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EXPERIMENTAL PHYSIOLOGY 



are not so conspicuous as in fig. 135. This is due to the smaller 

 magnification obtained in this instance. The heart beats are, however, 

 more conspicuous and show greater oscillations than those caused by 

 respiration. If measured they are seen to be lh mm. high in a total 

 blood pressure measured by 16^ mm., i.e. the variation caused by 

 each beat is T xth of the mean pressure. If we make similar measure- 

 ments in the trace yielded by the mercury manometer we find each 

 beat is about 1 mm. high in 75 mm. mean pressure, i.e. the variation 

 in pressure recorded as due to each beat is T l -,th of the mean pressure. 

 Hence in this particular experiment the Hiirthle manometer was seven 

 times more sensitive in recording rapid variations of pressure than the 

 mercury manometer. Tracing n of fig. 148 is a further illustration 

 of the different effects of normal and artificial respiration upon the 

 blood pressure. This piece of tracing also demonstrates the fact that 

 in artificial respiration a fall of pressure is produced by inflation of 

 the lungs. The production of this result we have already explained 

 on p. 174. 



In fig. 149 is reproduced a tracing of the effect of stimulation of 

 the peripheral end of the vagus of sufficient strength to cause slowing 



Fig. 149. — Stimulation of the Periphebal End of the Vagus. Rabbit. 

 Hurthle Manometer. The Uppeb Horizontal Line marks Zeeo Pbessure. 



of the beat but not stoppage. This should be contrasted with those 

 given in figs. 140 and 141. 



The most interesting results to be gained by the use of this instru- 

 ment are, however, to be seen when the record is more extended by 

 making the movement of the blackened surface more rapid. Such a 

 record is given in fig. 150. It shows on the blood-pressure tracing 

 practically the same variations as those obtained in a sphygmogram 

 (see fig. 131, p. 167). Thus from a to b is the rise in pressure due to 



