LXni.] CAPILLARY BLOOD-PRESSURE. 305 



(h.) In every kymograph tracing, notice the smaller undulations 

 due, each one, to a single beat of the heart, and the larger ones due 

 to the respiratory movements (fig. 229). In a blood-pressure trac- 

 ing taken from a dog with the vagi not divided, observe that tlie 

 size of the heart-beats on the descent of the respiratory wave is 

 greater, while the number of beats is less than on the ascent. 



(i.) Study blood-pressure tracings obtained by stimulation of 



(i.) The peripheral end of tlie vagus (fig. 229). 

 (ii.) The central end of the depressor, 

 (iii.) The central end of a sensory nerve. 



FlO. 229.— 5. p. Blood-pressure tracing of dog's carotid, stimulation of the vagus at 

 the indent in the line S ; T indicates time in seconds, and is the abscissa. 



6. Make a Glass Cannula. —Heat in the flame of a blowpipe a piece of hard 

 glass tubing about 5 mm. in diameter. When it is soft, take it out of the 

 flame, draw it out gently for about 3 cm. Allow it to cool ; make the gas-jet 

 smaller, heat the thin drawn-out part of the tube, and draw it out very slightly. 

 This makes a shoulder. With a triangular file just scratch the narrow part 

 obliquely beyond the second constricted part, and break it off. A cannula 

 with a shoulder and an oblique narrow orifice is thus obtained. Round off" the 

 oblique edges either by a file, rubbing them on a whetstone, or heating slightly 

 in a gas-flame. Tie a piece of india-rubber on the other end, and the cannula 

 is com[)lete. Instead of a straight glass cannula with a shoulder, the form 

 shown in fig. 230 may be used. It has a lateral tube, which is closed by means 

 of a caoutchouc tube, and is useful in this respect, that the large bulb prevents 

 clotting of the blood, while if clotting does occur, the clot can readily be 

 washed out by means of the pressure bottle through the lateral tube. 



6. Effect of Vagus on Heart. — The student is not permitted to do this 

 experiment on a living animal It can, however, bo shown on a rabbit or cat 



U 



