AQUATIC MAMMALS 



cat. 4 There is a complex of interacting conditions involved in this situa- 

 tion, some constituting cause and some effect. As the heel is bound down 

 to the body a long femur is not desirable unless really needed for other 

 purposes, and this is not the case, for the binding down of the leg pre- 

 vents great movement of the thigh from serving any useful purpose. 

 Furthermore, a short femur can be held abducted without projecting 

 from the side to an undesirable extent. Especially is this true of flexion, 

 and atrophy of the flexor muscles accompanies the shortening of the 

 ilium and the femur. With the flexors serving little useful purpose there 

 can be but slight need for muscles accomplishing only extension. Ac- 

 cordingly those extensors which usually insert near the knee have shifted 

 their insertions to other situations and because of the increase in post- 

 acetabular length of the innominate, the origins of the adductores, and 

 of the obturator externus, have shifted so as more effectively to accom- 

 plish what extension of the femur is needful, in addition to the regular 

 adduction and rotation. Because the actions involved in swimming by 

 the seal comprise relatively simplified but constantly repeated motions 

 there is apparently need only for simplified adductors, and these are re- 

 duced to two, whose position in respect to the external obturator is so 

 intimate that the three have previously been mistaken for one single 

 obturator. Probably for the reason that the sea-lion has need for very 

 intricate, though slight, movements of adjustment for steering, the ab- 

 ductores now occur in six slips, and the sartorius is double. As a re- 

 sult of the above conditions the arc of movement of the pinniped femur 

 in those specimens dissected seemed to be only through about 25 or 30 

 degrees. 



In this order the femur is relatively very broad transversely and nar- 

 row in the sagittal plane. The breadth may be attributable to the fact 

 that good leverage is needed through a greater trochanter which ex- 

 tends well away from the head, and a broad distal end for articulation 

 with the shank. Its thinness is permitted on the one hand by the fact 

 that little mechanical strength in flexion and extension is needed, and 

 promoted because a flat femur can accomplish a sharper angle of flexion 

 with the shank. The latter point appears to be of lesser consideration 

 in the seals. 



*I regret to say that in the last paragraph of page 124, Howell, 1929, an 

 error was made in stating the percentage of femoral length in relation to body 

 length, and the figures 22 and 29 as given are really those for the tibia. They 

 should read 11 and 12 respectively, as was stated on page 36. 



[294] 



