AQUATIC MAMMALS 



The pinniped shank is considerably shortened (length of tibia being 

 22 per cent of the body length in Zalophus, 29 in Phoca, and 36 per cent 

 in a cat) , and because it is so bound down by muscles and other tissue 

 its possible movement, judging by the specimens dissected, seems to be 

 through an arc of only about 15 degrees. In this order the fibular head 

 really is not concerned with the knee joint and its conformation has been 

 affected by the posture habitually assumed. In the Phoca studied the 

 angle which the femur formed with the shank was a trifle more than 90 

 degrees, while in the Zalophus it was considerably less — apparently about 

 45. Accordingly in the former animal the fibular head slopes moder- 

 ately, and in the latter quite steeply, enabling consequent sharper bend- 

 ing of the knee without mechanical interference. But there is some 

 slight generic and even specific variation in this detail. 



Upon examining the distal ends of the shank bones (in fig. 13 of 

 Howell, 1929) it will be found that there are no deep grooves in Zalo- 

 phus, but several in Phoca. Upon the lateral aspect of the latter there 

 is a deep one for the tibialis anticus and another for the peroneus 

 longus ; upon the medial aspect there are three, for the posterior tibialis, 

 flexor digitorum longus, and peroneus digiti quinti. As in the case of 

 the radius these grooves have doubtless resulted from the constantly re- 

 peated oscillations of the feet, made during swimming. 



The significance of the absence of the soleus in Phoca and of the lat- 

 eral gastrocnemius in Zalophus is unknown. In both, the plantaris does 

 not join the tendo calcaneus but passes mediad thereto. In the Phoca, 

 but not the Zalophus, the tendon of the strong flexor hallucis longus 

 passes over a posterior extension of the astragalus, as already described, 

 and it is the tension of this tendon alone which prevents the foot from 

 assuming an angle of 90 degrees with the shank. But this does not 

 necessarily mean that if it were not for the tension of this muscle the seal 

 could walk like a sea-lion. To do this it would have to bend the verte- 

 bral column so that the sacrum is vertical to the ground, and it might 

 find such a posture inconvenient, if not actually impossible of assump- 

 tion. 



In bony details the foot of the sea-lion is of especial interest in the 

 present connection in having the first and fifth toes approximately equal 

 in length and longer than the middle three, as in the seal, and the bony 

 elements are definitely flattened, especially the hallux. The question of 

 epiphyses has already been discussed in chapter ten, for the situation in 

 this respect in the manus applies equally, with minor variations, to the 

 pes. 



[296] ■ 



