The Skeletal System. 



33 



ence of the bone surfaces. This may be estimated either from the 

 rotations of the limbs in embryonic development, or by reference to an 

 ideal or neutral condition as indicated in the accompanying diagram 

 (Fig. 22). The neutral type (A) may be defined as one in which the 

 proximal segment (humerus or femur) is situated at right angles to the 

 median vertical plane, the middle segment directed downward parallel, 

 to the median plane, and the distal segment again at right angles to it. 

 In this condition the bones of the middle segment are parallel, with the 

 radius, or the tibia, according to the par ticular limb, anterior in position. 

 The first digit is also anterior. The entire anterior surface is indicated 

 in the accompanying diagram by shaded lines. The angles b. and c. 

 are "extension" angles; e. and f. "flexion" angles; a. is an "abduction" 

 angle, while d. is an " adduction" angle. 



In both limbs of a mammal, the entire appendage is rotated downward 

 to a position more or less underneath the body. In the anterior limb 

 (B) the proximal segment is rotated backward, the middle and distal 

 segments forward. The two divisions thus come to differ from one 



Fig. 22. Schematic representation of the respective positions of the segments 

 in the mammalian limbs: A, neutral; B, anterior limb; C, posterior limb. Ex- 

 planation in text: tr.p., transverse plane 



another to the extent of two right angles. Also, in the front limb, the in- 

 teresting condition is observable that the radius and ulna are crossed on 

 one another, the position of the former being chiefly anterior, although 

 its proximal end is lateral, and its distal end, like the first digit, medial. 

 In the rabbit, as in most mammals, the hand is thus fixed in a prone 

 position, while in man the hand may be rotated to a supine position in 

 which the radius is wholly lateral, or in which the two bones are parallel. 

 In the posterior limb (C) all three segments are rotated forward. Con- 

 sequently the bones of the middle segment retain their original parallel 

 position with reference to one another. The extension angles of the 

 knee and ankle are anterior, the flexion angles posterior. They differ 

 from the corresponding angles of the anterior limb only at the middle 

 joint, but here the difference amounts to two right angles. It will be 

 observed also that the distal or ankle joint of the posterior limb retains 

 a primitive condition, in which the foot is placed approximately at right 

 angles to the leg; in other words, it is of a plantigrade type, one in which 



