212 PHYSIOLOGY. 



sistance, be in balance with every other, and with the forces to 

 which they are severally exposed. 



CCXCVII. The extension of the arteries (CCXCV.) de- 

 pends upon the resistances which occur to the free transmission 

 of the blood through them, as in CLXX., and further, from a 

 resistance in, the veins. For, as a considerable portion of the 

 blood, by CCLXI., does not commonly pass into the smaller 

 branches of the arteries, but must pass very entirely into the 

 veins, so these, by their capacity constantly diminishing as 

 they approach nearer to the heart, and by their coats being of 

 a density and firmness sufficient to prevent further dilatation, 

 considerably resist the free passage of the blood from the arteries 

 into them. 



CCXCVIII. While these resistances continue, the arteries, 

 and with them almost every fibre of the body, must be extend- 

 ed at every systole of the heart, and with this extension, the 

 growth of every part will proceed ; but, as every part, by its 

 receiving an addition of solid matter, becomes more dense and 

 rigid, so it is less easily extended, and, perhaps, less readily re- 

 ceives an accretion of new matter than before. Hence it is, 

 that the more the body grows, it admits of any additional growth 

 more slowly ; and, unless the extending powers increase in the 

 same proportion with the increasing density of the solids, there 

 must be a period at which these two powers will balance each 

 other, and the growth will proceed no farther. But as it is 

 evident that the bulk and weight of the heart, and probably, 

 therefore, its force, does not increase with the increasing bulk of 

 the body, and that the action of the heart is the principal ex- 

 tending power in the system, it is also plain, that the extending 

 power does not increase in the same proportion with the in- 

 creasing density of the solids ; and, therefore, that these two 

 powers will, at a certain period, come to balance each other. 



CCXCIX. But not only is the force of the heart thus con- 

 stantly diminishing with respect to the resistance of the ar- 

 teries, but the force of the heart, though it were still subsisting, 

 has, from other causes, le.ss effect in extending the arteries. The 

 blood is more confined to the arteries, and extends them further 

 in proportion to the resistance in the veins, as in CCXCVII. ; 

 and this resistance in the veins, and the extension of the arte- 



