5S 



The blood vos- 

 *^3l> considered 

 as tvibes of in- 

 variable di- 

 mea5Lon5» 



J'orct with 

 ^'7hlch the 

 blood is pro- 

 p^litd from 

 the arteries 

 into ihe veins. 



• NTHE FUNCTIONS OF THE IIEAfet AND ARTERIES, 



When we consider the blood vessels as tubes of invariable 

 dimensions, we may suppose, in order to determine the ve- 

 locity af the blood in their different parts, and the resisr 

 tances opposed to its motion, that this motion is nearly uni- 

 form, since the alterations arising from the pulsation of the 

 heart do not materially affect the calculation, especially as 

 they are much less sensible in the smaller vessels than in the 

 larger anes, and the principal part of the resistance arises 

 troui these small vessels. We are to consider the blood in 

 the arteries as subjected to a certain pressure, by means of 

 whiel) it is forced into the veins, where the tension is much 

 less considerable; and this pressure, originating from the 

 contractions of the heart and continued by the tension of 

 the arteries, is almost entirely employed in overcoming the 

 frictiwi of the vessels: for the fo.r<:e required to overcome 

 the inertia of the blood is so inconsiderable, that it may, 

 without impropriety, be wholly neglected. We must there- 

 tbre inquire, what the magnitude of this pressure is, and 

 what degree of resistance we can suppose to arise from the 

 t'liction of the internal surface of the blood vessels, or from 

 any other causes of retardation. The magnitude of the 

 pressure has been ascertained by Hales's most interesting 

 experiments on a variety of animals, and may thence be 

 estimated with sufficient accuracy for the human body; and 

 foif determining the magnitude of the resistance, I shall 

 employ the theorems which 1 have deduced from my own 

 experiineiits on very minute tubes, compared with those 

 which had been made by former observers under different 

 circumstances; together with some comparative experi- 

 Uicnts on the motion of water and of other fluids iu tlie 

 same tubes. 



Dr. Hales infers, from his experiments on quadrupeds of 

 different sizes, that the blood in the human arteries is sub- 

 jected to a pressure, which is measured by a column of the 

 height of seven feet and a half : in the veins, on the con- 

 trary, the pressure appear-j to amount to about six inches 

 only : so that the force, which urges tiie blood from the 

 greater arteries through the minuter vessels into the large 

 veins, may be considered as equivalent to the pressure of a 

 column of seven feet. 



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