58 



General Biology 



right and one on the left side, and a muscular ventricle. There is also 

 a thick-walled tube called the truncus arteriosus ( ) 



arising from the base of the ventricle, and a thin-walled triangular sac, 

 the sinus venosus ( ) from the dorsal side. The 



heart is the central pumping station of the entire circulatory system, 

 which furthermore consists of all the arteries, veins, and lymphatic 

 structures in the body. Arteries always carry blood away from the 

 heart ; veins, to the heart. The fibers of the heart muscle run in every 

 direction, so that in systole, ( ) that is, when the 



heart contracts, its size is diminished, and the blood in the various cavi- 

 ties is forced out ; then in diastole, ( ) when the 

 heart again expands, the blood flows into it. The openings of both the 

 auricles and ventricles are guarded by valves, little flaps of membrane 

 which permit the blood to flow through the opening quite readily, but 

 close up when the blood begins to flow backward, as it would be bound 

 to do when the ventricle contracts, if the valves did not block the 

 passage. The large truncus arteriosus (the proximal portion of 

 which is called bulbus arteriosus), has two large branches, called 



Jfch entity 



Membrane (tfente) 

 fott'rnq 



Wedi* ■ 



/Free far Casrvttt/m 



Fig. 12. Femoral Nerve, Artery and Vein o£ Puppy. 



aortae ( ). The truncus receives the blood as it is 



forced out of the heart when that organ contracts. From here it is dis- 

 tributed to all parts of the body. The sinus venosus on the dorsal 

 surface of the heart is the cavity into which the veins bring back the 

 blood from all parts of the body. The sinus itself opens into the right 

 auricle and thus receives all the blood which flows back to the heart 

 from all parts of the body, except the lungs. 



