144 CONSTITUTIONAL DISEASES. [BOOK I. 



length reaches the fore part of the animal, and 

 acquires the name of the thoracic duct; here it 

 mixes with a portion of lymph, and is conveyed 

 immediately by a large vein to the heart. Ascend- 

 ing the pulmonary artery, as described in a former 

 section (37), the air entering the lungs, changes its 

 colour to a fine healthy scarlet, and at the next pul- 

 sation it is driven into the circulation, to mingle 

 with the mass, to impart its newly-acquired pro- 

 perties, and to return again and again, wasting 

 away, until at length it becomes used up and ex- 

 tinct, its place being supplied with other new ma- 

 terials for making blood by the continued process 

 of digestion. This wasting away, in what may be 

 termed the wear and tear of the animal system, is 

 constantly going on, whilst the new constantly sup- 

 plies the place of the old blood ; thus effecting a 

 change of substance, which is said to take place 

 with the whole body of the horse in the course of 

 every year and a half, or two years ; so that at no 

 time has he a particle of flesh, bone, hair, hoof, or 

 other matter which formed his body two years before, 

 and affording a fit subject for reflection and admira- 

 tion in us, as it invites those who have the care of 

 providing for his health to take advantage of the 

 well-known circumstances, the more securely to 

 effect those changes by gradual means, which too 

 frequently are attempted by violence, and fail. Na- 

 ture will not be forced ; rather seek her in her re- 

 cesses, and humour her ways. Those who act dif- 

 ferently, generally induce some lasting disorder to 



