DIGESTION 



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London (1906, etc.), using the Pavlov method of fistula? in various parts of the 

 alimentary canal, has found that proteins are practically entirely converted into 

 ammo-acids and absorbed as such in the small intestine. One of these, arginine, 

 a conjugated di-amino-acid, as we have seen, is further hydrolysed by an enzyme, 

 arginase, into urea and di-amino-valerianic acid. This enzyme was discovered 

 by Kossel and Dakin (1904). 



Digestion of Fats. A lipase -has been described by some as present in the 

 stomach, but its function is negligible compared with that of the pancreatic juice. 

 The action of this latter is assisted by the bile, which promotes fine emulsification, 

 owing to its power of reducing surface tension, and as we have seen', it also has 

 a direct effect on the activity of the enzyme. The bile also acts as a solvent for 

 the free fatty acids, especially important in the case of the higher ones, such as 

 stearic, etc. By this means, fats are hydrolysed into their constituent fatty acids 

 'and glycerol. 



Absorption of Fats. These fatty acids and glycerol are taken up by the cells 

 covering the villi and, in their interior, are synthesised into neutral fats again, 

 probably by the reverse action of lipase. In the form of fine droplets, the neutral 

 fats are passed into the central lymphatic space of the villus, and thence in the 

 stream of lymph into the lacteals and thoracic duct and so into the blood stream. 

 In the blood they can be observed by ultra-microscopic methods of illumination 

 as the " blood dust," with its vigorous Brownian movement. It is difficult to 

 see precisely why fats should be hydrolysed only to be resynthesised in the villi, 

 but it must clearly be for the purpose of facility of absorption. 



Absorption of Water and Salts. Water can be absorbed by the intestinal 

 mucous membrane to practically any extent. The regulation of the water content 

 of the organism is carried out by the kidneys. No water is absorbed from the 

 stomach, most in the small intestine and a certain amount in the large intestine. 



If it were pure water that is to be absorbed, the osmotic pressure of the 

 constituents of the blood-plasma would suffice to explain the fact ; but it actually 

 happens that isotonic saline solutions can be absorbed. Even hypertonic solutions 

 are ultimately absorbed after a preliminary dilution by pure osmotic action. 

 There must, therefore, be some active intervention on the part of the absorbing 

 epithelial cells, by which energy is consumed. This process is thus a kind of 

 inverse of that involved in secretion. It is perhaps most strikingly shown by the 

 fact that the animal's own serum can be absorbed. At the same time, physical 

 factors have considerable effect on the rate of absorption and a discussion of the 

 part played by these factors will be found in the article by Starling (1909). 



SUMMARY 



In animals, food requires treatment, mechanical and chemical, before it can be 

 absorbed. Digestion mechanisms ensure this, and also provide for efficient 

 absorption of the products. 



In unicellular organisms, the solid food particles, mostly living algje or 

 bacteria, are attacked inside the cell. They are first killed in acid medium, then 

 digested by enzymes in alkaline medium. 



Similar statements apply to the phagocytes of the higher, niulticellular 

 animals. There is no evidence of prehensile, pseudopodial attack ; chance 

 contact with bacteria leads to adhesion and engulfing by the action of surface 

 forces. " Opsonins," as specific chemical entities, in all probability have no 

 existence. 



The general plan of the digestive system in the higher animals is a long tube 

 with dilatations in places and arrangements for enclosing food, temporarily, in 

 various sections, in order to enable the enzymes, which are secreted into these 

 sections, to act for a sufficiently long time. 



Two kinds of movements are required. One to pass the food along the gut, 

 the other to send it backwards and forwards in a particular section. The former 

 is provided for by nerve centres in the wall of the alimentary canal itself; these 



