MECHANISM OF THE INFERIOR EXTREMITIES. 247 



anterior extremity of the metatarsus, whereby that side of the 

 pelvis is pushed forwards, and a rotation in advance impressed 

 upon it. By the latter impulse, the foot of that side is wholly 

 detached from the ground, the thigh being flexed at the same 

 moment at the hip joint, and the leg flexed at the knee, the 

 whole extremity is carried forward and fixed upon the ground, 

 after the manner described in the first step. Ordinary progres- 

 sion results, then, from the regular succession of the last mo- 

 tion in the two extremities. In regard to the impulsion of the 

 pelvis from the foot behind, this will probably take place in 

 every case, more or less; it may however, be reduced very 

 much by a certain extent of flexion at the knee joint; and the 

 want of it not be felt, because other powers concur to produce 

 the same impulsion; as certain muscles, and also the momen- 

 tum of swinging the lower extremity forward. 



An equality of length in the lower extremities is indispensa- 

 ble to graceful and regular progression. If one of them be 

 shortened from any cause whatever, it is manifested in the gait, 

 by an unusual sinking of the pelvis on the defective side, at the 

 moment the foot is brought to the ground, and from the conti- 

 nuity of the pelvis with the upper parts of the body, a conside- 

 rable lateral inclination is communicated to the latter in the 

 same instant. The pains frequently taken to conceal this de- 

 fect, disguise it very imperfectly, unless the shortness be only 

 such as may be supplied by a shoe with a sole thicker than 

 that of the other foot. Where the shortness arises from luxa- 

 tion upwards of the os femoris, a crutch is the best substitute 

 for sustaining that side of the pelvis. 



In running, the position of the feet is somewhat different from 

 what it is in walking; they are extended so as to support the 

 trunk on the phalanges alone, instead of on their soles: whereby 

 a double advantage is obtained, that of keeping the lower extre- 

 mities at their greatest possible length, and also of enabling them 

 to detach themselves quickly from the ground. The velocity 

 here is the principal difference between it and walking, yet there 

 are some peculiarities. 



The trunk of the body is kept continually and largely inclined 

 forwards, which enjoins the necessity of a quick successive ad- 

 vance of the lower extremities to prevent it from falling. This 



