Ijg on THE FUNCTIONS OP THE HEART AND ARTERIES. 



arteries may also have an immediate effect on the quantity 

 of blood contained in any part, although very little variation 

 could be produced in this manner by a change of the capa- 

 city of the larger vessels. 

 The state of According to this statement of the powers which are con- 

 the pulse de- ^^g^fjgj }„ tj^e Circulation, it must be obvious, that the na- 



peuds almost „ . , •. , , , i i 



wiioliy on the ture of the pulse, as perceptible to the touch, must depend 

 action of the almost entirely on the action of the heart, since the state 

 of the arteries can produce very little alteration in its qua- 

 Modifications lities. The greater or less tension of the arterial system 

 of it by the jjj^y indeed render the artery itself, when at rest, somewhat 

 state of the ar- , , . i •<» i i • i- i /»i 



tery. harder or softer; and, it the longitudinal hbres give way to 



the distending force, it may become also tortuous: possibly 

 too a very delicate touch may in some cases perceive a dif- 

 ference in the degree of dilatation, although it is seldom 

 practicable to distinguish the artery, in its quiescent state, 

 from the surrounding parts. But the sensation, which is 

 perceived when the artery is compressed, as usual, by the 

 finger, is by no means to be confounded with the dilatation 

 of the artery ; for in this case an obstacle is opposed to the 

 motion of the blood, against which it strikes, with the mo- 

 tnentum of a considerable column, almost in the same man- 

 ner as a stream of water strikes on the valve of the hydraulic 

 ram ; and in this manner, neglecting the difference of force 

 arising from the different magnitudes of the sections, the 

 pressure felt by the finger becomes nearly equal and simi- 

 kr to that which is originally exerted by the heart : each 

 pulsation passing under the finger, in the same time, as is 

 required for the contraction of the heart, although a very 

 little later; and more or less so, in proportion as the artery 

 is more or less distant ; the artery remaining then at rest 

 for a time equal to that in which the heart is at rest. When 

 iherefore an artery appears to throb, or to beat more 

 strongly tha^ usual, the circumstance is only to be ex- 

 plained from its greater dilatation, which allows it to re- 

 ceive a greater portion of the action of the heart, in the 

 •ame manner as an aneurism exhibits a very strong pulsa- 

 tion, without any increase of energy, either in itself, or in 

 the neighbouring vessels; -and on the other hand, when the 

 pupations o^f the artery of a paralytic arm become feeble, 



