Elementary Anatomy of a Hound 41 



The nearer the blood-vessels approach the heart 

 the larger they become, the smallest being those 

 most remote. After passing through the body the 

 impure (purple) blood is returned to the heart by 

 the veins, these uniting to form two main trunks, 

 the anterior and posterior vencB cavce, which pour 

 their blood into the right auricle, and this now con- 

 tracts so as to squeeze the blood (still impure) into 

 the right ventricle, the contraction of which forces 

 the blood into the pulmonary artery, whose origin 

 is at the base of the right ventricle. It (the blood) 

 now passes through the lungs for purification by 

 contact with the oxygen taken in during inspiration, 

 and then returns by the pulmonary veins (carrying 

 arterial or bright scarlet (pure) blood) to the left 

 auricle — the point from which we started. 



Arteries convey blood from the heart, whereas veins 

 convey it towards the heart. 



The capillaries are networks of small blood-vessels 

 interposed between arteries and veins, forming a 

 breakwater system as it were. Most of the veins 

 have valves to prevent regurgitation of blood, 

 though the latter does occur in certain diseased 

 states. 



At the beginning of this paragraph two circulations 

 of the blood have been referred to, viz., the systemic 

 or greater and the pulmonary or the less. It is im- 

 portant to bear this in mind, though the two are 



