738 PHYSIOLOGY 



hindered according to the physical conditions present. If these act against 

 the cells, e.g. if the fluid be hypertonic, the absorption is effected more slowly, 

 while with hypotonic solutions the physical conditions concur with the vital 

 activity of the cells in bringing about a very rapid transference of fluid from 

 the gut into the blood-vessels. Among these physical conditions we must 

 reckon the nature of the salts present in the solution. If these can pass 

 easily into and through the cells, e.g. ammonium salts, sodium chloride, 

 absorption is carried out rapidly. If, on the other hand, the salts in the 

 intestinal contents are but slightly diffusible or have very little power of 

 penetrating into the cells, the absorption of water by the cells causes an 

 increased concentration of the salts, and therefore an increased osmotic 

 pressure, which offers a resistance to any further absorption ; and the process 

 comes to an end, when the absorptive power of the cells is exactly balanced 

 by the increased osmotic pressure, or attraction for water, of the intestinal 

 contents. 



Cushny and Wallace, as the result of their experiments on the relative absorbability 

 of salt solutions from the gut, divide the salts into four main classes as follows : 



I II III IV 



Sodium chloride, Ethyl sulphate, Sulphate, phosphate, Oxalate, 



bromide, iodide, nitrate, lactate, sali- ferrocyanide, capry- fluoride, 



formate acetate, cylate, phthalate. late, malonate, succi - 



propionate, butyrate, nate, malate, citrate, 



valerianate, caprate. tartrate. 



Of these the first class contains those salts which are absorbed with great ease 

 from the intestine. The second group of salts are absorbed with somewhat greater 

 difficulty. The third group are absorbed so slowly, i.e. the salts retain the water in 

 which they are dissolved so long, that they increase peristalsis and act as laxatives or 

 purgatives. The members of the fourth class are not absorbed at all. It is evident 

 that this classification is independent of the diffusibility of the salts. Sodium acetate 

 has a much smaller dissociation value and a lower diffusibility than sodium chloride 

 or iodide, and yet is absorbed at approximately the same rate as these two salts. There 

 is, however, as Cushny pointed out, one physical or chemical character which apparently 

 determines the non-absorbability (relative or absolute) of the members of the third 

 and fourth classes. All these salts form insoluble compounds with calcium. This 

 common character is not an explanation of the permeability of the cell wall, but is 

 simply a general statement of one of the conditions which affect the power of the cells 

 to take up salts from their solutions, this power being absent in the caser of salts which 

 furnish an insoluble calcium compound. 



THE ABSORPTION OF FATS 



Fats administered to an animal in excess of its diurnal requirements 

 are stored up in the body in the form in which they are administered. Each 

 cell of the body probably possesses in itself the mechanism for the utilisation 

 of these neutral fats, and for effecting in them the various changes involved 

 in the successive stages of their disintegration and oxidation through which 

 they are finally converted to C0 2 and water. The problem therefore of fat 

 absorption is ultimately one of the simplest with which we have to deal, 

 and involves merely the transference of the neutral fat of the food to the 

 circulating fluids in such a form that it can be carried by them to the place 



