74 CIRCULATION OF THE BLOOD. 



when the tuhes are moderately distended, or when the tension of fluid is low, and vice versa. 

 He demonstrated, also, that a low tension favors dicrotism. In this latter observation, 

 he diminished the tension by enlarging the orifices by which the fluid was discharged 

 from the tubes, imitating the dilatation of the small vessels, by which the tension is di- 

 minished in the arterial system. He also demonstrated that an important and essential 

 element in the production of dicrotism is the tendency to oscillation of the fluid in the 

 vessels during the intervals between the contractions of the heart. This can only occur 

 in a fluid which has a certain weight and acquires a velocity from the impulse ; for, 

 when air was introduced into the apparatus, dicrotism could not be produced under any 

 circumstances, as the fluid did not possess weight enough to oscillate between the im- 

 pulses. Water produced a well-marked dicrotic impulse under favorable conditions; 

 and with mercury, the oscillations made two, three, or more distinct impulses. By 

 these experiments he proved that the blood oscillates in the vessels, if this movement 

 be not suppressed by too great pressure or tension. This oscillation gives the successive 

 rebounds that are marked in the descending line of the pulse, and is capable, in some 

 rare instances when the arterial tension is very slight, of producing a second rebound 

 of sufficient force to be appreciated by the finger. 



Without treating of the variations in the character of the pulse in disease, due to the 

 action of the muscular coat, we shall 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 antagonistic to the dis- 

 tending force of the blood, as is shown by opening a portion of an artery included be- 

 tween two ligatures in a living animal, when the contents will be forcibly discharged and 

 the caliber of that portion of the vessel is 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 

 characters 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 circum- 

 stances, the pulse, as felt by 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. 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 difference in the caliber of the 

 arteries at different periods of the day. The diameter of the radial has been found very 

 much greater in the evening than in the morning, producing, naturally, a variation in the 

 character of the pulse. We learn from these physiological variations, how, in disease, 

 when they become more considerable, they may give important information with regard 

 to the condition of the system. 



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 amount of constant pressure, by which the blood is con- 

 tinually forced toward the capillaries. The discharge of blood into the capillaries has a 

 constant tendency to diminish this pressure ; but the contractions of the left ventricle, 

 by forcing repeated charges of blood into the arteries, have a compensating action. By 

 the equilibrium between these two agencies, a certain degree of tension is maintained in 

 the arteries, which is called the arterial pressure. 



The first experiments with regard to the extent of the arterial pressure were made 

 by Hales, an English physiologist, more than a hundred years ago. This observer, 

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



