Cellular Aspects of Body Electrolytes and Water 29 



low temperature and then transferring the eye to a chamber 

 maintained at the higher temperature. In this case the ab- 

 sorbed water and salts are excreted back and the cornea 

 reacquires its normal hydration (Table IV). The secretory 

 activity that usually maintains the cornea in its normal state 

 of hydration — about 75 per cent water — may be due to both 

 the endothelium and epithelium, but whether it is due to an 

 active excretion of salt, e.g. sodium, or of water, remains to 

 be proved. The extraordinary tendency of the cornea to 

 take up water, by contrast with tendon or sclera, is presumably 



Table IV 

 The effect of subsequent warming on eyes maintained for 15-18 



HOURS AT 7° 



(Davson, 1955) 



Column A gives the water content after the period at 7° ; column B the water 



content after a further period of 6-8 hours at 31°. 



related to the large quantity of mucoid present as a coating 

 over the individual collagen fibrils (Schwarz, 1953), and it 

 seems likely that changes in hydration are really the conse- 

 quence of changes in hydration of this colloid, the collagen 

 fibrils being pushed apart by the swelling. The Gibbs- 

 Donnan sweHing of the collagen-mucoid system of skin and 

 subcutaneous tissues may well be a factor in determining the 

 water content and the turgescence of the tissues. Thus it 

 would seem from McMaster's (1946) studies that the extra- 

 cellular fluid may, in normal circumstances, be something of 

 an abstraction, the space between cells and collagen fibrils 

 being occupied by a mucoid gel ; only when excessive amounts 

 of fluid are filtered from the plasma, or under experimental 



