33-i 



OP THE CIRCULATION OF THE BLOOD. 



the blood itself. (See 259.) Again, in the above tracings it may be noticed 

 that the force of the impulse, or the extent of the vertical movement of the 

 lever, diminishes with increase of distance from the impelling organ, pro- 

 ducing the effect which Poiseuille described as "extinction of the wave." 



257. Since the blood, like other fluids, is almost completely incompressi- 

 ble, all force applied to it becomes perceptible by movement, or if this be 

 prevented, by lateral pressure exerted against the sides of the vessels, and 

 these two, the velocity of movement and the lateral pressure, however much 

 each may vary in amount, always together represent the impelling force. 

 The circumstances that chiefly affect the movement or velocity of the blood, 

 independently of variations in the amount of the propelling force, are those 

 which increase or diminish the friction between it and the vascular walls. 

 Thus it will be retarded by diminution of the diameter of the tubes, 1 or, as 

 was observed by Hunter, by curves, angles or divisions in their course, 2 or 

 by their rigidity, 3 or by increased viscidity in the blood itself; or lastly, by 

 augmentation of its attraction for the tissues, or for the walls of the vessels 

 through which it is coursing. These constitute the obstacles or resistance to 

 the passage of the blood ; and with their increase, if the impelling force 

 remain the same, whilst the velocity of its movement is retarded, the amount 

 of lateral pressure exerted is increased. The effects of diminished resistance 

 in diminishing pressure may be understood from a consideration of Fig. 137. 



For here the height to which 

 thefluidwill rise in thesrnall 

 tubes numbered 1 6, is the 

 expression of the pressure of 

 the liquid against the walls 

 of the tube ROC at different 

 points. If the fluid were sta- 

 tionary, from the closure of 

 the orifice c, it would, accord- 

 ing to a well-known law, rise 

 to the same height in all ; 

 but when a discharge is al- 

 lowed to take place from c, 



1 Thus Poiseulle (Mem. cle 1'Acad. des Sciences Sav. Etrang , t. ix, p. 513 et seq.) 

 found that the amount of fluid discharged by small tubes increasos, cceferis paribiis, 

 in proportion to the diameters of these tubes raised to the fourth power ; so that a 

 tube of T ^jyth mm. in diameter will discharge l<i times more fluid in a given time, 

 than one of ^^th mm. in diameter. The following facts are worthy of recollection 

 in connection with the physios of the circulation : 

 The circumference of a cylindrical tube = the radius x G.28. 



= the square of the radius x 3.14. 

 = the sectional area X the velocity 



of the current. 

 = the discharge -4- by the sectional area. 



FIG. 137. 



The sectional area of ditto 



The amount of discharge from ditto 



And lastly the velocity of the current 

 (See Vierordt, Pliys., 18(51, p. 88 ) 



2 So Budge (Physiologic, 1802), p. 309, found that a simple tube (a) would deliver 

 more llu id in a given time than a bifid one (/;), the sectional areas of whose arms 

 were together equal to a, and the bifid tube mor than a trittd one (c) in the propor- 

 tion of (a) o^'.O : (/>) 3400 : () 3100, the pressure being equal in all three ea^es. 



3 Thin Ma rev has shown that whilst elastic and rigid tubes of equal diameters will 

 discharge an equal quantity of fluid as long as the stream is continuous, the moment 

 the stream becomes intermittent an advantage is gained by the elastic tubes, appar- 

 ently owing to the diminution of friction in the latter, and the conversion of the 

 jetting movement of the fluid into a more continuous, uniform, and steady How; and 

 herein perhaps we may perceive an explanation of the hypertrophy of the Heart, so 

 frequently observed as a concomitant of a rigid aorta in old age. See Annal. des 

 Sri. .Nat. Zool., 1857, t. viii, p. l>30 ; and Ma rey's These Inaugurale, 1859. 



