244 EXPERIMENTAL PHYSIOLOGY. 



be traced upwards, a ganglion will be found on it. This is 

 merely to be regarded as an exercise for practice. 



(e.) In every case a base-line or line of no pressure must 

 be recorded on the continuous paper. This indicates the 

 abscissa, or when the mercury is at the same height in the 

 two limbs of the manometer. 



(/) Measure a blood-pressure tracing provided for you. 

 Lay the tracing on a table. Take a right-angled triangle 

 made of glass or wood, and place one of the sides bounding 

 its right angle upon the abscissa, the other side at right 

 angles to this has engraved on it a millimetre scale. Read 

 ort' the height in millimetres from the base-line to the lowest 

 point in the curve, and also to its highest point, take the 

 mean of the two, and multiply by two, this will give the 

 mean arterial pressure. Instead of measuring only two 

 ordinates, measure several, and take the mean of the number 

 of measurements. In all cases the result has to be multi- 

 plied by two. 



(g.) Study blood-pressure tracings obtained by stimula- 

 tion of 



(i.) The peripheral end of the vagus. 

 (iU The central end of the depressor, 

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



(7i.) In every kymograph tracing, notice the smaller undu- 

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

 larger ones due to the respiratory movements. In a blood- 

 pressure tracing taken from a dog with the vagi not divided, 

 observe that the 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. 



2. To Make a Cannula. Heat in the flame of a blow-pipe 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 



