INTRODUCTORY. 13 



circulating through the system. The nutritious fluid thus 

 added to the blood is gradually developed, and has imparted 

 to it the vital properties of the blood, and in due time is in a 

 fit state to become food for the tissues or to repair the loss 

 caused by the death of microscopical cells. 



11. The Circulatory System. This system carries the 

 blood back and forth from the heart to every part of the 

 body. It consists of the heart, arteries and veins, and hair- 

 like canals called capillaries. A second circulatory system 

 carries the blood to and from the lungs to the heart. The 

 first, or long circuit, is to feed the tissues and remove the 

 worn-out material ; the second, or short circuit, through the 

 lungs, is to cast out of the system the products of decay and 

 death of tissue, and to receive from the air taken into the 

 lungs the oxygen without which life cannot exist. 



12. The Respiratory System. More immediately neces- 

 sary for life than the circulation is the process of breathing, 

 and the two lungs, which in the lower animals are called the 

 lights, occupy a large space in the upper part of the body, 

 called the thorax. The bronchial tubes, which extend from 

 the trachea, or windpipe, to the inner surfaces of the lungs, 

 communicate with the external air through the nose. 



13. The Excretory System. Besides the lungs, there 

 are other organs, whose function it is to carry out of the 

 system various elements not only useless to sustain the body, 

 but more or less injurious to the process of life. These are 

 the intestines, the kidneys, and the skin. 



14. Alcohol, Tobacco, etc. This brief survey of the 

 human body, and the manner in which the functions of life 

 are carried on, will prepare us to consider more particularly 

 the structure of the various tissues and organs ; how they are 

 nourished and sustained, and what are the requirements to 

 keep the body in health. We will endeavor to show that 

 perfect health depends upon the care we give the body, the 



