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 have 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 1 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. 
