TIIK HOKSM IN MOTION. 57 



In order to aid the mind in understanding the actions of the muscles 

 upon their levers, the skeleton is mounted with movable joints, by 

 which means we are enabled to adapt it to. every position required. 

 By this means it is a comparatively easy matter for one to understand 

 the action throughout (See Plates II., XIV., XV.) 



Retraction begins by the relaxation of the gluteus maximus, the 

 vastus, semi-membranosus, and the great adductor. The triceps also 

 relaxes, and the tibia is free to respond to the contraction of the semi- 

 tt nclinosus lifting its lower extremity. The tensor vagina?, acting from 

 the hip upon the knee, the psoas magnus, iliacus, and sartorius from 

 the inner and upper wall of the pelvis, with the anterior branch of the 

 superficial gluteus from the hip, all act in concert to advance the thigh, 

 the knee becoming more flexed as it is advanced ; and with the knee, 

 or stifle, goes the hock joint, by the relaxation of the gastrocnemii and 

 the mechanical arrangement before described. 



The flexors of the foot act at the instant their tendons are released 

 from the forced service as ligaments, and continue their action until 

 the perpendicular from the centre of motion to the ground is reached, 

 which marks the point of greatest flexion of all the joints. The flexors 

 of the thigh, already mentioned, maintain their tension to keep the 

 Imver extremity of the femur in its advanced position. The semi- 

 tendinosus relaxes, while the triceps extends the tibia upon the femur 

 already well thrust forward, and the muscles of the calf, acting on the 

 point of the hock, extend the metatarsus synchronously with the feeble 

 action of the extensors of the foot. The perforans and perforatus do 

 not take part in this movement, as their action would counteract that 

 of the extensor. In this order the foot takes the ground, the heel 

 being the first to make the contact, and by its elastic frog it is pecul- 

 iarly fitted to receive the shock. It will be observed, by reference to 

 the plates, that the bones of the entire limb are at angles best adapted 

 to meet the contact with the ground. The toe is raised to avoid trip- 

 ping, and allow the elastic frogs of the foot to make the first contact. 



The instant of contact, when the foot is as far forward as possible 

 to sustain the centre of gravity, marks a sudden change. The flex- 

 ors of the thigh, the sartorius, tensor vaginae femoris, iliacus, and the 



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