56 THE MECHANISM OF LIFE 



elevation of the osmotic pressure increases with the number of 

 contractions. 



4. The intramuscular osmotic pressure increases with the 

 work done by the muscle. 



5. Fatigue is caused by the increase of osmotic pressure in 

 a contracting muscle. 



The Field <rf Diffusion. — Just as Faraday introduced the 

 conception of a field of magnetic force and a field of electric- 

 force to explain magnetic and electrical phenomena, so we may 

 elucidate the phenomena of diffusion by the conception of a 

 field of diffusion, with centres or poles of diffusive force. If we 



Fir.. 3. — Fields of diffusive force. 



(a) Monopolar field of diffusion. A drop of blood in a saline solution of higher 



concentralion. 

 (h) Bipolar field of diffusion. Two poles of opposite signs. On the right a grain 



of salt forming a hypertonic pole of concentration, on the left a drop ol 



blood forming a hypotonic pole of dilution. 



consider a solution as a field of diffusion, any point where the 

 concentration is greater than that of the rest may be considered 

 as a centre of force, attractive for the molecules of water, and 

 repulsive for the molecules of the solute. In the same way 

 any point of less concentration may hi' regarded as a. centre 

 of attraction for the molecules of the solute, and a centre of 

 repulsion for the molecules of water. 



A field of diffusion may be monopolar or bipolar. A 

 bipolar field has a hypertonic pole or centre of concentration, 

 and a hypotonic pole or centre of dilution. By analog} 

 witli the magnetic and electric fields we may designate the 

 hypertonic pole as the positive pole of diffusion, and the 

 hypotonic as the negative pole. 



