226 CIRCULATION OF THE BLOOD. 



a horizontal line touching it at the highest point; that the 

 line of descent much more oblique than that of ascent ter- 

 minates in the same way by gradually approaching a horizon- 

 tal line touching the curve at its lowest point. (See fig. 207.) 



37. Observation of the Expansive Movements 

 "which accompany the successive Changes of Arte- 

 rial Pressure above described. When an artery is ex- 

 posed in a living animal, as, e. y., when it is prepared in the 

 manner described in 34, two kinds of motion are seen. The 

 bit of artery which is separated from the surrounding parts 

 lengthens, and its diameter visibly increases each time it is 

 acted on by the contracting heart. Of these two phenomena, 

 the first is commonly called locomotion, because in certain 

 superficial arteries of the human body (especially when they 

 are enlarged in advanced life), the artery, as it lengthens, is 

 compelled to bend to one side or the other, and thereby 

 visibly changes its place each time that it is distended. The 

 other, viz., the expansive movement, is called pulsation, and 

 is practically of great importance, seeing that it is the only 

 phenomenon of the arterial circulation which admits of being 

 investigated without exposing the artery, and consequently 

 affords the only direct means by which we can judge of its 

 ever-varying conditions in man. 



Arteries being elastic, their changes of diameter express all 

 changes of the pressure exercised by their liquid inelastic con- 

 tents on their internal surfaces. If, therefore, the expansive 

 movements of an exposed artery were to be measured and re- 

 corded graphically, the record would correspond closety with 

 that of the pressure obtained by Fick's kymograph. For just 

 as in that instrument the variations of pressure are converted 

 by the C-shaped spring into nearly rectilinear movements, the 

 artery expands with every increase of pressure on its internal 

 surface, and contracts with ever}' diminution of it, so that any 

 point taken on its surface is constantly performing, in relation 

 to its axis, orderly successions of rectilinear movements in 

 opposite directions. 



In both cases that of the spring and that of the artery 

 the expansion, and the pressure which produces it, vary in 

 the same directions during the same times, but not in the same 

 fleyrce. As regards the spring, we can readily determine the 

 relation of expansion to pressure by the method of graduation 

 described in the preceding paragraph, and so use the former 

 as an expression for the latter. In the case of the arter}', ho 

 such empirical graduation is possible. The expansion of an 

 artery, or any other elastic tube, due to any given increase of 

 pressure against its internal surface, depends upon the degree 

 in which the tube is already distended at the commencement 

 of the act of expansion. The greater the original distension, 



