PRACTICAL EXERCISES i& 5 



19. Blood-pressure Tracing. (a) Put a dog under morphia (p. 58). 

 Set up an induction-machine arranged for an interrupted current 

 (Fig. 65, p. 175). Fill the U-shaped manometer-tube (if this has not 

 already been done) with clean mercury to the height of 10 to 12 c.m. 

 in each limb. Put half an inch of oil above the mercury in the distal 

 (straight) limb before putting in the float. This helps to keep the float 

 from sticking. 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. 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. 28, p. 99). 



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 

 drum, so that it hangs down outside of the writing-point of the 

 manometer-float and always keeps it in contact with the smoked 

 surface without undue friction. Or a piece of glass rod drawn out to 

 a fine thread in the blowpipe flame answers very well. Below the 

 writing-point of the float, and in the same vertical line with it, adjust 

 the writing-point of a time-marker beating seconds (Fig. 60, p. 170). 



Next, fasten the animal on a holder, back down. Give ether and 

 insert a tracheal cannula (p. 177). (The tracheal cannula is not 

 absolutely required for the experiment, but it is convenient, as the 

 animal is more under control, and artificial respiration can be begun 

 at any moment, should this be necessary.) Insert a glass cannula, 

 armed with a short piece of rubber tubing, into the central (cardiac) 

 end of the carotid artery (p. 58). Leaving the bulldog forceps on 

 the artery, fill the cannula and tube with the magnesium sulphate or 

 sodium carbonate solution. Slip the rubber tube over a short glass 

 connecting-tube. Fill this also with the solution, and connect it 

 with the manometer-tube, seeing that both are quite full of liquid, 

 so that no air may be enclosed. Now take off the bulldog forceps, 

 and allow the drum to revolve at slow speed. The writing-point of 

 the manometer float will trace a curve showing an elevation for each 

 heart-beat, and longer waves due to the movements of respiration. 



(6) Isolate the vago-sympathetic nerve in the neck. Ligature 

 doubly, and cut between the ligatures. Stimulate the peripheral 

 (lower) end, the heart will be slowed or stopped, and the blood- 

 pressure will fall. Stimulate the central (upper) end ; there may be 

 inhibition of the heart or acceleration, and the pressure may fall or rise. 



(c) Expose and divide the other vago-sympathetic while a tracing 

 is being taken. Again stimulate the central end of the nerve, and 

 observe whether there is any effect. 



(d) Expose the sciatic nerve in one leg, as follows. The leg having 

 been loosened from the holder, the foot is seized by one hand and 



