INTEODUCTION 11 



or iodide injected under the skin of the face of a horse could 

 be found after eight minutes in the urine (Colin). 



The Mechanism of AbsoFption.— The underlying physi- 

 cal principle governing absorption is that of osmosis. 



By osmosis is understood the passage of a dissolved sub- 

 stance through a membrane, such as parchment or the 

 protoplasmic cell wall of the living cell, or the epithelial 

 layers of the alimentary tract. In general, only relatively 

 simple, soluble, and crystalline substances, such as salts, 

 sugars, and urea, diffuse at all easily through such a mem- 

 brane. They are called crystalloids. Substances which are 

 of greater complexity, which are not crystalline, and which 

 tend to form viscous or mucilaginous solutions, or suspen- 

 sions in water, do not osmose, or, at any rate, only do so 

 very slowly. Typical of this class are starch, proteins 

 including enzymes, and glue. They are called colloids. 

 Amongst inorganic compounds, silicic acid forms a typical 

 colloidal solution. In the light of these facts one under- 

 stands why proteins and starches must be resolved into 

 simpler crystalloids — peptones and sugars — before absorp- 

 tion, and why toxines — e.g., of ricine, snake venom — are not 

 absorbed, or only very slowly, by the alimentary mucosa. 



But the epithelium is not only capable of separating, by 

 the process of osmosis, the crystalloid from the colloid, it is 

 also semi-permeahle — that is, will allow osmosis of certain 

 substances and not of others. This may be illustrated by 

 reference to salts. In water solution a salt (in part and to 

 an extent dependent on the strength of the solution and 

 temperature) is split up or dissociated into ions, which are 

 held to be the carriers of electric charges, and which do not 

 possess the ordinary properties of the atoms or groups 

 of which they are composed. In illustration, sodium 

 chloride splits into ions Na+ and C\~ not possessing the 

 ordinary qualities of sodium and chlorine, existing inde- 

 pendently of one another, and bearing charges of electricity, 

 positive on the metal, and negative on the acid ion 

 respectively. 



Too great emphasis cannot be laid on the essential point 



