170 SPECIAL PHYSIOLOGY. 



only to protect the soft parts covered by it from injury through the 

 sole, but also contributes to maintain the antero-posterior arch of the 

 foot. Thus constructed, the arch of the foot, strong, and yet elastic, 

 from the gliding movements of its joints, and possessing likewise, 

 without diminution of its strength, a slight lateral motion, at the ball 

 and socket joint, between the astragalus and the scaphoid, is capable 

 of adapting itself readily to the unevenness of the surfaces on which 

 we tread, and also of breaking the shock produced in walking, running, 

 leaping, or other movements of the body. 



At the ankle, the superincumbent weight of the body is borne, upon 

 the broad square surface of its topmost bone, the astragalus, Fig. 52, 

 1, from which the weight is transmitted in the standing posture, partly 

 backwards through the os calcis, 2, and partly forwards through the 

 other tarsal, 6, 7, and metatarsal bones, 8. The surface of the astrag- 

 alus is received into the deep quadrangular recess formed by the lower 

 end of the tibia, the inner ankle, or malleolar process of that bone, 

 and the outer ankle, or malleolar projection of the fibula. With the 

 exception of a slight lateral play, which is very limited when the leg 

 is at right angles to the foot, but is somewhat more free when the foot 

 is fully extended, giving a graceful turn to the limb, the chief move- 

 ment here permitted is of a hinge-like character, in which the tibia, or 

 main bone of the leg, may be said to rock backwards and forwards 

 upon the astragalus. 



At the knee-joint, the femur can be brought, in standing, in a 

 straight line over the tibia, so that the one bone is supported on the 

 other, like a pillar. The chief points of constructive adaptation here, 

 are the very broad and slightly hollowed surfaces of the upper end of 

 the tibia, the concave semilunar cartilages which deepen the bearing 

 surface of the joint, the large expanded condyles of the femur, the 

 strong lateral ligaments, and the still stronger internal crucial liga- 

 ments, both sets being attached behind the axis of motion ; and lastly, 

 the protective influence of the patella in front, and also the increased 

 leverage given to the muscles fixed to that bone. The knee-joint, 

 indeed, is the largest in the body, and, from its breadth and strength, 

 is admirably suited to bear the weight, and sustain the shocks, which 

 are continually brought to act upon it. One point, specially notice- 

 able, is the greater length of the inner condyle of the femur as com- 

 pared with the outer condyle a formation necessary to establish a 

 horizontal line of support, from side to side, between the femur and 

 tibia, at the knee-joint. At the upper end of the femur, the neck is 

 elongated, and placed at an angle with the shaft, so as to increase the 

 breadth of the body for the attachment of muscles at the hips ; and in 

 consequence of this, the thigh bones incline towards each other from 

 the pelvis to the knees, so that the leg and foot may be brought more 

 directly beneath the centre of gravity. If, with this inclination in- 

 wards of the thigh bone, its condyles had been of equal length, there 

 would either have been a certain interval between the internal condyle 

 and the head of the tibia, or, if the tibia had been elevated at that 

 edge to meet the femur, the bearing surface of the knee-joint would 

 have formed an inclined plane downwards and outwards, and so would 



