164 APPLIED PHYSIOLOGY 



as low as 90 millimetres Hg, with a diastolic pressure of 

 about 80. As age advances pressure tends to rise, on 

 account of lessened elasticity of the vessels. 



It must be noted, however, that the blood-pressure 

 differs from the body temperature, and resembles the 

 pulse rate in that it is not necessarily the same for all 

 healthy individuals of the same age. In some persons 

 and families the pressure tends to be habitually low; 

 in others it is always above the normal, and, just as in 

 the case of the pulse, there is no constant mental or 

 physical peculiarity which one can associate with these 

 differences. Seeing, however, that a high arterial 

 pressure must tend to determine a relatively large 

 amount of blood to the brain, owing to the uncontracted 

 cerebral arteries offering the path of least resistance 

 (see p. 175), one might expect an habitually high arterial 

 pressure to be favourable to the performance of mental 

 work. Families in whom such habitually high tension 

 is discernible are often * gouty,' and it is interesting in 

 this connection to remember that gout has been described 

 as the * disease of the intellectual ' the dominus mor- 

 borum, et morbus dominorum. 



There appears to be a slight diurnal variation of 

 pressure in health in the direction of a rise in the 

 morning and a fall at night, corresponding to the 

 morning fall and the evening rise in temperature and 

 pulse rate. 



Rest tends to lower pressure, whilst exercise increases 

 it at first, but ultimately lowers it. Thus, a man may 

 start out for a walk with a systolic pressure of 125, 

 which during the first half-hour may rise to 140 or 150 ; 



