HUMAN BIOLOGY 



This aid makes the blood flow faster and relieves the heart of part of 

 its work, so that it beats faster, just as a horse might trot faster if 

 another horse helped to draw the load (Exp. 3). The pressure of a 

 contracting muscle upon an artery does not aid the blood flow in the 

 artery because the latter is destitute of valves. 



How Breathing aids the Heart. Breathing is a blood-pumping pro- 

 cess as well as an air-renewing process. When the chest expands, 

 blood is drawn into it. When the chest con- 

 tracts, the flow of blood away from it is aided. 

 As the chest expands, the downward pressure 

 of a great, broad muscle, the diaphragm (Fig. 

 74) compresses the liver, stomach, and other ab- 

 dominal organs, and forces the venous biood up- 

 ward into the expanding chest, thus helping it 

 on its wav to the heart. But if the abdominal 

 wall is weakened by tight lacing or by the pres- 

 sure of belts and bands which support the cloth- 

 ing, the weak abdominal wall yields to the 

 downward pressure of the diaphragm, and no 

 compression of the liver or aid to the circulation 

 will result. 



How the Blood Vessels are Controlled. Evi- 

 dently the biood vessels are not regulated by the 

 will. We cannot voluntarily increase the beat- 

 ing of the heart, or cause it to slacken its action. Even an actor cannot 

 cause his face to turn pale or to blush at will. This is because the 

 tiny muscles in the walls of the blood vessels are involuntary muscles. 

 They are controlled by nerves of the sympathetic system called vaso- 

 motors. They are not subject to the will (see Fig. 25). The nerve cen- 

 ter which controls the biood vessels is located in- the top of the spinal 

 cord at the base of the brain. When cold air strikes the skin the 

 nerves near the arteries are stimulated, the arteries in the skin contract, 

 .uid the skin turns white. When the heat from a hot fire strikes the 

 skin, the nerves are soothed, the arteries relax, and the face becomes 

 red. When the stomach is filled with food, the heart beats faster 

 and sends more blood to aid in digestion. When we run fast, the 

 heart beats fast to supply more blood to the muscles, but it slows down 

 as sleep comes on, that the body and brain may rest. 



Parts of the Blood. The blood which flows from a cut 

 finger seems to be a bright red throughout. When a drop 

 of it is looked at through a microscope, however, the 



FIG. 59. THE VEN- 

 TRICLES OF A 

 DOG'S HEART 



relaxed (above), 

 and contracted (be- 

 low). 



