734 



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 ileoca3cal 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 



Lieberkiihn's 



Mucosa 

 Muscularis mucos* 



Submucosa 

 Lymphatic plexus 



Circular muscle 



^-^- ^eg^^ggp Lymphatic plexus 



Longitudinal muscle 



FIG. 349. 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. 349) 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 basilar 

 border. The villus offers two channels by means of which material, which 

 has passed through the epithelium, may be carried into the general circu- 

 lation. 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 mucosse. From 

 the superficial plexus communicating branches pass vertically to a corre- 

 sponding plexus lying in the submucosa. The central lacteal and the super- 

 ficial 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 



