204 VERTEBRATES 



and the left subclavian veins, and crosses at top of left 

 lung, emptying into the i^ight auricle. Make a diagram of 

 the venous circulation as now shown. 



Follow the left jugular vein and find, near its entrance 

 into the vena cava, a large white blood vessel opening into 

 it. This is the thoracic duct, which contains a large part of 

 the drain of the body and the unassimilated food from the 

 intestines. Trace it back to the mesenteries below the liver. 

 It is the principal duct of a great system of drainage and 

 cleansing, which works over the partially waste matter and 

 part of the new food by passing it through glands called 

 lymphatic glands. The pink gland just anterior to the 

 heart, the thymus, is one of them. The circulation through 

 this cleansing system is known as the lymphatic circulation. 



Find the dark blood vessel running from the heart to the 

 lungs. Where does it seem to leave the heart? It is called 

 the pulmonary artery. Find the place of division into right 

 and left pulmonary arteries. How many branches of these 

 arteries do you find in the lungs? Do you find returning 

 blood vessels from the lungs to the heart? They may be 

 distinguished in a specimen not injected, by a lighter color 

 and thinner walls. Trace these returning veins, pulmonary 

 veins, from both lungs into the heart. How many ? Where 

 do they enter? Make a diagram of the circulation of the 

 blood in the lungs. What work is done by the lungs? 



Tie the posterior vena cava between the heart and liver 

 in two places, about a half inch apart, and sever it between 

 the ties so as to prevent bleeding. In the same way tie the 

 vena cava above the kidneys. Remove the liver. 



Carefully cut away the thymus gland from above the 

 heart and expose the arteries, which are more firm in tex- 



