The Animal Body 109 
of the body, but to assist the bones in giving pro- 
tection to some of the more delicate structures and 
organs. Within the body is the digestive system, which 
receives and prepares the food for the use of the 
millions of cells which constitute the living body. To 
earry this prepared food to these cells is the work of the 
circulatory system, composed of the heart and the biood- 
vessels. The arteries carry the blood from the heart to 
the different parts of the body, the minute capillaries 
distribute it through the tissues, and the veins collect 
the impure blood and carry it back to the heart. 
Thence it passes to the lungs, where some of the 
waste matters are removed and cast out in the exhaled 
air. The respiratory system consists of the lungs, 
with their infinite number of air-cells, and the pas- 
sages leading to them—the windpipe and its branches 
—through which pure air is taken in and impure air 
breathed out. : 
The urinary system, consisting of the kidneys and 
bladder and the tubes connecting them, serves the pur- 
pose of taking waste matter from the blood and excret- 
ing it from the body. Closely associated with this 
system is the reproductive system, which consists, in 
the female, of the uterus, or womb, and ovaries, and 
in the male, of the testicles, with the passages leading 
from them. The function of this system is the re- 
production of the species. 
Covering the outside of the body is the skin. Its func- 
tions are to protect the body, to throw off waste ma- 
terials in the perspiration or sweat, and to serve as an 
organ of feeling or sensation. The nervous system con- 
