

 300 THE CIRCULATION 



pulse that would come from a low central resistance due, say, to an aortic- 

 valve insufficiency. A hard "pulse" (artery) might come from a very 

 vigorous systole with low capillary resistance or from a much weaker 

 systole with narrowed capillaries; in either case, however, it represents a 

 high arterial pressure, whatever the conditions giving rise to it. Again, 

 a very quick sudden systole would be partly masked were the arteries 

 somewhat inelastic from disease. The important elements of blood- 

 pressure lie largely in the capillaries, for there its effects are exercised, but 

 the conditions in the arteries must be well understood, because they supply 

 the capillaries with blood. 



Whether felt directly with the trained finger or observed indirectly in 

 the tracing made by a mechanical appliance (sphygmograph), these 

 several factors of the pulse produce effects which may be studied, meas- 

 ured, and compared, and especially in the permanent written trace, the 

 sphygmogram. 



Fro. 161 



P.Cetr. 



O.Resp. art 



Sphygmogram from a curarized dog kept alive by artificial respiration to show especially the 

 vaso-motor pressure-changes: Resp. art., an artificial respiration making the pressure rise and 

 fall; Vas-mot. and V. M., rises and falls of pressure from vaso-motor changes. (The smallest 

 waves are those of the pulse. Each division of the time-line is 2 seconds.) (Meyer.) 



VASO-MOTION is the narrowing and expanding of the arterial tubes by 

 the contraction and relaxation respectively of the smooth muscle-fibers 

 in their walls. It is then, so far at least as the constriction is concerned, 

 purely an active process. The expansion of the arteries is also an active 

 process in its nervous influences and probably also in the action of the 

 muscle fibers themselves. Vaso-motion then has to do with the tonus of 

 the arteries. It must be carefully distinguished from the purely passive 

 enlargement caused by the heart-beat and from the perhaps equally 

 passive narrowing of the arterial caliber by the elastic recoil of the walls 

 distended at each pulse. These distinctions, though basal, are easily 

 neglected, the result being mental confusion as regards the various 

 important forces of the circulation and the blood-pressure. Vaso-motion 

 is probably not in normal cases concerned with the circulation of the 

 blood, but it directly determines in large part the blood-pressure. On 

 the other hand, the arterial elasticity is an important cause of the circula- 

 tion, as we have seen, and is also a factor in blood-pressure. 



The exceedingly important functions of vaso-motion are those on 



