THE BLOOD 123 



the heart so easily. The flow of blood 

 towards the heart is also favoured by muscular 

 movements in the limbs pressing on the thin 

 walled veins, and as these are provided with 

 valves opening towards the heart, the blood 

 must flow onwards. Pressure on the veins 

 of the organs in the abdomen must also assist, 

 and so great is the capacity of the circulatory 

 system in the abdominal and pelvic cavities 

 that a quantity of blood equal to all the blood 

 in the body might be therein contained. If 

 there is more blood in one part of the body 

 there will be less in another. An adjustment 

 of local circulations is constantly going on, 

 according to the degree of physiological 

 activity of one organ or another. If there is 

 a large supply of blood to the abdominal 

 viscera, as during digestion, or to the skin, 

 as when exposed to heat, there will be less 

 blood in other internal organs. This may in 

 part account for the mental lethargy after a 

 full meal, and for the lassitude one feels 

 during hot weather. 



64. The circulation of the blood is thus 

 carried on in accordance with the physical laws 



