TRANSPORTATION SYSTEM 



131 



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CAPILLARY 



Fig. 104. — Diagram showing 

 lymph spaces adjoining capil- 

 lary and among cells. 



' capillaries, as happens in osmosis, and some other things may be added to 

 it by a sort of secretion as it passes through those walls. The white 

 corpuscles may crawl between the cells and escape (Fig. 101), and now and 

 then a red cell may also pass out. The fluid which escapes from the 

 capillaries is thus very little different from blood minus its red corpuscles 

 and minus about two-thirds of its proteins. 

 It is called lymph. 



The lymph carries with it most of the 

 blood substances which can be dissolved in 

 water, including most of the digested foods 

 and a small amount of oxygen. It bathes 

 the cells, which take any of the substances 

 that are required. These cells also lose to 

 the lymph any of their soluble \A»stes, 

 principally carbon dioxide and urea. There 

 is some diffusion of the various substances 

 directly through the protoplasm of the cells, 

 so that lymph is not the sole means of communication between the blood 

 capillaries and the surrounding tissues. 



Lymph cannot continue exuding from the capillaries unless it is some- 

 how removed, and it cannot return to the blood vessels from which it 

 came, because of the blood pressure. Instead, it is drained off by another 

 set of vessels known as the lymphatic system. Very small 

 lymph capillaries pass among the cells everywhere, and the 

 lymph moves into them, mostly by diffusion, though 

 minute solid particles are somehow able to get into them. 

 These capillaries collect into larger vessels, which even- 

 tually empty into a vein. In man there are two main 

 lymphatic trunks, one which receives lymph from the entire 

 lower portion of the body below the chest and from the left 

 side above that level, the other from the right side of the 

 chest and head and the right arm (Fig. 105). These large 

 vessels empty into certain veins, one at the base of the 

 neck, the other in the left shoulder (Fig. 99td, rid). The 

 lymph is thus returned to the blood system from which it 

 came. In the course of the lymph capillaries there are 

 valves (Fig. 106) which prevent backward flow, and there 

 are valves at the two points where the main lymph ducts 

 enter the veins. While these valves, together with pressure exerted 

 by muscles, help maintain the flow of the lymph, the main cause 

 of movement is the pressure of the blood behind it, and that is 

 furnished by the heart. Because the source of pressure is distant and 

 the resistance is great, the flow of lymph is sluggish. It takes an hour 



Fig. 105. 

 Very unequal 

 portions of 

 human body 

 supplied by 

 the two main 

 lymphatic 

 systems. 



