THE MECHANICS OF THE CIRCULATION 473 



(by means of the compression-clamp) that the pressure in the arterial 

 manometer is high. Under these conditions observe (1) that the outflow 

 into the bowl is constant. This condition (representing the constancy 

 of the capillary flow) is due to two factors present in the circulation : the 

 great elasticity of the arteries and the high resistance in the capillaries. 

 If either of these be lessened the flow is no longer constant, but inter- 

 mittent with each beat of the heart. Thus, with extremely relaxed 

 capillaries one sometimes observes a venous pulse. Note (2) the high 

 and relatively constant pressure in the arteries, and (3) the very low 

 pressure in the veins. 



Expt. 23. The normal sphygmogram is the pulse-record made with a 

 sphygmograph ("pulse- writer") on the kymograph-drum covered with 

 smoked paper. Demonstration of the important graphic method, and 

 of the sphygmograph. Fasten the glazed paper to the drum so that the 

 margins beyond the latter at both ends shall be the same. The layer 

 of soot need not be black, but it should be fairly uniform. Cut off the 



FIG. 259 

 <k I 



UL 



O 



A simple myogranijto show the deformation of the curve by the use of arc-levers and the 

 mode of j correcting the error. Each point of the curve save o is too far toward the right, but 

 can be put in its proper place by setting it to the left its respective distance as shown in the 

 angle o, o, a, b, x. (Weiss.) In practice the error is lessened by having the lever's pivot opposite 

 the middle of the curve instead of opposite its bottom. 



margins and never re-smoke the paper after this has been done. Keep 

 the kymograph wound up. Use a speed of the drum which will make 

 a sphygmogram one-half as long again as it is high. After applying 

 the receiving tambour to the artificial artery, open the pinchcock to 

 equalize the pressure within the sphygmograph. Insert one or more 

 wooden strips beneath the "artery" if necessary to obtain a good record. 

 The tinsel writing-pen should be at least two centimeters long on the end 

 of a straw lever about 15 centimeters in length. The whole lever should 

 be always held at a tangent to the cylinder's surface. 



It is fundamentally important that each of the conditions should be 

 normal except the one studied in each case. Thus the pulse-rate, the 

 capillary resistance, the leverage, the drum-speed, etc., all must be 

 exactly as in making the norm with which the abnormal sphygmograms 

 are to be compared. A normal curve must, then, accompany each 

 pathological curve. This singleness of variation is of course one of 

 the basal principles governing all biological research, as Francis Bacon 

 and Mill long ago pointed out. 



