100 SPECIAL PHYSIOLOGY 



will look quite white. A sufficient weight should be put on to make 

 the area distinctly paler, but not white. 



(1) What is the diameter of the area from which the capillary 

 circulation is excluded? 



(2) What is the area expressed in square millimetres? 



(3) What weight was added to the apparatus? 



(4) What is the total weight resting on the computed area? 



(5) What is the weight in milligrams resting upon each square 

 millimetre of surface? 



(6) How high would a column of water be in milligrams that 

 would represent this same pressure per square millimetre? 



(7) How high would a column of mercury be that would represent 

 this same pressure? 



(8) What is the capillary pressure in the volar surface of the ring 

 finger in the different members of the class? 



(9) Is the capillary pressure modified by a variation of the position 

 of the arm? 



(10) Is the capillary pressure modified by variation in the posture 

 of the subject: lying, sitting, standing? 



B. The Plethysmograph. 



This instrument is designed to determine the tissue pressure 

 in contradistinction to the arterial pressure in larger arterial 

 trunks. 



When an arm, leg, or finger is thrust into a case just large enough 

 to accommodate the member, any change in the volume of the 

 tissues will change the amount of space between the limb and the 

 case, and this change in volume may be easily traced with a record- 

 ing tambour. 



Such a case is really a modified receiving tambour and is called a 

 plethysmograph. One of these adapted to the finger is shown in 

 Appendix, 12. 



1. Appliances. Plethysmograph; recording tambour, smallest size 

 for finger, medium size for arm; kymograph. 



2. Preparation. Pass the finger or the bare arm through the 

 rubber collar of the receiver. The collar should fit the arm above 

 the elbow, or the index finger around the first phalanx tightly enough 

 to prevent any escape of air between the tissue and collar, but not 

 tightly enough to prevent ready return of venous blood. 



The tube leading from the plethysmograph to the recording 

 tambour should have a side vent, which should be left open while 

 the adjustment of the apparatus is in progress. 



Closing the vent, one should find that the tracing lever of the 

 recording tambour rises and falls rhythmically, showing a rhythmic 

 change in the size of the limb. 



