CH. XXII.] OSMOTIC PRESSURE OF PROTEIDS 327 



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 proteids, 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 proteids of the blood-plasma as 

 equal to 30 mm. of mercury. We should from the theoretical standpoint find it 

 difficult to imagine that a pure proteid can exert more than a minimal osmotic 

 pressure. It is made up of such huge molecules that, even when the proteids are 

 present to the extent of 7 or 8 per cent. , as they are in blood-plasma, there are 

 comparatively few proteid molecules in solution. Still, by means of this weak but 

 constant pressure it is possible to explain the fact that an isotonic or even a hyper- 

 tonic 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 proteid constituents into such simple materials as urea (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 time attain there a greater concen- 

 tration than in the blood, and so they will diffuse towards the blood, by which they 

 are carried to the organs of excretion. 



But, again, we have a difficulty with the proteids ; they are most important for 

 the nutrition of the tissues, but they are practically indiffusible. We must pro- 

 visionally assume that their presence in the lymph is due to filtration from the blood. 

 The plasma in the capillaries is under a somewhat higher pressure than the lymph 

 in the tissues, and this tends to squeeze the constituents of the blood, including 

 the proteids, through the capillary walls. I have, however, already indicated that 

 the question of lymph-formation is one of the many physiological problems which 

 await solution by the physiologists of the future. 



B. Moore and W. H. Parker confirm Starling's hypotheses that colloids in 

 solution exert a small osmotic pressure, but the direct method is the only one 

 available to determine it, the variations in freezing or boiling points are too small. 

 It was hoped that this pressure could be used for determining the molecular weight 

 of colloids like proteids, but it is found that in the case of substances of known 

 molecular weight such as soaps, the apparent molecular weights are from 20 to 60 

 times too large. There must thus be a physical union or association of molecules 

 to form a single osmotic unit. It is, therefore, possible that the chemical molecule 

 of a proteid is not so large as has been supposed, but its apparent size is due to a 

 physical aggregation of many molecules. Moore doubts whether the differences in 

 osmotic pressure are sufficiently great to explain absorption, lymph production, or 

 the formation of urine. If this is so, the physiological factor, the so-called vital 

 activity of the cells, must be called in to explain these phenomena. 



Waymouth Reid finds that absolutely pure proteids exert no osmotic pressure ; 

 the pressure observed is due to saline and other materials from which it is difficult to 

 disentangle the proteid. 



Dr C. J. Martin has suggested to me a way of illustrating the so-called selective 

 action of living membranes. Suppose a number of fishes are swimming about in a 

 tank, like moving molecules or ions in solution ; across the tank is a wall which 

 divides it into two parts ; the fishes are all in one compartment of the tank. 

 Suppose, next, the wall has in it a number of holes guarded by valves, so arranged 

 that the fish can pass through into the second compartment, but cannot return. 

 After a time, as the fish discover these holes, there will be an equal number of fish 

 in both compartments ; but this is not the end, for on waiting further, more fish will 

 find their way through, and as none are able to return, they will all in time accumu- 

 late in the second compartment. It is not difficult to grasp the idea that the arrange- 

 ment of molecules in a living membrane is possibly such that the orifices through 

 which other molecules pass are valvular, and such a conception is useful if it 

 merely serves to rob the word "vital" of its mystery. 



