The Muscular System. 



65 



determined by a study of the embryonic relations of the limbs, 

 but may be referred more easily, if "not so accurately, to the neutral 

 plan as illustrated in Fig. 35. 



The neiutral 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 down- 

 ward 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 

 particular limb, anterior in position. The first digit is also anterior. 

 The entire anterior surface is indicated in the accompanying dia- 

 gram by shaded lines. The angles b and c are "extension" 



Fig. 35. Schematic representation of tlie respective positions of the segments 

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

 in text: tr.p., transverse plane. 



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 another to the extent of two right angles. Also, 

 in the front limb, the interesting 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 



