PHYSIOLOGY. 213 



ries depending upon it, will be more or less, according to the 

 respective density of these two sets of vessels. But it appears 

 from the experiments of Sir Clifton Wintringham, that the den- 

 sity and firmness of the veins, with respect to their correspon- 

 dent arteries, is much greater in young animals than in old ; 

 and thence it appears, that, during the growth of animals, the 

 arteries are requiring an increase of density in a greater propor- 

 tion than the veins are at the same time ; and, therefore, that 

 the resistance in the veins with respect to the arteries, must be 

 constantly diminishing ; that the veins will therefore receive a 

 greater proportion of blood ; that in the same proportion the ar- 

 teries will be less extended ; and, lastly, that the diminished 

 resistance in the veins, concurring with the diminished force of 

 the heart, will the sooner bring the increasing rigidity of the ar- 

 teries, and, therefore, of every fibre of the body, to be in ba- 

 lance with the extending powers ; at least, so far as to prevent 

 their producing any further growth. 



CCC. This account of the change of the resistances in the 

 arteries and veins, with respect to one another, is agreeable 

 to phenomena, which shew that the arteries are larger, and 

 contain more blood in proportion to the veins in young animals 

 than in old ; that arterial haemorrhagies occur most frequently 

 in young persons, and that congestions in the veins, with hae- 

 morrhagies, or hydropic effusions depending upon them, occur 

 most frequently in old age. 



CCCI. It is probable, that the resistance both of arteries 

 and veins goes on increasing, while the force of the heart is 

 not increasing at the same time ; but it appears also, that from 

 the diminished force of the heart, and the compression which 

 the smaller vessels are constantly exposed to from the distention 

 of the larger, from the action of the muscles and other causes, 

 the number of small vessels, and therefore the capacity of the 

 whole system is constantly diminishing so much that the heart 

 may still, for some time, be sufficient for the circulation of the 

 blood. But, while the resistances in the vessels are constantly 

 increasing, the irritability of the moving fibres, and the energy 

 of the brain, are at the same time constantly diminishing ; and 

 therefore the power of the heart must at length become un- 

 equal to its task, the circulation must cease, and death ensue, 



