56 CIRCULATION OF THE BLOOD 



oscillation gives the successive rebounds that are marked in the descend- 

 ing line of the pulse, and is capable, in some rare instances when the 

 arterial tension is slight, of producing a second rebound of sufficient 

 force to be felt with the finger. 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 degree of contraction, which 

 is antagonistic to the distending force of the blood. This is shown by 

 opening a portion of an artery included between two ligatures in a living 

 animal, when the contents will be forcibly discharged and the calibre of 

 that portion of the vessel be much diminished. Too great distention of 

 the vessels by pressure of blood seems to be prevented by this constant 

 action of the muscular coat; and thus the conditions are maintained 

 that give to the pulse the characters just described. 



By excessive and continued heat, the muscular structure in the 



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



force of the heart. Under these conditions, the pulse, as felt with the 



finger, will be found to be larger and softer than normal. Cold, either 



general or local, has an opposite effect ; the arteries become contracted, 



and the pulse assumes a harder and more wiry character. As a rule, 



/ prolonged contraction of the arteries is followed by relaxation, as is seen 



in the full pulse and glow of the surface observed in reaction after expos- 



I ure to cold. 



PRESSURE OF BLOOD IN THE ARTERIES 



The reaction of the elastic walls of the arteries during the intervals 

 of the heart's action gives rise to a certain degree of pressure, by which 

 the blood is continually forced toward the capillaries. The discharge of 

 blood into the capillaries has a constant tendency to diminish this pres- 

 sure ; but the contractions of the left ventricle, by forcing repeated charges 

 of blood into the arteries, have a compensating action. By the equilib- 

 rium between these two agencies, a certain tension is maintained in the 

 arteries, which is called the arterial pressure. 



The first experiments in regard to the extent of the arterial pressure 

 were made by Hales, and were published in 1733. This observer, 

 adapting a long glass tube to the artery of a living animal, ascertained 

 the height of the column of blood that could be sustained by the arte- 

 rial pressure. In experiments on the carotid of the horse, the blood 

 mounted to the height of eight to ten feet (243 to 304 centimeters). 



If a large artery, like the carotid, is exposed in a living animal, and 

 a metallic point, connected with a vertical tube of smaller calibre and 

 seven or eight feet (213 or 243 centimeters) long by a bit of elastic tub- 

 ing, is secured in the vessel, the blood will rise to the height of about 



