118 



PHYSIOLOGY OF FARM ANIMALS 



[CH. 



If however the bone is not at right angles to AP but is, say, in 

 the position FA' or FG the mechanical 

 advantage is not measured by 



P' {=P)xFA' or P'xFC 

 but by P' X FB where FB is at right 

 angles to the line of action of P' or P. 

 FB is greatest when it is equal to FA, 

 that is the mechanical advantage is 

 greatest when the force exerted by the 

 muscular contraction acting at its 

 attachment is at right angles to the 

 bone. Acting on this principle the 

 cart horse, with a view to obtaining 

 the utmost mechanical advantage when 

 drawing a load, will endeavour to move 

 4, ^' the levers of its limbs in such a way 



that the power is in each case as 

 nearly as possible at right angles to the lever. Thus the best 

 results will be obtained by only slightly bending the joints and 

 consequently taking short steps. On the other hand a horse 

 when galloping will require the power of straightening out its 

 limbs to their utmost capacity, and thus will obtain speed at a 

 lavish expenditure of muscular exertion. 



It was formerly believed that the muscular attachment of the 

 fore limb to the trunk showed that the body was simply slung 

 between the fore legs which acted as props while the hind legs 

 did the work. Photography however has shown that this view 

 is quite erroneous, and that the fore legs as well as the hind act 

 as propellers to the body. This is especially well shown in 

 photographs of horses galloping when the fore leg may propel 

 the animal 10 feet forward, and in so doing raise it four inches 

 vertically from the ground. 



Joints. Where two bones with moving surfaces come in con- 

 tact joints occur. There are three chief kinds of joints : (1) ball 

 and socket joints (as in the hip), (2) hinge-like joints (as in the 

 hock), and (3) sliding joints (as in the knee). All these are covered 

 with cartilage which is much more yielding than bone. The 

 cartilage is covered by the synovial membranes which secrete 

 the synovia or "joint-oil.' This lubricates the joints and so 

 facilitates easy and rapid movements. 



