318 THE BLOOD AND ITS CIRCULATION 



have no ducts to carry off their secretions. These internal secre- 

 tions seem absolutely necessary for the health of the body. 

 Several glands, the thyroid, adrenal bodies, the testes, and ovaries, 

 as well as the pancreas, give off these remarkable substances. 



The Amount of Blood and its Distribution. — Blood forms, by weight, 

 about one sixteenth of the body. This would be about four quarts to a 

 body weight of 130 pounds. Normally, about one half of the blood of 

 the body is found in or near the organs lying in the body cavity below 

 the diaphragm, about one fourth in the muscles, and the rest in the 

 head, heart, lungs, large arteries, and veins. 



Blood Temperature. — The temperature of blood in the human body 

 is normally about 98.6° Fahrenheit when tested under the tongue by a 

 thermometer, although the temperature drops almost two degrees after 

 we have gone to sleep at night. It is highest about 5 p.m. and lowest 

 about 4 A.M. In fevers, the temperature of the body sometimes rises to 

 107° ; but unless this temperature is soon reduced, death follows. Any 

 considerable drop in temperature below the normal also means death. 

 Body heat results from the oxidation of food, and the circulation of blood 

 keeps the temperature nearly uniform in all parts of the body. 



Cold-blooded Animals. — In animals which are called cold-blooded, 

 the blood has no fixed temperature, but varies with the temperature of 

 the medium in which the animal lives. Frogs, in the summer, may sit 

 for hours in water with a temperature of almost 100°. In winter, they 

 often endure freezing so that the blood and lymph within the spaces 

 under the loose skin are frozen into ice crystals. This change in body 

 temperature is evidently an adaptation to the mode of life. 



Circulation of the Blood in Man. — The blood is the carrying 

 agent of the body. Like a railroad or express company, it takes 

 materials from one part of the human organism to another. This 

 it does by means of the organs of circulation, — the heart and 

 blood vessels. These blood vessels are called arteries where they 

 carry blood away from the heart, veins where they bring blood back 

 to the heart, and capillaries where they connect the larger blood 

 vessels. The organs of circulation thus form a system of con- 

 nected tubes through which the blood flows. 



The Heart ; Position, Size, Protection. — The heart is a cone- 

 shaped muscular organ about the size of a man's fist. It is 

 located immediately above the diaphragm, and lies so that the 



