PRACTICAL EXERCISES 195 



(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 applying 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 \vith 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. 



21. 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 

 observation 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}. 



22. Blood-pressure Tracing. (a) Put a dog under morphine (p. 55). 

 Set up an induction machine arranged for an interrupted current 

 (Fig. 81, p. 184). 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 

 size of the float properly adjusted to that of the tube, this is not 

 necessary, and is to be avoided. Then, tilting the tube carefully, 

 fill the proximal limb (i.e., the limb which is to be connected with 

 the bloodvessel) with a saturated solution of sodium carbonate or 

 a half-saturated solution of magnesium sulphate, or what is better 

 for most purposes, a 2 per cent, solution of sodium citrate. This is 

 easily done by means of a pipette furnished with a long point. Now 

 attach a strong rubber tube to the proximal end of the manometer, 

 and fill it also with the solution. All air must be got out of the 

 manometer and its connecting-tube. Raise the end of the rubber 

 tube and blow into it, so as to cause a difference of about 10 cm. in 

 the height of the mercury in the two limbs of the manometer, and, 

 without releasing the pressure, clamp the tube with a pinchcock or 

 screw clamp (Fig. 34, p. 101). 



Now smoke a drum, and arrange the writing-point of the mano- 

 meter-float so that it will write on it. Suspend a small weight by a 

 piece of silk thread from a support attached to the stand of 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. 



132 



