536 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



The same condition applies in the arterial system of animals : the nearer the artery 

 to the heart, the greater will be its expansion on the systole of the ventricle and 

 the stronger will be the pulse, while the greater the distance the less will be the 

 expansion and the weaker, consequently, will be the pulse. 



It has been mentioned that the descending limb of the curve described by 

 such levers is ordinarily broken into a number of secondary undulations. These 

 may be due to various causes. When by the injection of a mass of fluid the 

 walls of the tube are distended, as they regain their position of equilibrium inertia 

 carries them still farther, until the point of equilibrium has been passed ; a recoil 

 then takes place, and so on. In other words, each point of the tube is the seat of 

 a series of oscillations following each succeeding wave, and which are due simply 

 to the inertia of the walls of the tube. 



FIG. 223. MAREY'S SPHYGMOQRAPH. (Yeo.) 



The frame B, B, B, is fastened to the wrist by the straps at B, B, and the rest of the instrument lies on 

 the forearm. The end of the screw, V, rests on the spring, R, the button of which liee on the radial artery. 

 Any motion of the button at R is communicated to V, which moves the lever L up and down. When in 

 position, the blackened slip of glass is made to move evenly by the clock-work, H, so that the writing 

 point draws a record of the movements of the lever. 



Again, in the artificial scheme of the circulation, if the resistance be consider- 

 able, or if the end of the tube be completely obstructed, the wave will be 

 reflected from the distal end of the tube, and will again cause a secondary wave. 

 If a lever (the sphygrnograph, Fig. 223) be so placed on an artery in a living 

 animal as to record the movements of its walls, various breaks will also be seen 

 in the descending limb of the pulse-curve (Fig. 224). The most important of 

 these is the so-called dicrotic wave, which is more or less marked in every pulse r 

 although it may be so exaggerated as to produce the impression of a double 

 impulse, or may, on the other hand, be scarcely perceptible. Anything which 

 reduces the tension in the arterial system will facilitate the development of the 

 dicrotic wave. Anything which increases the rigidity of the arteries reduces the 

 degree of the dicrotic wave. It is, therefore, evident that the dicrotic wave is 



FIG. 224. TRACING DRAWN BY MAREY'S SPHYGMOGRAPH. (Yeo.) 



mainly a wave of oscillation, due to the inertia of the walls of the vessels, possibly 

 being reinforced by a wave of expansion reflected from the closure of the aortic 

 valves. When the conditions are especially favorable for producing such waves 

 of oscillation, or, in other words, when the walls of the arteries are especially 

 relaxed, we will then sometimes find that the pulse-curve in its descent will be 

 marked by two breaks, the first of which is then spoken of as the pre-dicrotic 

 and the second as the dicrotic wave. 



5. THE CIRCULATION IN THE CAPILLARIES. The capillaries consist 

 of minute tubules whose walls are constituted by a single layer of trans- 

 parent, thin, nucleated, endothelial cells joined to each other by their 



