260 CIRCULATION. 



Without treating of the variations in the character of the 

 pulse in disease, due to the action of the muscular coat, we 

 will consider some of the external modifying influences which 

 come within the range of physiology. The smallest vessels 

 and those of medium size possess to an eminent degree what 

 is called tonicity, or the property of maintaining a certain 

 continued amount of contraction. This contraction is antag- 

 onistic to the distending force of the blood, as is shown by 

 opening a portion of an artery included between two ligatures, 

 in a living animal, when the contents will be forcibly dis- 

 charged and the caliber of that portion of the vessel very 

 much diminished. Too great distention of the vessels by 

 the pressure of blood seems to be prevented by this constant 

 action of the muscular coat ; and thus the conditions are 

 maintained which give the pulse the character we have just 

 described. 



By excessive and continued heat, the muscular tissue of the 

 arteries may be dilated so as to offer less resistance to the 

 distending force of the heart. Under these circumstances, 

 the pulse, as felt by the finger, will be found to be larger and 

 softer than normal. Cold, either general or local, has a pre- 

 cisely opposite effect ; the arteries become contracted, and 

 the pulse assumes a harder and more wiry character. Usually, 

 prolonged contraction of the arteries is followed by relaxation, 

 as is seen in the full pulse and glow of the surface which 

 accompany reaction after exposure to cold. 



It has been found, also, that there is a considerable differ- 



the accuracy of results to be obtained when it is used with skill and care ; but the 

 very perfection and nicety of the instrument present almost insurmountable 

 difficulties in the way of its use by the general practitioner. Results, regarding 

 the amplitude of pulsations especially, should be received with great caution, from 

 the extreme difficulty of adjusting the lever so as to give the maximum of the 

 impulse. It does not appear, however, how these drawbacks to the general use 

 of the instrument can be obviated; for its construction leaves nothing to be 

 desired, and the delicacy of its adjustment, like that of a fine balance, is indis- 

 pensable. In the hands of Marey, its results, we conceive, are to be fully ac- 

 cepted. 



