396 FOODS CIRCULATED AND USED IN THE BODY 



from it. The upper chambers are called the right and left auricles 

 respectively ; the lower chambers, the right and left ventricles. Which 

 have the thicker walls? What is probably the use of these walls? 



Notice the position of the valves and the direction of their move- 

 ment. In what direction do arteries lead? Veins? 



Do the chambers all connect with one another? Write a paragraph 

 describing the structure of the heart. 



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 



artery to right lung-i 

 vein 



right 

 auricle 



right 

 ventricle. 



vein. 



,artery to 



ery to 

 le/ft lurng 



__, veins from 

 lurigs 



left 

 _- auricle 



left 

 ..ventricle^ 



.aor-toc 



How does the blood get from the left ventricle to the right ventricle ? 



membranous bag called the pericardium, 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 within the heart no connec- 

 tion with the other side. 



Practical Exercise 3. To make an apparatus that will demonstrate the fact 

 that the heart is a double force pump. 



The heart in action. The heart is constructed on the same 

 plan as a force pump, the valves preventing the reflux of blood 

 into the auricles when it is forced out of the ventricles. Blood 

 enters the auricles from the veins because the muscles of that part 

 of the heart relax; this allows the space within the auricles to 



