98 ELEMENTS OF PHYSIOLOGY 



pounds of blood, for the blood is T ^ of the total weight of 

 the body. There is not enough blood in the body to dis- 

 tend all of the blood vessels at once. The skin alone with 

 all of its blood vessels distended could contain two thirds 

 of all the blood in the body. The veins have twice the 

 capacity of the arteries ; they could contain every drop 

 of blood in the body. When the brain works, it requires 

 more blood. When digestion is in progress, the lining of 

 the digestive organs blushes a rosy red and the digestive 

 fluids are poured out. During the digestion of a hearty 

 meal, one will not do his best thinking. When a muscle 

 is used, the dilator nerves act, the blood tubes in the 

 muscle become enlarged, and its supply of blood increases 

 to serve it during its. action. The impulses that run 

 along the vasomotor nerves arise in the enlargement at 

 the top of the spinal cord called the spinal bulb. The 

 part of the spinal bulb that in this way regulates the 

 caliber of the arteries is called the vasomotor center. It 

 is constantly sending impulses along the constrictor fibers 

 so as to keep the muscles of the arteries slightly con- 

 tracted. The vasomotor center thus keeps a rein upon 

 the arteries, holding them in a condition of tone, as this 

 slight contraction is called. Sudden paleness, due to fear, 

 is brought about by extra strong impulses from the vaso- 

 motor center, causing the muscular walls of the small arter- 

 ies of the face to grip the vessels tight and drive the blood 

 from the face. Alcohol destroys the tone of the blood 

 tubes. From temporary drinking the face becomes red ; 

 from habitual drinking the " rum blossom," the purple 

 swollen blood tubes in the nose, results. 



162. How the Flow of Blood through the Whole Body 

 is regulated. The amount of blood passing through one 

 organ may be increased by the vasomotor nerves, but it 

 is only because the amount going through the enlarged 

 vessels of that organ has been withdrawn from other 



