THE VASCULAR SYSTEM. 159 



vein to the vertebral, so that the blood from the vein 

 may return partly by the latter vessel. 



The pulmonary veins convey the pure blood from the 

 lungs to the left auricle. There are three main trunks 

 from each lung. These six veins are arranged in pairs 

 (Fig. 69), constituting a left pair from the left lung; a 

 median pair, composed of one branch from the right and 

 one from the left lung; and a right pair from the right 

 lung. Each pair forms a common trunk before entering 

 the auricle. In order to demonstrate these veins one 

 must remove the heart and lungs from an injected cat, 

 and carefully dissect away the fatty and connective 

 tissues enveloping the vessels. 



The peripheral connection between the arteries and 

 veins is by means of capillaries, which are microscopic 

 vessels with an extremely thin wall composed of a single 

 layer of epithelial cells. These capillaries are so abun- 

 dant everywhere in the flesh that a needle cannot be 

 inserted without penetrating some. They form a kind of 

 mesh or network, so that every cell may be supplied 

 with oxygen and food and discharge its waste matter. 

 All the blood carried to any portion of the body by the 

 arteries is not returned by the veins, as a considerable 

 amount of the plasma and some of the white corpuscles 

 escape through the thin capillary walls and are returned 

 to the circulation by a system of vessels called lymphatics 

 (Fig. 78). 



PRACTICAL QUESTIONS AND SUGGESTIONS. 



1. What holds the heart in place? 



2. Draw the ventral aspect of the heart with the pericardium re- 

 moved and label all features. 



3. When the heart is detached from the body, how do you distin- 

 guish the dorsal from the ventral aspect? 



4. How many veins open into each auricle? 



5. What arteries lead from each ventricle? 



