CURRENT IN THE CAPILLARIES. 161 



ance at its periphery, be sufficient to bring the elasticity of the tube into action. 

 This can be done by putting a narrow cannula in the outflow end of the tube, or 

 by placing a clamp on it so as to diminish the exit aperture. This apparatus 

 converts the intermittent flow into a continuous current.] The fire-engine is a 

 good example of the conversion of an intermittent inflow into a uniform outflow. The 

 air in the reservoir is in a state of elastic tension, and it represents the elasticity 

 of the vascular walls. When the pump is worked slowly, the outflow of the water 

 occurs in jets, and is interrupted. If the pumping movement be sufficiently rapid, 

 the compressed air in the reservoir causes a continuous outflow, which is distinctly 

 accelerated at every movement of the pump. 



Current in the Capillaries. In the capillary vessels the pulsatile 

 acceleration of the current ceases with the extinction of the pulse- 

 wave. The great resistance which is offered to the current towards 

 the capillary area causes both to disappear. It is only when the 

 capillaries are greatly dilated, and when the arterial blood-pressure 

 is high, that the pulse is propagated through the capillaries into the 

 beginning of the veins. A pulse is observed in the veins of the sub- 

 maxillary gland after stimulation of the chorda tympani nerve, which 

 contains the vascular or vaso-dilator nerves for the blood-vessels of 

 this gland. If the finger be constricted with an elastic band so as to 

 hinder the return of the venous blood, and to increase the arterial 

 blood-pressure, while at the same time dilating the capillaries, an inter- 

 mittent increased redness occurs, which corresponds with the well- 

 known throbbing sensation in the swollen finger. This is due to the 

 capillary pulse. [Eoy and Graham Brown found that pulsatile pheno- 

 mena were produced in the capillaries, by increasing the extra-vascular 

 pressure (p. 173). Quincke called attention to the capillary pulse which 

 can often be seen under the finger nails. Extend the fingers completely, 

 when a whitish area appears under the nails. A red area near the free 

 margin of the nail advances and retires with each pulse-beat. It is 

 well-marked in some diseased conditions of the heart, and is probably 

 produced by increased extra-vascular pressure,] 



82. Schemata of the Circulation. 



E. H. Weber constructed a scheme of the circulation. It consisted of a force- 

 pump with properly arranged valves to represent the heart, portions of gut for 

 the arteries and veins, and a piece of glass tubing containing a piece of sponge to 

 represent the capillaries. Various schemes have been invented, including the very 

 complicated one of Marey [and the thoroughly practical one of Rutherford]. 



83. Capacity of the Ventricles, 



Since the right and left ventricles contract simultaneously, and 

 just the same volume of blood passes through the pulmonary as 



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