rno^ 



232 ESSENTIALS OF PHYSIOLOGY. 



viscera, and the total capacity of these vessels is so large that the 

 amount of blood contained in them forms a great proportion of the 

 total blood in the body. Further, the general arterial pressure is more 

 markedly altered by section or stimulation of the splanchnic nerves, 

 than of any other nerve in the body. 



Hence the maintenance of the mean arterial pressure at a constant 

 level, in spite of the varying influences which are brought to bear upon 

 the vaso-motor centre in daily life, is largely effected by changes in 

 the degree of constriction of the arterioles supplied by the splanchnic 

 nerves, and known as the splanchnic area. For example, when the 

 depressor nerve is stimulated, the fall of pressure which occurs is due 

 mainly to the dilatation of the arterioles in the splanchnic area. When 

 the blood supply to the centre is deficient, the resulting rise of blood 

 pressure is caused primarily and chiefly by constriction of the arterioles 

 of the abdominal organs. During muscular exercise an increased flow 

 of blood through the skeletal muscles is required and takes place, and 

 the vessels in the splanchnic area are constricted, more blood being 

 diverted into the muscular system. On the contrary, during digestion 

 the digestive organs require an abundant blood supply, and the vessels 

 of the skin are contracted, while the arterioles of the digestive tract are 

 relaxed. It is for this reason that severe exercise taken immediately 

 after a meal tends to disturb digestion. 



When the arterioles of the abdominal organs constrict in response to- 

 stimulation of the splanchnic nerve, they naturally contain less blood 

 than before, and the blood thus squeezed out of them by their con- 

 striction has to be accommodated in other parts of the vascular system. 

 Much of it passes into the arteries, thereby distending them more fully, 

 and so raising the arterial pressure. This factor is an additional 

 cause of the rise of blood pressure which is brought about by stimula- 

 tion of the splanchnic nerves. 



Influence of Gravity on the Circulation. If a thin- walled, 

 cylindrical rubber bag is filled with fluid and held with its long axis 

 vertical, the fluid, under the influence of gravity, tends to accumulate 

 at, and to distend, the lower end of the bag. In the body also, the 

 blood tends to accumulate in the most dependent parts, and if a rabbit 

 is held up by its ears the blood accumulates in the abdominal area, 

 particularly in the large veins, and the arterial blood pressure falls 

 (fig. 91). As a result, the amount of blood passing through the brain 

 in a given time is inadequate, and its functions are seriously interfered 

 with, so much so that it is said to be possible to kill a hutch rabbit by 

 holding it up in this position for a short time, death being due to 

 ansemia of the brain. 



