THE CIRCULATION 165 



bat as he continues his walk, and the peripheral arteries 

 dilate, it drops to 100 or less. Sudden or violent effort 

 raises 'the pressure abruptly (though briefly), and is a 

 fertile cause of apoplexy in persons whose tension is 

 abnormally high. Mental work also raises the pres mre 

 considerably, and those who use their brains much are 

 often the subjects of high tension. Excitement is an 

 even more potent elevator of blood - pressure, and 

 explains the frequency with which cerebral haemorrhage 

 is associated with anger ; hence the expression ' apo- 

 plectic with rage.' The possible disturbing influence 

 of excitement should always be borne in mind when 

 making blood-pressure observations on patients. Posture 

 is also of considerable influence, the pressure being 

 about 10 millimetres less when lying down than when 

 standing up. This will be referred to again when we 

 come to speak of the effect of gravity on the circulation. 



Cold contracts the peripheral arteries, and raises the 

 pressure, so that the first onset of cold weather in winter 

 is apt to be attended by a crop of apoplexies. Warmth 

 has the opposite effect. Hence during a spell of hot 

 weather an arterial pressure may appear quite normal 

 which in ordinary conditions would exceed the upper 

 physiological limit (Oliver). 



A meal has much the same effect on pressure as 

 exercise, raising it at first, and causing a fall later. 

 Sleep has the reverse effect, the pressure falling during 

 the first few hours and then rising gradually up to the 

 time of waking. 



The effect of all these influences is a transient one, 

 for, as has been already pointed out, the mean blood- 



