400 BIOLOGY FOR BEGINNERS 



blood toward this organ. Their walls are thinner than the arteries, 

 having little elastic or muscular tissue, but many of the larger ones 

 are provided with cup-like valves to prevent backward flow of 

 blood. Veins are often just beneath the skin and can be easily 

 seen on the back of the hand where the dark color of their blood is 

 conspicuous; enlargements show the location of the valves. Veins 

 have no pulse wave and the blood pressure is lower than in the 

 arteries. Except for the pulmonary veins, their blood is dark (de- 

 oxygenated) as compared with the redder, arterial blood. However, 

 this is of little use in deciding whether a wound has cut a vein or 

 artery, as on exposure to ah*, blood absorbs oxygen and brightens 

 in color. 



Bleeding from an artery, if large enough to be serious, is in 

 pulse-like spurts, while the flow from veins is steady. This and 

 the location of the wound are the best means of distinguishing 

 the source of blood flow. 



Lymph Circulation. A part of the blood plasma that diffuses 

 through the capillary walls into the spaces between the cells does 

 not return to the capillaries directly but is collected into the lymph 

 capillaries. 



These tiny tubes connect all the lymph spaces together and unite 

 to form the lymph veins which eventually join to empty into the 

 blood stream near the left jugular (neck) vein. Thus, a part of the 

 plasma, instead of following the usual route (artery capillary 

 vein) may return as follows, artery capillary lymph space 

 lymph capillary lymph vein true vein. It is in the form 

 of this lymph that the blood actually nourishes the tissues and the 

 lymphatic circulation is just as necessary as that of the blood 

 as a whole. 



Each cell of the body is practically an island surrounded by 

 lymph. This lymph has passed, by osmosis, through the capillary 

 walls, bearing in solution the digested food-stuffs from the ali- 

 mentary tract, and oxygen from the lungs. 



These the cell uses in its life activities and throws off carbon 

 dioxide, water, and other wastes into the lymph, and thence into 

 the blood of the vein capillaries. 



