324 



THE BLOOD AND ITS CIRCULATION 



tissues. Extremely numerous in the extremities and in the muscles 

 and among other tissues of the body, they, like the branches of a 



tree, become larger and unite with each other as they 



approach the heart. 



If the wall of a vein is carefully examined, it will be 

 found to be neither so thick nor so tough as an artery wall. 

 When empty, a vein collapses ; the wall of an artery holds 

 its shape. If you hold your hand downward for a little 

 time and then examine it, you \vill find that the veins, 

 which are relatively much nearer the surface than are the 

 arteries, appear to be very much knotted. This appear- 

 ance is due to the presence of tiny valves within. These 

 valves open in the direction of the blood current, but 

 would close if the direction of the blood flow should be 

 reversed (as in case a deep cut severed a vein). As the 

 pressure of blood in the veins is much less than in the 

 arteries, the valves thus aid in keeping the flow of blood 

 in the veins toward the heart. The higher pressure in 

 arteries and the suction in the veins (caused by the enlarge- 

 ment of the chest cavity in breathing) are the chief factors 

 which cause a steady flow of blood through the veins in 



Valves in a 

 vein. No- 

 tice the thin the body, 

 walls of the 

 vein. 



Lymph Vessels. - The lymph is collected from 

 the various tissues of the body by means of a number 

 of very thin-walled tubes, which are at first very tiny, but after 

 repeated connection with other tubes ultimately unite to form 

 large ducts. These lymph ducts are provided, like the veins, 

 with valves. The pressure of the blood within the blood vessels 

 forces continually more plasma into the lymph ; thus a slow 

 current is maintained. On its course the lymph passes through 

 many collections of gland cells, the lymph glands. In these glands 

 some impurities appear to be removed and colorless corpuscles made. 

 The lymph ultimately passes into a large tube, the thoracic duct, 

 which flows upward near the ventral side of the spinal column, and 

 empties into the large subclavian vein in the left side of the neck. 

 Another smaller lymph duct enters the right subclavian vein. 



The Lacteals. - We have already found that part of the digested 

 food (chiefly carbohydrates, proteins, salts, and water) is absorbed 



