LECTURES ON MOUSES. 
675 
the loins being the parts from which, when the hind feet have been 
projected forward and placed to serve as fulcra upon the ground, 
the spring is made which impels the whole machine onward, and 
the haunches being the chief agents in the propulsion. In racers, 
as has been observed on a former occasion, the loins and hindquar- 
ters are considered as of paramount importance ; in greyhounds, in 
the deer species, in hares, rabbits, &c., in fine, in all quadrupeds of 
speed, the same conformation is remarkably characteristic ; plainly 
shewing whereabouts the power for fast and efficient galloping 
should be lodged. 
Although we are unable to account for the production of muscu- 
lar motion, the principles directing its agency on the framework of 
the skeleton are clearly those of mechanics ; the lever being the 
power according to the laws of which locomotion may be said to 
be effected. The bones constitute “ a series of levers,” on which 
the muscles operate with more or less advantage and effect, de- 
pending upon their length, their position, their prominences or pro- 
cesses, &c. Of levers we know there are three kinds ; and of each 
of them examples may be found in the animal economy. 
For instance, the extension of the fore-limb is effected on the 
principle of that description of lever in which the fulcrum , or axis, 
or centre of motion, is situated between the moving power and the 
resistance or part to be moved ; whereas, in the flexion of the limbs, 
both fore and hind, in general, the power holds the intermediate 
place. When the arm is extended, the elbow-joint becomes the 
fulcrum ; the point of the elbow, to which the muscles are attached, 
the power ; and the limb itself the weight or resistance. When 
the arm is flexed, the elbow-joint is still the fulcrum, but the power 
is now transferred to the radius, the resistance being the same : 
thus furnishing us with an example of a lever of another kind, one 
in which the power is intermediately placed. A third kind of lever 
is exemplified in the extension of the hock, the foot being upon the 
ground : the foot now becoming the fulcrum, the point of the hock 
the seat of the power, and the resistance or weight to be moved 
forward, falling down the shaft of the tibia operating’upon the hock- 
joint. The same lever operates in the extension of the fetlock 
after it has been flexed in action for the purpose of pointing and 
fixing the toe in the ground, which then becomes the fulcrum, 
the power being exerted at the summit of the sesamoids, and the 
resistance bearing upon the large pastern. These laborious duties 
in the work of progression which the hind fetlocks, in concert with 
the hocks, have to perform, acccount for their failure in horses 
doing much heavy draught, or that have hunted or raced much in 
heavy grounds. 
It being a law in mechanics, that any deviation of the direction 
