ABSORBENT VESSELS 



135 



one or more larger tubes or lyniph capillaries. Like the 

 blood vessels, these latter are branches of larger vessels 

 in the submucoiis layer; and fluid which once enters the 

 lymph capillaries of the villus flows first into these larger 

 lymph vessels, while 

 these in turn are col- 

 lected and fuse into a 

 single tube about the 

 diameter of a goose 

 quill, called the thoracic 

 duct. This duct passes 

 up in front of the spinal 

 column and finally opens 

 into a large vein in the 

 neck under the left collar 

 bone. Owing to the 

 milky color of the flxiid 

 which is in these vessels 

 and the absence of blood 

 they have received the 

 name of the lacteals, 

 and this name also 

 serves to distinguish 

 them from other parts 

 of the lymphatic sys- 

 tem. The blood capilla- 

 ries as already mentioned are collected together and finally 

 pour their contents into the portal vein. From this descrip- 

 tion it must be clear that the digested food of the stomach 

 and intestine in order to enter the blood and lymph must 

 pass through two very thin layers, namely, the single layer 

 of mucous cells, which form the lining of the tracts, and 



Fig. 46 — Section of a villus ; a, layer of 

 mucous memlrane cells which absorb food 

 and carry it to the tubes within ; 6, an 

 artery ; c, blood capillaries ; d^ a lacteal. 



