144 PHYSIOLOGY. 



sure plays a part. When salt or glucose is injected into the blood- 

 vessels, the first effect will be a stream of water from the cells of 

 the tissues to the blood, and the production of an excess of water 

 in the blood-stream. But soon the salt or glucose passes out into 

 the tissues outside the blood-vessels, and then they draw the water 

 from the blood-stream. In nutrition, the cells of the tissues use up 

 the materials which are supplied by the extra-cellular lymph. By 

 the concentration the extra-cellular lymph is lowered and a stream 

 of material is set up from the blood to the cells outside the blood- 

 vessel. At the same time, the cells of the tissues are undergoing 

 metabolic changes, the proteid molecule is breaking up into simple 

 molecules of the character of crystalloids, such as urea, phosphates, 

 and sulphates, which pass into the extra-cellular lymph, increase its 

 molecular concentration, and by their greater osmotic pressure draw 

 water from the blood to the lymph; thus they increase the produc- 

 tion of lymph. But as the broken-down materials from the proteids 

 accumulate in the lymph, increasing its molecular concentration, so 

 that it is greater than that of the same substances in the blood, then 

 they will diffuse toward the blood, and pass out in the excretions. 

 In absorption from tne intestine, it is found that the living cells of 

 the intestinal wall modify absorption, so that it does not follow the 

 law of diffusion through a dead membrane. 



In the pathological condition known as dropsy, there is pre- 

 sented a partial example of filtration. It is characterized by a 

 transudation of the watery portion of the blood through the mem- 

 branous walls of the capillaries and small veins into the surround- 

 ing connective tissues, producing oedema. This watery element has 

 been literally squeezed through the vessel-walls. 



Loeb explains this oedema by a greater osmotic pressure in the 

 tissues than in the blood or lymph. Chemical changes in the mus- 

 cle take place which increase the osmotic pressure. These chemical 

 conditions are the result of a diminished supply of oxygen caused 

 by deficient circulation. 



Absorption by the Stomach and the Intestines. 



The stomach does not absorb water, but alcohol. Water and 

 the salts dissolved in it are absorbed throughout the small intestine, 

 from the pylorus to the ileo-cascal valve, and partly by the large 

 intestine. So that the watery chyme leaving the stomach becomes 

 gradually thicker as it travels down the intestines. The relatively 

 rapid absorption of water by the intestines removes from the putre- 



