208 THE CIRCULATION OF THE BLOOD AND LYMPH 



21. Plethysmographic Tracings. Connect the vessel D (Fig. 36, 

 p. 128), directly with a recording tambour by the tube F, omitting for 

 simplicity the recording arrangement in the figure. Place the arm 

 in the plethysmograph, and adjust the indiarubber band to make 

 a watertight connection. Support D so that the arm rests easily 

 within it, and fill it with water at body temperature. No water must get 

 into the tambour, and it is well to insert a piece of glass tubing in the 

 connection between it and the plethysmograph, so that it may be seen 

 when the water is rising too high. A T-piece with a short piece of 

 rubber tubing on the stem should be inserted in the course of the tube 

 leading to the tambour. All adjustments are made with the T-piece 

 open, and when a tracing is to be taken the short rubber tube is closed 

 by a clip. Arrange a time-marker to write half or quarter seconds 

 (Fig. 88, p. 193). Adjust the writing-point to write on a drum, and 

 close the upper tubulure C with a cork. The quantity of blood in the 

 arm is increased with every systole of the left ventricle, diminished in 

 diastole. The lever will therefore rise when the ventricle contracts, 

 and sink when it relaxes. 



(1) Take tracings with the arm (a) horizontal, (b) hanging down. 



(2) With the arm horizontal, take tracings to show the effect (a) of 

 closing and opening the fist inside the plethysmograph;* (b) of apply- 

 ing a tight bandage round the arm a little way above the indiarubber 

 band; (c) of inhaling 2 drops of amyl nitrite. 



Instead of the arm plethysmograph, a small plethysmograph to hold 

 a finger may be employed. It consists of a glass tube drawn out at 

 one end. The wide end is provided with a rubber collar. The narrow 

 end is connected by a small rubber tube with a very small and sensitive 

 recording tambour, a T-piece being inserted on the connection as before. 

 With the T-piece closed fill the tube with water. Then, holding up the 

 wide end of the tube, the tip of the finger is put in so as just to close 

 the tube. The T-piece is then raised and opened, and the finger pushed 

 in as far as it will go. The collar must fit the finger so as to form a 

 watertight joint. Now get the proper pressure in the tambour by 

 blowing into the T-piece, and close the clamp. A time -tracing can be 

 taken as before. 



22. Pulse-Rate. (i) Count the radial pulse for a minute in the 

 sitting, supine, and standing positions. Use a stop-watch, setting it 

 off on a pulse-beat and counting the next beat as one. Make three 

 observations in each position. 



(2) Count the pulse in a person sitting at rest, and then again in the 

 sitting position immediately after active muscular exertion. Note how 

 long it takes before the pulse-rate comes back to normal. 



(3) Count the pulse in a person sitting at rest. Repeat the observa- 

 tion while water is being slowly sipped, and note any change. 



(4) With one hand over the thorax of a rabbit, count its pulse. Then 

 notice the effect (a) of suddenly closing its nostrils, (b) of bringing a 

 small piece of cotton-wool sprinkled with ammonia or chloroform in 

 front of the nose (reflex inhibition of the heart] . 



23. Blood-Pressure Tracing. (a) Put a dog under morphine (p. 63). 

 Set up an induction machine arranged for an interrupted current 

 (Fig. 93, p. 198). Fill the U-shaped manometer tube (if this has 

 not already been done) with clean mercury to the height of 10 to 

 12 cm. in each limb. If the float tends to stick, half an inch of oil 

 may be put above the mercury in the distal (straight) limb before 

 putting in the float. But where the mercury is clean and dry, and the 



* Closing the fist causes a fall in the curve i.e., a diminution in the volume 

 of the arm. On opening the hand, the curve regains its level. 



