112 



PHYSIOLOGY 



CHAP. 



the alternate activity of the psoas-iliac muscles, which tend to 



bend the trunk forward, and the dorsal and lumbar muscles, which 



tend to incline it backward. 



The common line of gravity of the head, trunk, and thighs, also 



passes behind the knee-joints ; and some arrangement is necessary 



when the individual is in the upright 

 posture to prevent falling backwards 

 owing to flexion of the knees. This is 

 provided for by the tension of the ileo- 

 femoral ligaments which rotate the 

 femora inwards, and thus prevents the 

 slight external rotation which is neces- 

 sary for the flexion of the knees. The 

 hip- and knee-joints are thus both fixed 

 by the weight of the trunk, which 

 throws the ileo-femoral ligaments into 

 tension. Owing to this mode of fixation 

 of the knee-joints, the active interven^ 

 tion of the extensor quadriceps muscle 

 is not necessary, and indeed the patellar 

 ligament does not seem to be more tense 

 in ,the vertical posture than in other 

 positions. 



The line of gravity of the whole body 

 falls on the ground in a plane somewhat 

 anterior to the line between the two 

 tibio-astragalic articulations, and the 

 body tends to fall forwards. This is 

 avoided by the fact that the plane of 

 flexion in this joint is very oblique with 

 that of the other side ; the two planes 

 of flexion form an angle of 60 open to 



FIG. 72. Curve normally presented t ne f r0 nt. In Order that flexion at 

 by the anterior median profile of . . 



the vertebral column in the these tWO JOintS should be pOSSlble, it 



is therefore necessary for the two knees 

 to be moved a ? art from each other > and 



dorsal vertebra ; /, lower border flexed. When flexion of the kn66S is 

 of 2nd lumbar vertebra ; p, pro- j p n p j 



; &-, symphysis ossium prevented, falling forward owing to 

 ' flexion of the tibio-astragalic articula- 



tions is also prevented. As the fixation 

 of the hip-joint determines the fixation of the knee, the fixation 

 of this joint leads to the fixation of the ankle. Here again the 

 gastrocnemius, soleus, posterior tibial, and posterior peroneal 

 muscles also take part in maintaining fixation. 



The tarsal and metatarsal bones, which constitute the skeleton 

 of the foot, form an arch which rests on the ground by the 

 tuberosity of the heel, and the heads of the first and fifth meta- 



