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 necessaiy 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 



