THE CIRCULATION 



the lighter color where a fresh cut is made ? Blood 

 going from the lungs to the tissues is bright red, about 

 the color of a piece of new beef when the knife cuts into 

 it. The blood going from the tissues to the lungs is 

 darker, about the color of the outside of the piece of raw 

 beef some time after it is cut. 



143. The heart is situated in the chest, between the two 

 lungs. It is a hollow muscle, and has the remarkable 

 power of contracting and relaxing itself with periodical 

 regularity. The movement of contraction is called the 

 systole ; the relaxation is called the diastole. 



144. The heart is divided by a vertical partition into 

 halves (Fig. 84). The right half receives the dark blood 



PULMONARY 



FIG. 84. Diagrammatic Section of Heart. 



from the body and sends it to the lungs. The left half 

 receives the bright blood from the lungs and sends it to 

 every part of the body. Each half is also divided by a 

 horizontal partition, but unlike the vertical ones, these par- 

 titions are pierced with openings for communication. The 

 heart therefore contains four chambers ; the two upper 

 ones are called auricles, the two lower ones ventricles. 

 The right auricle communicates with the right ventricle just 



