INTRODUCTORY. POSITIONS OF THE LIMBS. 29 



inwards. In the Walrus and, to a certain extent, in the Sea- 

 lions the anterior limb remains throughout life in this position. 

 The posterior limb is also rotated, but the rotation in this 

 case takes place forwards, so that the thigh lies nearly parallel 

 to the body, the knee-joint pointing forwards ; the crus down- 

 wards and the pes forwards. The pre-axial surface of the 

 whole limb with the tibia and hallux looks towards the middle 

 of the body, the postaxial surface with the fibula and fifth toe 

 looks outwards. This is the position in which the hind limb 

 is carried in nearly all mammals. 



In nearly all mammals a further change takes place in the 

 position of the anterior limb. The radius and ulna have 

 hitherto been parallel to one another, but now the lower end 

 of the radius, carrying with it the manus, comes to be rotated 

 forwards round the ulna, so that the manus, as well as the 

 pes, comes to be forwardly-directed, and its pre-axial surface 

 faces inwards. 



In the majority of mammals the radius and ulna are per- 

 manently fixed in this, which is known as the prone position, 

 but in man and some other mammals the manus can be 

 pronated or turned into this position at will. When the 

 radius and ulna are parallel throughout their whole length 

 the manus is said to be in the supine position. 



The extensor side of a limb is that to which the muscles 

 which straighten it are attached, the flexor side is that to 

 which the muscles which bend it are attached. 



