172 APPLIED PHYSIOLOGY 



reaching from the top of the head to the sole of the 

 foot, is equal to 140 millimetres Hg (Hill). In order 

 to meet the greater strain to which the walls of the 

 bloodvessels of the lower limbs are subjected in conse- 

 quence of the haemostatic pressure, they are provided 

 with stouter walls than those of the upper part of the 

 body. 



As the vascular system is not filled to distension with 

 blood, its contents would tend to sag down into the most 

 dependent parts when the body is upright were there not 

 some mechanism provided to prevent its doing so. As 

 a matter of fact, such a mechanism is provided by the 

 vasomotor system, for when the erect position is assumed 

 the fall of pressure in the medulla stimulates the vaso- 

 motor centre and the splanchnic set of vessels, and those 

 of the lower limbs are tightened up to prevent ' pooling ' 

 of the blood. The tension exerted by the abdominal 

 muscles in standing and the action of the respiratory 

 group aid in this process. 



If the abdominal muscles are very flaccid, however, as 

 they often are, for example, in women, or if the vaso- 

 motor centre be exhausted and irresponsive, the sagging 

 down of blood under the action of gravity is able to take 

 place to a considerable extent, and faintness, or even 

 syncope, may ensue. There can be no doubt, indeed, 

 that the ability to support a standing position for a long 

 time without faintness depends upon the efficiency of 

 the counteracting mechanism. In feeble subjects, in 

 conditions of great fatigue, and after long confinement 

 to bed, the efficiency and promptitude of the mechanism 

 are impaired, and in such circumstances if an individual 



