264 THE CIRCULATION OF THE BLOOD 



until a record has been obtained which passes the maximal amplitude. Now 

 release the pressure from the arm. 



The first point in the decreasing pressure at which the pulse tracing be- 

 gins to increase is known as the systolic pressure; the point in the pressure 

 which records the highest point in the amplitude of the pulse wave is known 

 as the diastolic pressure. The systolic pressure will vary from 120 to 150 

 mm. of mercury; the diastolic from 90 to 120 in different individuals of the 

 average physiology class, see figures 189 and 190. 



FIG. 217 a. Two pressure curves from the human forearm. The pressure scale is 

 given to the left. The new apparatus of Recklinghausen was used. The diastolic pres- 

 sure points (lower cross lines) and the systolic pressure points (upper cross lines) are 

 marked. The changes in the character of the pulse records here is abrupt and character- 

 istic. The brackets mark the zones of larger, L, and smaller, s, oscillations. 



To the right is introduced a reconstruction tonogram built up from the measure- 

 ments of these curves. (Recklinghausen.) 



21. The Vaso-motor Changes in the Finger, the Plethysmogram. 



Insert the finger in the Porter finger plethysmograph, fill the tube with 

 water, and connect it with a small-sized air tambour. The variations in 

 volume of the finger are slight, so that one must use a rather long, delicately 

 balanced recording lever. Take a tracing on a recording cylinder moving 

 at a slow speed, i mm. per second. The finger and its plethysmograph 

 should be supported by a swinging support so that no mechanical move- 

 ments will destroy the accuracy of the record. Observations through 

 several minutes will usually show variations in volume of the finger, which 

 will be rcorded by the tambour. Cold air in the face or cold water on the 

 hand will usually be marked by a decrease in volume indicating vaso- 

 constriction. Application of heat to other fingers of the same hand will lead 

 to increase in volume indicating vaso-dilatation. 



22. The Vaso-motors of the Frog's Web. Prepare a frog for ob- 

 servation of the circulation of the web under the microscope, as described 

 above; but give it just enough i per cent, curari to destroy voluntary 

 movements. Quickly dissect the sciatic nerve in the thigh, using extreme 

 care not to interfere with the circulation. Mount the preparation, pick out 

 an active field of capillaries, small arteries, and veins with the low power of 



