(J3 ON THE FUNCTIONS OF THE HEART AND ARTERIE§. 



in the case of water: and supposing it about four times as 

 great, instead of 20 inches, wesliall have 80, for the measure 

 of a column of which the pressure is capable of forcing 

 the blood, in its natural course, tlirongh the smaller arteries 

 and veins, which agrees very well with Hales's estimate. 

 The calcvila- This determination of the probable dimensions of the arte- 

 lation founded Y\.J^\ system, and of the resistances occasioned by its different 



on preceding . . „ , , . ^ , - . 



observations, parts, is in some tew respects arbitrary; at the same time 

 that it cannot be materially altered, without altering either 

 the whole quantity of blood contained in the body, the dia- 

 meters of the smallest capillary vessels, the mean number 

 of bifurcations, or the magnitude of the resistance, all of 

 which are here assumed nearly as they h^ve been laid down 

 by former observers; the estimation of the length of the 

 successive segments only is made in such a manner, as to 

 reconcile these data with each other, by means of the expe- 

 riments and calculations relating to the friction of fluids in 

 Thecurrciture pipesi- The effect of curvature in increasing the resistance 

 fnc'eases tvfft ^^^ ^^^" hitherto neglected; it can be sensible only in the 

 resistance very larger vessels: and supposing the flexures of these to 

 ' *' be equivalent to the circumferences of two circles, each 



two Inches in diameter, the radius q being 1, we have r 



— i ^ =: -0000045 X 720 X 64 =: -207, or 



about one tifth of an inch, for the additional resistance 



arising from this cause in the case of water, or four fifths for 



blood, which is a very inconsiderable part of the whole. 



Objections to It might be questioned whether the experiments, which 



t eexperi- X have made, with tubes yl^ of an inch in diameter, are 

 ments answer- ... . 



ed. sufficient for determining, with accuracy, the degree in 



which the resistance would be increased in tubes, of which 

 the diameter is only one sixth part as great ; and it may be- 

 doubted whether the analogy, derived from these experi- 

 ments, can be safely employed as a ground for asserting, 

 that so large a portion of the arterial pressure is employed 

 in overcoming the resistance of the very minute arteries. 

 But it must be remembered, that these experiments are at 

 least conclusive with respect to the arteries larger than the 

 tube employed in them, and even those which are a little 

 smaller ; so th?it the remaining pressure, as observed in ex- 

 periments 



