for extension of the glenovertebral angle by the teres major and (or) 

 serratus magnus, which latter in Monodon is especially extensive. In at 



[215] 



