386 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



is, therefore, distended with accumulated blood ; so, in producing the low capil- 

 lary pressure, diminished power is met by diminished resistance, outflow is 

 relatively easy, accumulation is slight, and the elasticity of the delicate wall 

 is but little called upon. 



The Extinction of the Arterial Pulse. But why is the capillary pres- 

 sure pulseless, as the microscope shows ? To explain this, no new factors need 

 discussion, but only the adjustment of the arterial elasticity to the intermittent 

 injections from the heart and to the total friction which opposes the admission 

 of blood to the capillaries. This adjustment is such that the recoil of the 

 arteries displaces blood into the capillaries during the ventricular diastole at 

 exactly the same rate as that produced by the ventricular contraction during 

 the ventricular systole. Thus, through the elasticity of the arteries, the car- 

 diac pulse undergoes extinction ; and this becomes complete at the confines 

 of the capillaries. The respiratory fluctuations become extinguished also, and 

 the movement of the blood in the capillaries exhibits no rhythmic changes. 

 This conversion of an intermittent flow into one not merely continuous but 

 approximately constant affords a constant blood-supply to the tissues, at the 

 same time that the cardiac muscle can have its diastolic repose, and the ven- 

 tricular cavities the necessary opportunities to receive from the veins the 

 blood which is to be transferred to the arteries. 



A simple experiment will illustrate the foregoing. Let a long india-rubber 

 tube be taken, the wall of which is thin and very elastic. Tie into one end 

 of the tube a short bit of glass tubing ending in a fine nozzle, the friction at 

 which will cause great resistance to any outflow through it. Tie into the 

 other end of the rubber tube an ordinary syringe-bulb of india-rubber, with 

 valves. Expel the air, and inject water into the tube from the valved bulb 

 by alternately squeezing the latter and allowing it to expand and be filled 

 from a basin. The rubber tube will swell and pulsate, but if its elasticity 

 have the right relation to the size of the fine glass nozzle and to the amplitude 

 and frequency of the strokes of the syringe, a continuous and uniform jet will 

 be delivered from the nozzle, while the injections of water will, of course, be 

 intermittent. 



The Venous Pressure and its Causes. The pressure in the peripheral 

 veins is less than in the capillaries and declines as the blood reaches the larger 

 veins. Very close to the chest the pressure is below the pressure of the 

 atmosphere, and may sometimes vary from negative to positive, following the 

 rhythm of the breathing. These respiratory fluctuations will be considered later. 

 The low and declining pressures under which the blood moves through the 

 venules and the larger veins are due to the same causes as those which account 

 for the capillary pressure. It is still the force generated by the heart's con- 

 tractions, and made uniform by the elastic arteries, which drives the blood 

 into and through the veins back to the very heart itself. As the blood moves 

 through the veins, what resistance it encounters is still that of the friction 

 ahead. But the friction ahead is progressively less; the conversion of kinetic 

 energy into heat is progressively greater. The venous wall possesses elas- 



