VASO-MOTOR CHANGES IN THE FINGER 241 



be used. Adjust the rubber bag and leather sleeve of the Erlanger appara- 

 tus, figure 189, to an arm, and connect it to the sphygmomanometer, set the 

 valve and quickly pump the pressure up to a point which occludes the pulse. 

 Adjust the writing point of the recording tambour to the smoked paper on 

 the cylinder, then lower the pressure by lo-mm. steps until the first pulse 

 appears. Now proceed with care, changing the pressure by 5-mm. steps 

 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 begins to increase is known as the systolic pres- 

 sure; the point in the pressure which records the highest point in the ampli- 

 tude 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. 



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 

 recorded 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 in- 

 crease m volume. 



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 move- 

 ments. 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 the 

 microscope, then adjust the high power to a field which shows one or more 

 small arteries. Make a drawing of a diameter of these arteries, using pig- 

 ment cells for land-marks. Now quickly stimulate the exposed sciatic nerve 

 while keeping the selected artery under constant observation. After a short 

 stimulation the diameter of the vessels will be seen to decrease considerably, 

 sometimes to the point of complete occlusion. When the stimulation ceases, 

 the vessel will remain contracted for a few seconds, then will slowly regain 

 its usual caliber, figure 201 . This is an exceptionally good method for direct 

 observation of the vaso-motor changes in the smaller vessels. 



23. The Plethysmogram of the Kidney. Anesthetize a dog or cat, 

 see experiments 12 and 19 above, and take blood-pressure tracings on the con- 



16 



