THE HEART 



169 



.Connective 

 tissue. 



Epitlielial. 

 layer 



Muscle..^ 

 cells 



A vein 



Epithelial 

 layer. 



An artery 



A capillary 



which are ultimately passed out of 



the body by means of the lungs, 



skin, and kidneys. 



Circulation of the Blood in Man. 



— The blood is the carrying agent 



of the body. Like a railroad sys- 

 tem, it takes materials from one 



part of the human organism to an- 

 other. This it does by means of 



the organs of circulation, — the 



heart and blood vessels. These 



blood vessels are of three kinds : 



the arteries which carry blood away 



from the heart, the veins which 



bring blood back to the heart, and 



the capillaries which connect the 



smallest arteries with the veins. 



The organs of circulation thus 



form a system of connected tubes 



through which the blood flows. 



The Structure of the Heart. — The heart is a cone-shaped mus- 

 cular organ about the size of the fist. It is surrounded by a loose 



membranous bag called the pericar'dium, 

 the inner lining of which secretes a fluid 

 in which the heart lies. 



If we should cut open the heart of a 

 mammal down the midline, we could 

 divide it into a right and a left side, each 

 of which has no internal connection with 

 the other. Each side has an upper thin- 

 walled portion with a rather large in- 

 ternal cavity, the auWicle, which opens 

 into a lower portion with heavy muscu- 

 lar walls, the ven'tricle. Communication 

 between auricles and ventricles is regu- 

 lated by little flaps or valves. The 



auricles receive blood from the veins and pass it into the ventricles, 



which pump the blood into the arteries. 



Structure of artery, vein, and 

 capillary. 



Diagram of the heart, showing 

 the front half cut away. 



