338 THE MECHANICAL FUNCTIONS, 



The limbs of quadrupeds constitute four columns of sup- 

 port to the trunk, which is placed horizontally above them; 

 but the whole weight of the body, together with that of the 

 head and neck, does not bear equally upon them; the fore 

 extremities almost always sustain the greater- part of that 

 weight, both because the fore part of the trunk is itself hea- 

 vier than the hind part, and because it is loaded with the 

 additional weight of the head and neck. Hence, in the usual 

 attitude of standing, the pieces of which the fore limbs are 

 composed are required to be placed more in a straight line 

 than those of the hinder limb: for the power of a column to 

 support a weight is the greater in proportion as it approaches 

 to the perpendicular position. The hind limbs are composed 

 of exactly the same number of divisions; but the separate 

 portions are usually longer than those of the fore extremity, 

 and consequently if they had been disposed vertically in a 

 straight line, they would have elevated the hinder part of 

 the trunk to too great a height compared with the fore part. 

 This is obviated by their forming alternate angles with one 

 another. As the pelvis connects the spine with the joint of 

 the hip, and even extends farther backwards, the thigh bone 

 must necessarily be brought forwards; then the tibia and 

 fibula, which compose the bones of the leg, must be carried 

 backwards to their junction with the bones of the foot; and, 

 again, the foot must be turned forwards in its whole length 

 from the heel to the extremities of the toes. On comparing 

 the positions of the corresponding divisions of the anterior 

 and posterior extremities, we observe that they incline, 

 when bent, in opposite directions; for in the former we find, 

 in following the series of bones from the spine, that the sca- 

 pula proceeds forwards, the humerus backwards; the radius 

 and ulna again forwards, and the fore foot backwards, posi- 

 tions which are exactly the reverse of the corresponding 

 bones of the hind limb. (See Fig. 218, page 350.) 



The weight of the body, in consequence of this alternate 

 direction of the angles at the successive joints, must always 

 tend, while the quadruped is on its legs, to bend each limb; 



