THE MUSCULAR SYSTEM 6^ 



foot more nearly into line with the leg is extension (or plantar- 

 flexion). The muscles accomplishing the former movement, how- 

 ever, are classed as extensor muscles, and in so far as they are 

 inserted distally in the digits actually do straighten these; while 

 the muscles which extend the foot at the ankle are flexors and,, 

 when inserted distally in the digits, do bend the latter. 



At the hip joint, movement of the thigh forward is flexion,, 

 movement backward is extension. 



The corresponding surfaces and muscles and in general the 

 differences presented by fore and hind limbs may be determined 

 by a study of their embryonic relations, but it is simpler, even if 

 less accurate, to refer the differences to the common basis of a 

 more or less primitive or neutral type as illustrated in Fig. 38. 

 In lower vertebrates, such as reptiles and amphibians, it is easily 

 observable that the front and hind limbs are more nearly similar 

 to each other than in mammals, especially in respect of the setting 

 out from the body of the elbow and knee, so that the limbs are held 

 nearly at right angles to the body axis. This common tendency is- 

 further expressed by the existence of a plantigrade condition of 

 the hand and foot, and by a parallel arrangement of the bones of 

 the forearm and leg. There is thus an approximation to a neutral 

 plan as indicated in (A), where the animal is considered to be facing 

 the observer or the limb viewed from in front. This neutral plan 

 is, however, never quite realized, because, even in primitive verte- 

 brates, where the limbs can scarcely be said to support the body^ 

 the adjustments for forward progression have already altered the 

 respective positions of the segments. 



It will be noted that as regards surfaces and angles in the 

 neutral type, the radial side of the fore limb and the tibial side of 

 the hind limb, shaded in the diagram, are anterior in position. 

 Apart from the possible movements of the limb as a whole or of 

 the segments upon one another, it will be seen that there are 

 certain angle surfaces, a, h, c, dorsal and lateral in position, which 

 may be identified as extension angles and certain others, d, e, /,. 

 ventral and medial, or flexion angles. In the relation of the limb 

 respectively to dorsal and ventral surfaces of the body, a is likewise 

 an "abduction" angle w^hile d is an "adduction" angle. (Ab- 

 duction may signify either the movement of a limb away from the 

 median plane of the body or the movement of a digit away from 



