PHYSIOLOGY OF MOVEMENT. 



743 



the contraction of the adductors, the femur is approached to the ilium 

 by the contraction of the rectus and lumbar and iliac psoas muscles, the 

 knee being elevated anteriorly by the ischio-tibial muscle and the hock- 

 joint flexed by the contraction of the metatarsal flexor, whose tendinous 

 portion, when the stifle-joint is flexed, becomes tensed and mechanically 

 repeats the action on the joint below, the digital region being flexed on 

 the metatarsus. As a consequence of these flexions, produced almost 

 simultaneously, the axis of the limb becomes shorter, is raised from the 

 ground, and advanced in a more or less oblique line. That the foot may 

 again be placed on the ground the above muscles must relax and their 

 antagonists contract. First the femur is extended on the pelvis by the 

 action of the gluteus maximus, whose principal trochanteric branch acts 

 as a lever of the first class. The leg is then extended on the thigh by 



Fig. 305.— Oscillation of the Anterior Extremities. (Colin.) 



The figure shows that while one fore leg is describing the pendulum motion the other is acting as a 

 support, while the right forefoot describes the arc, '/ h, the left shoulder describes the arc, af lit rt, from the 

 impulse given to the centre of gravity of the bodv through the extension of the hind legs. Then the right 

 shoulder describes the arc, '/' d jl- The six positions of the left leg in one complete step are shown at 

 abed ef, the centre of gravity having been advanced from m to n. 



the rotuleus muscles, the metatarsus in its turn regaining its position on 

 extension through the contraction of the gastrocnemius, the digital region 

 being extended by the phalangeal extensors. The lever action of the 

 muscles which produce these motions is seen in Figs. 249 and 251. 



In movements of progression each hind limb alternately serves to 

 give an impetus to the body by passing into a condition of extreme ex- 

 tension, the foot being on the ground, and then, passing into a condition 

 of flexion, describes a pendulum motion through the air until the foot 

 again strikes the ground in front of the position which it left. The pen- 

 dulum motion commences with flexion of the liip-joint and forward motion 

 of the lower end of the femur followed by flexion of the other joints. 

 The greater the advance of the knee the greater the tension of the ex- 

 tensors, until the limb becomes extended and its increased length brings 



