DIGESTION 131 



lymph-plexus beneath the mucous membrane, which, again, 

 communicates with a coarser plexus outside the muscular coat. 

 From the peri-intestinal plexus vessels lying in the mesentery 

 converge to the receptaculum chyli, the bulbous commence- 

 ment of the thoracic duct, which lies at the back of the abdomen 

 in front of the bodies of the vertebrae. The thoracic duct runs 

 up the front of the vertebral column, through the thorax, and 

 then hooks over to pour the fluid which it conveys into the 

 great veins shortly before they join the heart. After a meal 

 containing fat the fluid in the lymphatic vessels of the mesen- 

 tery, the lacteals, has, as already stated (p. 43), the appearance 

 of milk. The fat absorbed by the epithelium covering a 

 villus is passed on into its lymph-space. From this into the 

 central lacteal receptacle, thence to the submucous and peri- 

 intestinal plexuses, the lacteal vessels of the mesentery, the 

 thoracic duct. Absorbed fat does not pass through the 

 liver, but is carried into the heart ; thence through the lungs, 

 and back to the heart, which pumps it to all parts of the body. 

 In addition to the lacteal radicle, the villus contains long 

 capillary bloodvessels, and the arteriole and venule in which 

 they commence and end. These traverse the lymph-spaces of 

 the connective tissue, which contains, not only the fat which 

 the epithelial cells have passed into it, but the other products of 

 digestion also. None of the fat traverses the walls of the 

 bloodvessels ; but the other products diffuse from the lymph, 

 through the walls of the vessels, into the blood. Many nerve- 

 fibres are found in the core of the villus on their way to epi- 

 thelial cells, or to one or two plain muscle-fibres which are 

 disposed in the direction of its long axis. For each villus is a 

 little pump. By the contraction of the muscle-fibres it is 

 shortened, and the fluid in its lacteal radicle is forced into the 

 submucous vessels. 



Two problems have to be considered : First, in what form 

 and by what mechanism are the several kinds of food ab- 

 sorbed ? Secondly, what becomes of them after they have 

 been absorbed ? 



Clearly, the epithelial cell is the absorbing mechanism. It 

 is not a membrane governed by the laws which regulate diffu- 

 sion of fluids through membranes, but a living cell. There 

 is hardly any limit to its power of selecting the food which it 



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