170 APPLIED PHYSIOLOGY 



This local control of the blood- supply is apt, as might 

 be expected, to become deranged with resulting disease. 

 Sometimes, for example, as in the syncopal stage of 

 Raynaud's disease, the arterioles pass into a state of 

 spasm, and the blood-supply to the part is entirely cut 

 off, so that it goes white and cold. A similar state of 

 things on a larger scale occurs in the condition known 

 as ' intermittent limp.' On the other hand, a permanent 

 dilatation of the arterioles is the cause of the local capillary 

 congestion known as erythromelalgia. In neurasthenic 

 subjects the local control of the circulation by the vaso- 

 motor system seems to be peculiarly feeble, and many 

 of the symptoms of which they complain, such as throb- 

 bings and flushings, and probably many more obscure 

 symptoms as well, are the result. 



That the veins of a part as well as its arteries are 

 subject to nervous control there can be little doubt. In 

 no other way could one explain, for example, the spasm 

 of veins which seems to be the cause of the asphyxial 

 stage of Raynaud's disease. Both the inflow and outflow 

 of blood, therefore, can be regulated through the nervous 



system. 



/ 



The Influence of Gravity upon the Circulation. 



The pressure effects exerted by the influence of gravity 

 on the contents of the bloodvessels may be spoken of as 

 the h&mostatic pressure, as opposed to the luemodynamic 

 pressure which results from the action of the heart. The 

 actual pressure at any point of the vascular system is 

 equal to the sum of these two (see Scheme). In a man 

 of 6 feet the hydrostatic pressure of a column of blood, 



