3 o8 PROD UCTION AND ABSORPTION OF L YMPH. 



ment of osmotic pressure has shown that the osmotic pressures of salts 

 and other crystalloids are enormously higher than those of colloids 

 such as albumin, and it has therefore been supposed that the osmotic 

 pressure of the proteids in the serum, being so insignificant, must be of 

 no account in physiological processes. The reverse is, however, the 

 case. Whereas the enormous pressures of the salts and crystalloids in 

 the various fluids of the body are of very little importance for most 

 physiological functions, the comparatively insignificant osmotic pressure 

 of the albumins is of great importance and for this reason. It has 

 been shown that bodies in solution behave in most respects like gases. 

 Now, there can be no difference in pressure between two gases in a vessel 

 which are not separated or are only divided by a screen freely permeable 

 to both gases. In the same way, if we hap^e two solutions of crystallised 

 substances separated by a membrane which offers free passage to the 

 water and the salts on either side, there can be no enduring difference of 

 the osmotic pressure on the two sides, especially if a free agitation of 

 the fluids on both sides is kept up. The pressures on the two sides will 

 be speedily equalised, and then any flow of fluid from one side to the 

 other will cease. Now, the capillaries in the living body represent such 

 a membrane. Leathes l has shown that, within five minutes after the 

 injection of sugar or salt into the blood vessels, their osmotic pressures in 

 the blood and lymph have become equal. Supposing, however, that we 

 have on one side of this membrane a substance to which the membrane 

 is impermeable, this substance will exert an osmotic pressure and will 

 attract water from the other side of the membrane with a force propor- 

 tional to its osmotic pressure. This attraction of fluid must go on until 

 all the fluid has passed through the membrane to the side where the 

 indiffusible substance is. 



Now the capillaries of the limbs are almost impermeable to proteids. 

 In consequence of this impermeability, the fluid which is transuded 

 from the capillaries under pressure contains very little proteid. From 

 what I have just said, it follows that the proteids left in solution within 

 the capillaries must exert a certain osmotic attraction on the salt 

 solution outside the capillaries. It is easy to measure the value of this 

 attractive force. If we place blood serum in a small thistle funnel, over 

 the open end of which is stretched a layer of peritoneal membrane 

 soaked in gelatine, and immerse the inverted funnel into salt solution 

 which is isotonic or even hypertonic as compared with the serum, 

 within the next two days fluid will pass into the funnel and will rise in 

 its capillary stem to a considerable height. I have found that the 

 osmotic pressure of the non-diffusible portions of blood serum, measured 

 in this way, may amount to about 30 mm. Hg. The importance of 

 this fact is obvious. Although the osmotic pressure of albumin is so 

 insignificant, it possesses an order of magnitude comparable to that of 

 the capillary pressures; and whereas capillary pressure determines 

 transudation, the osmotic pressure of the proteids of the serum 

 determines absorption. Moreover, the osmotic attraction of the serum 

 for the extravascular fluid will be proportional to the force expended in 

 the production of this extravascular fluid, so that at any given time 

 there must be a balance between the hydrostatic pressure of the blood in 

 the capillaries and the osmotic attraction of the blood for the surround- 

 ing fluids. With increased capillary pressure we shall have increased 



] Journ. PhysioL, Cambridge and London, 1895, vol. xix. p. 1. 



