126 



ANIMAL PHYSIOLOGY. 



wall are contracted, and so offer resistance to the heart. 

 Some of the peculiarities of the pulse, which cannot be 

 appreciated by pressure of the finger on the wrist, are 

 exhibited with the aid of the sphygmograph (Marey), an 

 instrument which is fastened to the wrist, and in which a 

 spring, pressed against the radial artery, causes a light 

 lever, carrying a pen, to move up and down. The pen is 

 in contact with a slip of paper or smoked glass set in 

 motion by clock-work, and produces a tracing which 

 indicates the pulsations by elevations, and the element of 

 time by horizontal distance. Such a tracing shows, that in 

 health the distension of the vessel takes place with almost 

 instantaneous suddenness, commencing and finishing abruptly ; 



1.1 



3. 



Fig. G8. SPHYGMOGRAPHIC TRACINGS of the pulses of three persona 

 all healthy. In 1, the arterial resistance is greatest; in 2, the 

 dilatation of the vessel has taken place with such force as to jerk 

 the lever of the instrument from its rest, and hence the sharp 

 points at the tops of the waves; 3, is a distinctly dicrotous pulse. 



while the gradual character of the recoil is shown by its 

 making a long sloping line. When the arterial resistance is 

 great, as it is in the most robust health, it counteracts the 

 distending impulse given by the heart, so that the rise of the 

 tracing is not so considerable as it would otherwise be; and 

 in these circumstances there is a moment's continuance of the 

 distension, then a gradual and but slightly undulating de- 

 scent. But when the arterial resistance is slight, the secondary 

 distending impulses given by the elastic recoil of the larger 

 vessels produce more effect on the tracing, and one particular 

 rise becomes prominent, which appears to be caused by the 

 walls of the commencement of the aorta, redistended by the 



