312 THE CAUSES OF THE xi 
the contractions of its valves, keeps the blood 
constantly circulating in one direction, never 
allowing it to rest; and then, by means of this 
circulation of the blood, laden as it is with the 
products of digestion, the skin, the flesh, the hair, 
and every other part of the body, draws from it 
that which it wants,and every one of these organs 
derives those materials which are necessary to 
enable it to do its work. 
The action of each of these organs, the per- 
formance of each of these various duties, involve 
in their operation a continual absorption of the 
matters necessary for their support, from the 
blood, and a constant formation of waste products, 
which are returned to the blood, and conveyed by 
it to the lungs and the kidneys, which are organs 
that have allotted to them the office of extracting, 
separating, and getting rid of these waste products ; 
and thus the general nourishment, labour, and 
repair of the whole machine are kept up with order 
and regularity. But not only is it a machine 
which feeds and appropriates to its own support 
the nourishment necessary to its existence—it is” 
an engine for locomotive purposes. The horse~ 
desires to go from one place to another; and to- 
enable it to do this, it has those strong contractile — 
bundles of muscles attached to the bones of its” 
limbs, which are put in motion by means of a sort : 
of telegraphic apparatus formed by the brain and ~ 
the great spinal cord running through the spine or 
