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PHYSIOLOGY 



to quench thirst has first to be passed from the stomach into the small 

 intestine before it can be absorbed and relieve the needs of the tissues. The 

 intestinal contents at the ileocaecal valve contain relatively nearly as much 

 water as they do at the upper part of the jejunum. Their absolute bulk is 

 however much smaller, so that only a small proportion of the water that 

 has been taken in by the mouth remains to be absorbed in the large gut 

 an amount probably much less than that which has been added to the 

 contents of the small intestine in the form of secretion by the stomach, 

 liver, pancreas, and intestinal tubules. 



The main problem before us is therefore the mechanism of absorption 

 of water and saline fluids by the villi of the small intestine. By means 



Epithelium of 



villus 



Central lacteal 



Lieberkiilm's 



follicle 



Muscosa 



Muscular is muscosae 



Submucosa 

 Lymphatic plexus 



Circular muscle 



Lymphatic plexus 

 Longitudinal muscle 



FIG. 360. Diagrammatic section through wall of small intestine to show vascular and 

 lymphatic arrangements of mucous membrane. (After MALL.) 



of these structures the absorbing surface of the intestine is largely increased. 

 It has been calculated that each square millimetre of intestine represents 

 an absorbing surface of 3 to 12 mm. 2 Each villus (Fig. 360) consists of a 

 framework of reticular tissue containing many leucocytes in its meshes, 

 separated from the lumen of the gut by a continuous layer of columnar 

 epithelial cells. These cells rest on an incomplete basement membrane and 

 present on the side turned towards the lumen of the gut a striated border. 

 The villus offers two channels by means of which material, which has passed 

 through the epithelium, may be carried into the general circulation. In 

 the centre of the villus is the central lacteal, a club-shaped vessel bounded 

 by a complete layer of delicate endothelial cells. This leads into a plexus 

 of lymphatics placed superficially to the muscularis mucosee. From the 

 superficial plexus communicating branches pass vertically to a correspond- 

 ing plexus lying in the submucosa. The central lacteal and the superficial 

 plexus are free from valves, which however are present in abundance 

 in the deeper plexus, so that fluid can pass easily from the lacteal to the 



