274 HUMAN PHYSIOLOGY 





amount can vary only at the expense of the cerebro-spinal 

 fluid. This is small in amount, and permits of very small 

 variations in the volume of blood. Increased arterial pres- 

 sure in the body does not therefore increase the amount of 

 blood in the brain, but simply drives the blood more rapidly 

 through the organ. There seems to be no regulating nervous 

 mechanism connected with the arteriolcs of the brain, and 

 the cerebral pressure simply follows the changes in the 

 general arterial pressure. The splanchnic area is the great 

 regulator of the supply of blood to the brain. Since the 

 cerebral arteries are supported and prevented from dis- 

 tending by the solid wall of the skull, the arterial pulse 

 tends to be propagated into the veins, and in these veins 

 the respiratory pulse is very well marked (Fig. 129). 



2. Circulation in the Lungs. Vaso-motor nerves seem 

 to be absent, and hence drugs like adrenalin fail to cause a 

 constriction of the arterioles. The amount of blood in the 

 lungs is regulated by the blood pressure in the systemic 

 vessels. 



3. Circulation in Heart Wall. A peripheral vaso-motor 

 mechanism is not present in the arterioles of the coronary 

 vessels (see also p. 231). 



Extra-Cardiac Factors Maintaining Circulation. 



In considering the flow of blood through the vessels, due 

 to the distribution of pressure in arteries and veins, it must 

 be remembered that the central pump or heart is not the 

 only factor maintaining it (Fig. 129). 



The thorax is to be looked upon as a suction pump of 

 considerable power, which draws blood into the heart during 

 inspiration. Again, the abdominal blood vessels are to be 

 regarded as the great blood reservoir, and when the abdo- 

 minal muscles are tightened and the respiratory movements 

 of the thorax are increased, as in the panting which accom- 

 panies intermittent muscular exercise, the blood is partly 

 pressed, and partly sucked from the abdomen into the heart, 

 and so forced on into the arteries. Even expiration helps 

 in this, for the blood which has filled the vessels of the lungs 

 in inspiration is driven on into the left side of the heart in 



