APPENDIX 243 



vital. Probably those are most correct who admit a certain amount of truth 

 in both views ; the main facts are explicable on a physical basis, but there 

 are also some puzzling data which show that the pulmonary epithelium is 

 able to exercise some other force as well, which interferes to some extent with 

 the known physical process. Take again the case of absorption. The object 

 of digestion is to render the food soluble and diffusible ; it can hardly be sup- 

 posed that this is useless ; the readily diffusible substances will pass more 

 easily through into the blood and lymph : but still, as Waymouth Reid has 

 shown, if the living epithelium of the intestine is removed, absorption comes 

 very nearly to a standstill, although from the purely physical standpoint 

 removal of the thick columnar epithelium would increase the facilities for 

 osmosis and filtration. 



The osmotic pressure exerted by crystalloids is very considerable, but 

 their ready diffusibility limits their influence on the flow of water in the 

 body. Thus, if a strong solution of salt is injected into the blood, the first 

 effect will be the setting up of an osmotic stream from the tissues to the 

 blood. The salt, however, would soon diffuse out into the tissues, and would 

 now exert osmotic pressure in the opposite direction. Moreover, both effects 

 will be but temporary, because excess of salt is soon got rid of by the 

 excretions. 



Osmotic Pressure of Proteins. It has been generally assumed that proteins, 

 the most abundant and important constituents of the blood, exert little or no 

 osmotic pressure. Starling, however, has claimed that they have a small 

 osmotic pressure ; if this is so, it is of importance, for proteins, unlike salt, do 

 not diffuse readily, and their effect therefore remains as an almost permanent 

 factor in the blood. Starling gives the osmotic pressure of the proteins of 

 the blood-plasma as equal to 30 mm. of mercury. By others this is attributed 

 to the inorganic salts with which proteins are always closely associated. 

 Moore, for instance, finds that the purer a protein is, the less is its osmotic 

 pressure ; the same is true for other colloidal substances. It really does not 

 matter much, if the osmotic force exists, whether it is due to the protein 

 itself, or to the saline constituents which are almost an integral part of a 

 protein. It is merely interesting from the theoretical point of view. We 

 should from the theoretical standpoint find it difficult to imagine that a pure 

 protein can exert more than a minimal osmotic pressure. It is made up of 

 such huge molecules that, even when the proteins are present to the extent 

 of 7 or 8 per cent., as they are in blood-plasma, there are comparatively few 

 protein molecules in solution, and probably none in true solution. Still, by 

 means of this weak but constant pressure it is possible to explain the fact that 

 an isotonic or even a hypertonic solution of a diffusible crystalloid may be 

 completely absorbed from the peritoneal cavity into the blood. 



The functional activity of the tissue elements is accompanied by the 

 breaking down of their protein constituents into such simple materials as uvea 

 (and its precursors), sulphates, and phosphates. These materials pass into 

 the lymph, and increase its molecular concentration and its osmotic pressure ; 

 thus water is attracted (to use the older way of putting it) from the blood to 

 the lymph, and so the volume of the lymph rises and its flow increases. On 

 the other hand, as these substances accumulate in the lymph they will in 



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