272 A MANUAL OF VETERINARY PHYSIOLOGY 



cent, of common salt which has the same osmotic pressure as blood- 

 serum exerts a pressure of 5,000 mm. mercury. 



Blood-serum is taken as the standard in physiological inquiry ; 

 any solution which has the same molecular concentration as serum, 

 and consequently exerts the same osmotic pressure, is termed 

 isotonic ; if it has a greater osmotic pressure, it is termed hyper- 

 tonic ; and if less, hypotonic. For example, on p. 6 the action of 

 salt solution on red corpuscles has been described ; if the addition 

 of a solution causes no effect, it may be known that it is isotonic 

 to the material within the corpuscles ; if it causes the corpuscles 

 to shrink and become crenated, it has a greater osmotic pressure 

 than the cells, and is hypertonic ; if it causes them to swell and 

 discharge their pigment, it has a smaller osmotic pressure, and is 

 hypotonic. 



It is not difficult to see how the physical factors of osmotic 

 pressure and diffusion may be turned to account in explaining 

 the formation of lymph, absorption from the intestines, secretion 

 of urine, and such-like processes. The danger, as has been pre- 

 viously indicated, lies in their uncompromising application, and in 

 the liability to forget that the living body is neither a parchment 

 membrane nor a vessel with a semi-permeable lining. It has 

 been urged that it is impossible to disregard the value of osmotic 

 currents in restoring equilibrium of composition between the 

 blood and the tissues or the tissues and the blood ; for example, 

 if a strong solution of common salt be injected into the blood- 

 stream, a current is created from the tissues into the blood, by 

 which the tissues may lose water ; but later on diffusion will 

 come into play, and the tissues will draw water from the blood. 

 It is suggested that constant and rapid osmotic changes are 

 occurring between the blood and the tissues ; so rapid, indeed, 

 may these be that if the osmotic equilibrium be upset by injecting 

 a large dose of dextrose, within half a minute it is readjusted, 

 We have seen that the lymph in the spaces must necessarily be 

 undergoing constant change in its composition and concentration, 

 as the result of tissue activity, with its attendant chemical 

 changes ; and it is readily conceivable that osmotic or diffusion 

 currents may be set up, water being drawn from the blood to the 

 tissue spaces, and crystalloid bodies, such as would result from 

 the breaking up of the protein molecule, passing by diffusion 

 from tissue spaces to blood, and so being got rid of by the ex- 

 cretory organs, of which the precursors of urea are a good example. 



Experimental inquiry, however, shows that it is not easy to 

 explain lymph formation by osmosis or diffusion, and we have 

 previously seen how far filtration has failed. The injection into 

 a vein of a strong solution of common salt, urea, or dextrose, is 

 followed by an immediate increase in lymph ; whereas the 

 osmotic pressure exerted by sodium chloride, for example, 

 should diminish the secretion by setting up osmotic currents 



