CIRCULATION OF BLOOD AND OF LYMPH 161 



vision, too; for the different organs of the body need different 

 amounts of blood at different times, and if this blood supply 

 were dependent on our conscious regulation of it, we could 

 accomplish very little else, and even that function would be 

 fmperfectly performed. 



The dilator influences, however, do not arise in the same 

 place as do the constrictor; the nerves which carry messages 

 causing constriction of blood vessels rise in the medulla, or 

 hind brain. From there they go down the spinal cord, and 

 are thence distributed in various directions. The vaso-con- 

 strictor center, therefore, is in the medulla, or hind portion of 

 the brain. No definite place can be determined, however, as 

 the centre from which messages go out to cause vessels to en- 

 large. Some fibres apparently arise in the vaso-constrictor 

 center, while others do not. 



IMPORTANCE OF A VIGOROUS CIRCULATION 



We seldom realize the fact that the whole body is traversed 

 in every direction by arteries, veins and capillaries and that 

 blood, a living stream under considerable pressure, is running 

 through it every minute of our lives. 



A swift brook, dashing down a steep hill, takes along with 

 it everything but the large pebbles and stones; all the loose 

 mud and dirt are carried along, and are left behind only when 

 the stream becomes sluggish and slow. Then the materials 

 in the muddy water catch on the grass and sticks in the stream, 

 and everything under the water is covered with dirt and refuse. 

 So when the blood flows strongly and steadily, it can pick up 

 many of the wastes of the body and carry them away. This, 

 however, is not due solely to the swiftness of .the stream, but 

 because, by means of this rapid flow, more blood is brought to 

 and carried away from every spot in the body; and the more 

 blood there is coming into contact with any tissue, the more 

 effectually that tissue is fed and the more perfectly it is 

 cleansed of its waste debris. In the business world there is 



