780 COMPARATIVE ANATOMY 



within the blood vessels, and lymph as soon as it has seeped through the 

 walls of the blood vessels and bathed the surrounding tissues. It is 

 gathered up from here by the various lymphatic vessels which unite to 

 form the large lymph duct. This duct empties into one of the veins of 

 the neck. The blood which has remained within the blood vessels and 

 passed through the capillaries is taken up by the venous capillaries and 

 passes toward the heart either directly or indirectly through a portal 

 system. 



It is essential that one appreciate that the arteries supply all parts 

 of the body with nourishment and that the veins do the draining. It 

 follows, then, that the arteries begin as vessels of some size and become 

 smaller and smaller as the blood supply from the heart becomes dis- 

 tributed more and more, while veins begin as capillaries and continu- 

 ally increase in size. An artery and a vein often lie side by side, but 

 the blood current in the vessels is running in an opposite direction. 



In addition to the systemic circulation there is also the pulmonary 

 circulation, which is the name given to the blood stream leaving the 

 right ventricle of the heart, passing through the pulmonary arteries to 

 the lungs, and after being aerated returning, through the pulmonary 

 veins, to the left auricle, from whence it flows downward into the left 

 ventricle and is then again ready for the systemic circulation. ^^ 



Whenever a vein splits up into capillaries so that the venous blood 

 must pass through an organ on its way back to the heart (either to have 

 waste substances removed as in the kidneys or to take up new sub- 

 stances as in the liver), and this blood is then again collected by venous 

 capillaries and sent on its way, a portal system is formed. 



The renal-portal system and the hepato-portal system are the two 

 important ones in the body's economy. 



When the circulatory system of the frog was discussed it was stated 

 that one must not forget that the material with which the heart works 

 is blood, but that the heart is similar to a pump or an engine, and, that, 

 consequently, just as a pump or an engine which is used for the purpose 

 of forcing water through a great hydraulic system requires water in two 

 places and in two ways to continue its work, so the heart requires blood 

 in two places and in two ways to do its work. 



The engine requires water in its boiler so that steam can be pro- 

 duced. This steam then supplies the force for its work of pumping 

 water, let us say, through the water-pipes of the building in which it 

 is installed. So, too, the heart must have a blood supply to furnish it 

 with energy just as the engine requires water to manufacture its steam. 

 Therefore, there are blood vessels running into the heart-walls and into 

 the walls of blood vessels themselves so as to furnish these with material 

 to produce the required energy to continue their pumping power. The 

 blood vessels that supply the heart walls are known as coronary vessels. 

 The coronary arteries leave the aorta immediately after the aorta, in 



