336 The Outline of Science 



Every year, indeed, brings new discoveries of a remarkable 

 kind. We have already noticed that certain chemicals called 

 "hormones," of which we shall speak more fully immediately, 

 are produced in various structures (ductless glands) of the body, 

 and "posted" in the blood, as it were, for a distant organ. One 

 of these "hormones" comes into play in connection with the blood. 

 When a man is setting about some prolonged and strenuous 

 exercise, nerve-messages go to stimulate certain glands near the 

 kidneys called the adrenal or suprarenal glands. They supply 

 one of these chemicals (adrenalin) to the blood, and it passes 

 round the circulating system until it reaches the small arteries. 

 It closes the cocks and shuts down the supply to organs which 

 are not at the time required to be active, and thus ensures a fuller 

 supply to those organs which are called into strenuous exercise. 



When the blood has passed through the tissues given up 

 its nutriment and received the waste carbonic acid gas and the 

 soluble nitrogenous waste the blood turns back towards the 

 heart. It passes into a new set of fine tubelets or capillaries, and 

 these unite in the veins. The veins have thinner walls than the 

 arteries, as there is now less pressure, but they have a remarkable 

 series of valves along their course. The blood cannot flow back 

 cannot go wrong. You can actually trace one or two of the 

 valves on the veins of your arms if you try to force the blood 

 back to your fingers. Little knots stand up here and there. So 

 the blood courses steadily back and is poured into the opposite 

 side of the heart to that from which it started. It enters the 

 right auricle, and passes on into the right ventricle ; and the next 

 beat of the heart sends it to the lungs, where it gives up its 

 carbonic acid and gets a fresh supply of oxygen. 



7 



How and Why We Breathe 



The blood has many functions. It takes fluid food to the 

 organs and, in its red corpuscles, it carries oxygen. It has also 



