PRODUCTION OF THE PULSE. 253 



increases the caliber of the vessel, causes an elongation 

 even more marked. 



The finger placed over an exposed artery, or one which 

 lies near the surface, experiences a sensation at every beat 

 of the heart, as though the vessel were striking against it. 

 This has long been observed, and is called the pulse. Ordi- 

 narily it is appreciated when the current of blood can be 

 subjected to a certain amount of obstruction, as in the radial, 

 which can readily be compressed against the bone. In an 

 artery imbedded in soft parts, which yield to pressure, the 

 actual dilatation of the vessel being very slight, pulsation is 

 felt with difficulty, if at all. When obstruction is complete, 

 as in ligation of a vessel, the pulsation above the point of 

 ligature is very marked, and can be readily appreciated by 

 the eye. The explanation of this exaggeration of the move- 

 ment is the following : Normally, the blood passes freely 

 through the arteries, and produces, in the smaller vessels, 

 very little movement or dilatation ; but when the current is 

 obstructed, as by ligation, or even compression with the 

 linger, the force of the heart is not sent through the vessel to 

 the periphery, but is arrested, and therefore becomes more 

 marked and easily appreciated. In vessels which have be- 

 come undilatable and incompressible from calcareous deposit, 

 the pulse cannot be felt. The character of the pulse in- 

 dicates, to a certain extent, the condition of the heart and 

 vessels. We have spoken, when treating of the heart, of 

 the varying rapidity of the pulse, as it is a record of the 

 rapidity of the action of this organ ; but it remains for us 

 to consider the mechanism of its production, and its various 

 characters. 



Under ordinary circumstances, the pulse may be felt in 

 till arteries which are exposed to investigation ; and as it is 

 due to the movement of the blood in the vessels, the prime 

 cause of its production is the contraction of the left ventricle. 

 The late very interesting experiments of M. Marey have 

 shown that the impulse given to the blood by the heart is 



