200 



right the pressure in the post-tibial artery in the leg is higher than the 

 pressure in the brachial artery by the height of the column of blood 

 which reaches from one artery to the other, about 70 mm. Hg. 



In the horizontal posture the pressures are the same. With the 

 body in an L. -shaped position, or in the head-down posture, there is 

 also a difference in pressure between arm and leg. These differences 

 are well exemplified in the following figures. It will be noticed that 

 it is the leg pressure which alters, not the brachial to any great extent : 



In changes of posture, then, the pressure in the brachial artery 

 that is, in the root of the aorta is maintained at practically a con- 

 stant height by the tone of the splanchnic arterioles and action of 

 the respiratory pump. If the splanchnic arterioles are in an efficient 

 state of tone, and if the abdominal veins are supported by the tone 

 of the abdominal wall, then the splanchnic vessels will not dilate under 

 the hydrostatic stress of gravity. The nervous mechanism involved 

 is probably of the simplest nature, for if the arterial pressure sud- 

 denly rise or fall at the moment of change in posture, the bulbar 

 centres are thereby either directly or reflexly excited to increased or 

 decreased activity. A sudden fall of arterial pressure provokes 

 acceleration of the heart, amplified respiration, and increased vaso- 

 constriction. A sudden rise of pressure, on the other hand, provokes 

 a slow heart, shallow respiration, and vaso -dilatation. 



When the compensatory mechanism is abolished by destruction, 

 exhaustion, or inhibition of the bulbar centres, the circulation fails, 

 and becomes inadequate to maintain life in the vertical feet-down 

 posture. The blood passes into the capacious reservoirs of the tone-l 

 .less abdominal veins, the heart empties, and the cerebral circulation I 

 ceases. There can be no doubt that the control of this compensatory' 

 mechanism is one of the most important and necessary functions of 

 the group of bulbar centres a function which must have been evolved 

 to its highest point as man in his evolution assumed the erect posture. 



During the course of each day the compensatory mechanism 

 becomes exhausted ; especially is this so after severe muscular 

 exertion. By sleep the compensatory power is restored. In condi- 

 tions of neurasthenia, weakness and exhaustion after disease, shock 

 after severe injury or haemorrhage, this power may be almost entirely 

 lost. In this connection we must bear in mind the supply of ad- 



