92 



FLOW IN ELASTIC TUBES. 



(4) The velocity of the current is proportional to the pressure and to the square of the 

 diameter, and inversely proportional to the length of the tube. 



(5) The resistance in the capillaries is proportional to the velocity of the current. 



64. FLOW IN ELASTIC TUBES. (1) When an uninterrupted uniform current flows through 

 an elastic tube, it follows exactly the same laws as if the tube had rigid walls. If the propel- 

 ling power increases or diminishes, the elastic tubes become wider or narrower, and they behave, 

 as Far as the movement of the fluid is concerned, as wider or narrower rigid tubes. 



(2) Wave-Motion. If, however, more fluid be forced in jerks into an elastic tube, i.e., in- 

 terruptedly, the first part of the tube dilates suddenly, corresponding to the quantity of fluid 

 propelled into it. The jerk communicates an oscillatory movement to the particles of the fluid, 

 which is communicated to all the fluid particles from the beginning to the end of the tube ; a 

 positive wave is thus rapidly propagated throughout the whole length of the tube. If we imagine 

 the elastic tube to be closed at its peripheral end, the positive wave will be reflected from the 

 point of occlusion, and it may be propagated to and fro through the tube until it finally dis- 

 appears. In such a closed tube a sudden jet of fluid produces only a wave-movement, i.e., only 

 a vibratory movement, or an alteration in the shape of the liquid, there being no actual trans- 

 lation of the particles along the tube. 



(3) If, however, fluid be pumped interruptedly or by jerks into an elastic tube filled with 

 fluid, in which there is already a continuous current, the movement of the current is combined 

 with the wave movement. AVe must carefully distinguish the movement of the current of the 

 fluid, i.e., the translation of a mass of fluid through the tube, from the wave-movement, the 

 oscillatory movement, or movement of change of form in the column of fluid. In the former, 

 the particles are actually translated, while in the latter they merely vibrate. The current in 

 elastic tubes is slower than the wave-movement, which is propagated with great rapidity. This 

 last case obtains in the arterial system. The blood in the arteries is already in a state of con- 

 tinual movement, directed from the aorta to the capillaries ; by means of the systole of the left 

 ventricle a quantity of fluid is suddenly pumped into the aorta, and causes a positive wave, the 



pulse-wave which is propagated with great rapidity to the 

 terminations of the arteries, while the current of the blood 

 itself moves much more slowly. 



Rigid and Elastic Tubes. If a quantity of fluid be 

 forced into a rigid tube under a certain pressure, the same 

 quantity of fluid will flow out at once at the other end of 

 the tube, provided there be no special resistance. In an 

 elastic tube, immediately after the forcing in of a quantity 

 of fluid, at first only a small quantity flows out, and the 

 remainder flows out only after the propelling force has 

 ceased to act. If an equal quantity of fluid be periodically 

 injected into a rigid tube, with each jerk an equal quantity 

 is forced out at the other end of the tube, and the outflow 

 lasts exactly as long as the jerk or the contraction, and 

 the pause between two periods of outflow is exactly the 

 same as between the two jerks or contractions. In an 

 elastic tube it is different, as the outflow continues for a 

 time after the jerk ; hence, it follows that a continuous 

 outflow current will be produced in elastic tubes, when 

 the time between two jerks is made shorter than the dura- 

 tion of the outflow after the jerk has been completed. 

 When fluid is pumped periodically into rigid tubes, it 

 causes a sharp abrupt outflow synchronous with the in- 

 flow, and the outflow becomes continuous only when the 

 inflow is continuous and uninterrupted. In elastic tubes, 

 an intermittent current under the above conditions causes 

 a continuous outflow, which is increased with the systole 

 or contraction. 



* 



Fig. 68. 

 Coats of a small artery, a, endo- 

 thelium ; b, internal elastic lamina 



65- STRUCTURE AND PROPERTIES OF 

 THE BLOOD-VESSELS. In the body the large 

 vessels carry the blood to and from the various 



circular muscular fibres of the tissues and organs, while the thin-walled capillaries 

 middle coat ; d, the outer coat. bring the bk)od {ntQ intimate relation with the 



tissues. Through the excessively thin walls of the capillaries the fluid part of the 

 blood transudes, to nourish the tissues outside the capillaries. [At the same time 

 fluids pass from the tissues into the blood. Thus, there is an exchange between the 



