THE FLOW OF BLOOD IN THE BODY 



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body. They unite again and again to form about twenty 

 main trunks for each limb. Each trunk extends upward, 

 and most of them finally unite to form a tube of the size 

 of a goose quill, called the thoracic duct. 



The thoracic duct lies upon the spinal column, and 

 extends upward into the neck, where it opens into a large 

 vein. The lymphatics have numerous valves, all opening 

 toward the heart. They prevent the backward flow of lymph. 



312. Lymph nodes. At irregular intervals the lym- 

 phatics open into small, baglike bodies composed of a 

 spongy network of fibers filled with 



cells, some of which become white 

 blood cells. Each body is called a 

 lymph gland or node. The lymph 

 flows through these nodes as water 

 flows through a filter. They strain 

 out matters which are injurious to 

 the system, while their cells en- 

 velop and destroy poisons and dis- 

 ease germs, and so they protect the 

 rest of the body. 



The lymph nodes may be felt in 

 the neck and groins and armpits, as 

 small kernels about the size of a 

 grain of wheat or corn. When the 

 lymph carries certain kinds of poi- 

 sons, they swell up and produce the 

 disease called scrofula. In boils, 

 erysipelas, and other inflamma- 



Lymph node and vessels 



tions, they swell and become very (xio). 



tender and sometimes break down and form abscesses. 



313. Flow of the lymph. A little pressure transmitted 

 from the blood in the capillaries is exerted upon the lymph, 



