70 CIKCULATION OF THE BLOOD IN THE VESSELS. 



observations on the arteries of the human subject and the lower animals ; and 

 by means of a " schema" representing the arterial system by elastic tubes 

 and the left ventricle by an elastic bag provided with valves and acting as a 

 syringe, he established the conditions of tension etc., necessary to their pro- 

 duction. In this schema, the registering apparatus, simpler in construction 

 than the sphygmograph, could be applied to the tubes with more accuracy 

 and ease. He demonstrated by experiments with this system of tubes, that 

 the amplitude of the pulsations, the force of the central organ being the same, 

 is greatest when the tubes 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 con- 

 tractions of the heart. This can only occur in a fluid which has a cer- 

 tain weight and acquires a velocity from the impulse ; for when air was 

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

 conditions, as the fluid did not possess weight enough to oscillate between 

 the impulses. Water produced a well marked dicrotic impulse under favor- 

 able 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 of the arteries, it will be use- 

 ful to 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 degree of contraction. This contraction 

 is antagonistic 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 discharged and the caliber of that 

 portion of the vessel be 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 

 to the pulse the characters 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 conditions, 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 



