CIRCULATION 



69 



its tube-like character is soon obscured, for it grows 

 faster than the space in which it is situated and becomes 

 twisted upon itself in such a way that four chambers are 

 formed. 



From the heart branch large tubes which become 

 smaller and smaller through repeated division. The 

 small tubes later reunite, forming larger and larger 

 tubes which lead back to the heart. The blood is thus 

 able to travel from the heart through the tubas to all 

 parts of the body and back to the heart, making a com- 

 plete circulation. 



Mechanical Factors which Control the Circulation. 

 (1) The heart. The passage of the blood through the 

 body is controlled by purely 

 mechanical features. The 

 heart furnishes the motive 

 power. It pushes the blood 

 forward through the tubes by 

 means of its rhythmical con- 

 traction. It is a hollow body 

 divided by a partition into 

 two unconnecting chambers 

 each of which is subdivided 

 into two connecting cham- 

 bers called respectively the 

 right and left auricle, and 

 the right and left ven- 

 tricle. Its walls are made 

 entirely of short, powerful 

 muscle cells so interlaced that they are strong enough to 

 send the blood throughout the body and firm enough to 



Fig. 45.— Muscle cells of heart. JB, 

 Connecting branch ; C, Cement 

 substance; JV, Nucleus. (From 

 Fitz.) 



