2;o A MANUAL OF VETERINARY PHYSIOLOGY 



passed through the wall of one vessel and become altered in 

 composition ; in the tissue space it is certain that further change 

 in its composition occurs in consequence of tissue activity, and 

 another vessel wall has to be negotiated before the lymph-stream 

 is reached ; this passage may occasion further change in its nature. 

 The question of the actual changes in the composition of the 

 lymph in its passage from blood capillary to lymph capillary does 

 not immediately concern us here, but it helps to explain why 

 some physiologists have such difficulty in accepting purely 

 physical reasoning where living tissues are concerned, even when 

 the process concerned is as simple as that of filtration. Nor must 

 it be considered that the presence of an increase of pressure in 

 the capillaries necessarily results in the formation of more lymph. 

 In the horse it has been shown that the flow of lymph from the 

 parotid gland is not appreciably increased when the gland passes 

 from a condition of rest to that of secretory activity, and yet we 

 know that the capillary pressure at the time is greatly increased. 

 Other evidence will be quoted later showing that serum may be 

 absorbed from the bowel when the pressure in the capillaries is 

 greater than the pressure in the intestines ; it may be urged that 

 the case of the bowel and bloodvessel is not comparable with that 

 of the tissue space and bloodvessel, but the object is rather to 

 show that filtration may be non-existent at the moment when the 

 necessary physical conditions for its activity are present. It 

 has been shown, therefore, that there are features in lymph 

 formation which cannot be entirely explained on the theory of 

 filtration. 



Diffusion and Osmosis. — For many years it has been known 

 that if two fluids — one containing salt, and the other pure water 

 — be separated by a membrane of parchment, one passes into 

 the other through the pores of the membrane, until the same 

 amount of salt exists on both sides of the diaphragm. Fluids 

 which thus pass through are termed crystalloids ; those which 

 refuse to pass through are known as colloids ; while the entire 

 process is termed osmosis or dialysis. We shall see presently 

 that physical chemistry of the present day gives a more restricted 

 definition to the term ' osmosis.' 



It is known that perfectly pure water is not a conductor of elec- 

 tricity, and the same may be said of solutions of sugar, urea, albumin, 

 and other bodies. The explanation afforded by modern chemistry 

 is that the molecules of pure water and molecules of sugar undergo 

 no dissociation into their constituent ions. Substances which dis- 

 solve in water and undergo dissociation are conductors of electricity. 

 For instance, sodium chloride is broken up in water into sodium ions 

 and chlorine ions, each group being charged with an opposite form 

 of electricity, the sodium ions being positively, the chlorine ions 

 negatively, charged. 



