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 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 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”’ 
G 
Ur|p 
A 

¢ 
| f 
Fic. 35. -Schematic representation of the 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 
