Il8 CURRENT IN THE CAPILLARIES. 



the elastic pump, the water will flow out at the other end of the tube in jets, while during the 

 intervals of pulsation no water will flow out. As the walls of the tube are rigid, just as much 

 fluid flows out as is forced into the tube. If a similar arrangement be made, and a long elastic 

 tube be used, a continuous outflow is obtained, provided the pulsations occur with sufficient 

 rapidity and the length of the tube, or the resistance at its periphery, be sufficient to bring the 

 elasticitv 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. [The ordinary spray- 

 producer is another good example.] 



[Thus, there are two factors a central one, the heart, and a peripheral one, 

 the amount of resistance in the arterioles. Either or both may be varied, and as 

 this is done, so will the pressure and velocity vary.] 



Current in the Capillaries. In the capillaries 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 venous pulse is observed in the veins of the sub-maxillary gland 

 after stimulation of the chorda tympani nerve, which contains the 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. [Roy and 

 (ifaham Brown found that pulsatile phenomena were produced in the capillaries by 

 increasing the extra- vascular pressure ( 86). Quincke called attention to the 

 capillary pulse, which can often be seen under the finger-nails. Extend the fingers 

 completely, when a w r hitish 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, especially in incompetence of the aortic 

 valves, and is probably produced by increased extra-vascular pressure.] 



82- SCHEMATA OF THE CIRCULATION. E. H. Weber constructed a scheme of the cir- 

 culation. 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, [the extremely ingenious one of v. Thanhoffer, 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 through the systemic circulation, it follows that the right veutricle 

 must be just as capacious as the left. The capacity of the veutricles has been esti- 

 mated in the following ways : 



Methods. (1) Directly, by filling the dead relaxed ventricle with blood or an injection mass. 

 This method is unsatisfactory and inaccurate. 



(2) Indirectly. Volkmann (1850) estimated it thus : Estimate the sectional area of the aorta, 

 and the velocity of the blood-stream in it ( 1 ). From this calculate the amount of blood passiug 

 through the aorta in the unit of time. As the total quantity of blood in the body is known 

 ( -^ of the body-weight), we can easily calculate how long this takes to flow through the aorta. 

 "We must also know the number of beats during the time of the circulation. From these data, 

 and from experiments on animals, Volkmann estimated the volume of blood discharged at each 

 systole by the ventricle to be ,fo of the body-weight. For a man weighing 75 kilos, this is 

 187 '5 grams. This estimate still leaves much to be desired. 



Place calculates it in the following manner : A man uses about 500 litres of in 24 hours. 

 To absorb this into the venous blood (which contains about 7 vols per cent, less than arterial), 



