302 GENERAL ZOOLOGY 



metabolic processes, though they take no direct part in 

 the digestion of food. 



The circulatory system of man is a highway for the 

 transfer of food, waste products, gases, and the leucocytes 

 which destroy undesirable materials. The food in the 

 liquid portion of the blood is largely protein. Carbohy- 

 drates and fats are stored in convenient places within the 

 body and enter the blood in small quantities as needed. 

 The red corpuscles carry oxygen from the lungs to the 

 tissues, and the blood returns to the lungs laden with carbon 

 dioxide. The heart pulsates rhythmically for the entire 

 life of a man, without rest, except what it snatches between 

 beats. It forces blood through the arteries to all parts 

 of the body, and is continually filled from the veins. 

 Though the rhythm of pulsation is under the control of the 

 nervous system, the tendency to beat is inherent in certain 

 portions of heart muscle and movements may be induced 

 or modified by the presence of certain salts in the blood or 

 by other stimuli. Through the delicate walls of the capil- 

 laries, which connect the arteries and veins, food and 

 oxygen are supplied to the tissues and waste material 

 enter the blood. Though the amount of blood passing 

 through the heart may be the same at different times, the 

 vasomotor nerves vary the diameter of the small vessels 

 so that different parts of the body have more or less blood 

 as their necessities demand. 



The respiratory system is concerned primarily with the 

 supplying of oxygen to the tissues and the elimination of 

 carbon dioxide. Air is drawn into the lungs w 7 hen the space 

 in the thoracic cavity is increased by the contraction and 

 descent of the diaphragm together with the raising of the 

 ribs by the shortening of the muscles between them. 

 Four hundred and forty liters (888 gallons) of blood pass 

 through the lungs each day. To aerate this an adult man 

 requires about 85,000 liters (3000 cubic feet) of air per hour. 

 The atmosphere may contain 1 to 2 per cent, of carbon 



