304 



PRACTICAL PHYSIOLOGY. 



[LXIII. 



(e. ) In every case a base line or line ot 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. 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. Or use a millimetre scale as in fig. 228. Read off the height 

 in millimetres from the base line to the lowest point in the curve 



FiG. 228. Blood-Pressure Tracing of the Carotid of a Dog, taken with Ludwig's 



Mercurial Manometer. 



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 multiplied by two. 



(y.) Measure the blood-pressure tracing (fig. 229) of the carotid 

 of a dog from the base line T. It represents the effect of stimula- 

 tion of the vagus, and the arrest of the heart-beat, and the con- 

 sequent great fall of the blood-pressure. 



