732 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



falls within the base of the figure. The mechanism of standing differs in 

 bipeds and quadrupeds. In man the centre of gravity lies within the 

 pelvis, about one and a half millimeters in front of the promontory of the 

 sacrum. In the erect attitude of man the feet are directed outward 

 (forming an angle of about fifty degrees), so increasing the base of 

 support, the heels touching, the knees extended, the thighs rotated 

 externally, and the pelvis and trunk bent slightly backward, the arms 

 hanging at the side. 



In the act of standing, the body not being rigid, balancing must 

 be aided by the assistance of the contraction of various muscles. In a 

 certain number of joints the action of ligaments in the erect position 

 assists the maintenance of the upright posture ; thus, in the attitude 

 already described, where the knees are extended to the utmost, the 

 trunk thrown back, and the head balanced, the anterior hip-ligaments 

 are rendered tense, and the knee- and hip-joint remained fixed without 

 any effort upon the part of the joint-muscles. In the position known as 

 " standing at ease'' the weight of the body rests mainly on one leg, the 

 other forming simply a support to assist the muscles around the support- 

 ing ankle. In this position the joints are not kept locked by the tension 

 of the ligaments, for the pelvis is now somewhat oblique, so as to bring 

 it directly over the head of the femur. Varying tension in the muscles 

 serves to preserve the balance and prevent fatigue. In the erect posture 

 the ankle supports the weight of the body ; the line of gravity falling 

 slightty in front of the axis of rotation of the ankle-joint, the tendene}- 

 is thus for the body to fall forward at the ankles ; this is, however, 

 checked by the calf-muscles, which keep the parts nearly in position of 

 exact equilibrium. In the erect position, the ankle-joint being neither 

 flexed nor extended to the utmost forward or backward, motion must be 

 prevented by muscular contraction. Lateral motion at the ankle-joint 

 is prevented by the malleoli. When the knee-joint is completely ex- 

 tended no muscular action is required to prevent it from bending, be- 

 cause the line of gravity then passes in front of the axis of rotation and 

 the weight of the body tends to bend the knee backward. Although the 

 ligaments which exert their contraction behind the axis of rotation tend 

 to render this impossible, ordinarily the position is maintained by mus- 

 cular action so exerted that the line of gravity passes slightly behind 

 the axis of the knee, the tendency thus produced of the knee to bend 

 being checked by the extensor muscles of the thigh. 



In the hip-joint the line of gravity falls behind the line uniting the 

 joint. When the person is erect, the tendency thus produced of the 

 body to fall backward is prevented by the ileo-femoral ligament. If, 

 however, the knee is not extended to the full extent the line of gravity 

 passes a little behind the axis of rotation of that joint, and the pelvis 



