I J jj oil THE FUNCTIONS OF THE HEART AND ARTERIES. 



mentutn of the blood, which accompanies the dilatation, en- 

 abiingf it to enter the minute arteries with equal ease, al- 

 though assisted by a tension somewhat smaller : so that the 

 same mean velocity may be sustained, as if the arteiies were 

 simply elastic, and a little smaller in diameter, with a very 

 Kut theoistri- little less exertion of the heart. But the distribution of the 

 button of the ^jJ^qJ could never be materially diversified by any operation 

 blood not ma- ^ ,.,.,,. .f. , ' ^ .-. i j 



teriaUy affected of this kind: ior 11 any artery were lor a moment oistended 



in this way. \^y gu^^j^ ^ variation, so as to exceed its natural diameter by 



oi^ hundredth part only, a pressure would thence arise 



equivalent to that of a column about two inches high, which 



would, in spite of all resistances, immediately dissipate the 



So that there blood with a considerable velocity, and completely prevent 



car. be no local ^^ local accumulation, unless the elastic powers of the 



accumuu- , • ,« i- • • i i i i • • i ^ 



tion, if the Vessel Itself were diminished ; and this is, perhaps, the most 



elasticity of the important, as well as the best established inference from the 



<iiiniDished.° doctriiie that I have advanced. 



Circulation in It appears, that a mola has sometimes been found in the 



a roola without ^terus, totally destitute of a heart, in which the blood must 

 a heart • . 



have circulated in its usual course through the veins and 



arteries: in this case it cannot be ascertained, whether there 

 was any alternate pulsation, or whether the blood was carried 

 en in a uniform current, in the same manner as the sap of 

 a vegetable probably circulates. If there was a pulsation, 

 it may have been maintained by a contraction of the artery, 

 much more considerable, and slower in its progress than 

 usual ; and with the assistance of a spontaneous dilatation : 

 the resistance in the extreme vessels being also probably 

 much smaller than usual : if the motion was continued, it 

 would lead us to imagine, that there may be some structure 

 in the placenta capable of assisting in the propulsion of the 

 blood, as there may possibly be some arrangement in the 

 roots of plants, by which they are calculated to promote 

 and in animals t^e ascent of the sap. The circulation in the vessels of the 

 without a more imperfect animals, in which a great artery supplies 

 * the place of a heart, is of a very different nature from that 



of the more perfect animals: the great artery, which per- 

 forms the ofFice of the heart, is here possessed of a muscular 

 power commensurate to its functions, and seems to propel 

 the blood, though much more slowly than in other cases, by 



meaD6 



