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 ( ).. 



which arises 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 fibres of the heart muscles 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 th pas- 

 sage. The large truncus arteriosifs (the proxim,al portion of which is 

 called bulbus arteriosus), has two large branches called aorta'e ( ). 



___- inter-no I . /as? it 

 jne (fienle) 



Fig. 12. Femoral Nerve, Artery and Vein of Puppy. 



The truncus receives the blood as it is forced out of the heart when it 



^-ct>ntracts, and from whence it is distributed to all parts of the body. 



\Tjie sinus venosus on the dorsal surface of the heart is the'cavity into 



which the veins bring back the bloocT 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. 



