166 



LYMPHATIC SYSTEM 



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near the heart. Valves similar to' those found in the veins are 

 along the larger lymphatics and prevent the back flow of the lymph. 

 In addition to the lymphatics that drain the tissue fluid, there are 



some called lacteal? 

 that are in the villi of 

 the small intestine. 

 These lymphatics aid 

 in the absorption of di- 

 gested nutrients, par- 

 ticularly fats. After 

 a meal, the lacteals 

 are filled with a white 

 fatty substance. At 

 other times, the fluid 

 filling the lacteals is 

 very similar to the 

 lymph in the other 

 lymphatics. Lymph 

 is kept moving by the contraction of skeletal muscles squeezing 

 the lymphatics. This is similar to the effect of the skeletal 

 muscles on the veins. The flow^ of lymph is also controlled by the 

 muscular movements of the body, and by the release of pressure 

 on the thoracic duct during each inspiration. This release causes 

 the h^mph in vessels where the pressure is greater, to flow into the 

 thoracic duct. 



The spaces formed between the membranes and the various 

 organs of the body which they cover are similar in structure to 

 lymphatics, and may be thought of as expanded lymph spaces. 

 The fluid found in the pleural, the pericardial, and the peritoneal 

 sacs is similar to the lymph in the lymphatics. 



Lymph nodes are expanded portions of the lymphatics found 

 in all parts of the body. Large collections of them are located 

 under the arms and in the neck. Others, in fewer numbers, occur 



Liquid materials soak through the walls of capillaries and 

 bathe the tissue cells. Spaces among the cells are called 

 lymph spaces. The excess Ijmiph is gathered up by special 

 tubes, lymph tubes. A lymph tube is shown in the diagram 

 to be parallel to a blood vessel (this representation is dia- 

 grammatic). 



