EPITHELIUM 167 



to a coefficient which is specific for each pair of substances provided tem- 

 perature is kept constant. It may be profoundly modified by the slightest 

 alteration in one or more of the physical or chemical conditions of one or 

 both of the phases forming the contact surface. 



The form of a cell depends in great part on the magnitude of 

 the surface forces brought to bear on it. If it is surrounded by 

 exactly similar cells then it will tend to assume a more or less 

 spherical form. This is exactly what one finds in the centre of a 

 mass of soap bubbles or in the middle layers of stratified squamous 

 epithelium. The cells are not absolutely spherical in shape, not 

 only because the cells in mass are not absolutely similar but because 

 the cells have to fit the space. No vacant spaces occur. Now, 

 according to the principle of Le Chatelier, the surface energy will 

 manifest itself by tending to reduce the area of contact. Mathe- 

 matical proof has been given that the least possible area of contact 

 surface is attained when the partition walls meet together in 

 groups of three, at equal angles, i.e. at angles of 120. 



The outer and inner layers differ markedly in shape from one 

 another and from the middle layers. The outer layer is exposed 

 to air (skin) or to the free external fluids of the body (mouth and 

 gullet) on the one side, but is in contact with cells on all other 

 sides. In addition, the outer surface is liable to undergo chemical 

 changes oxidations, etc., and physical changes adsorption, etc. 

 These again affect tension. The result is that the outer layers 

 are flattened and scale-like. 



The inmost layer of cells is in contact on one side with the 

 structure on which the epithelium is laid and from which it 

 takes its origin and its nourishment. These younger cells are 

 more or less elongated in shape. Their form is governed by 

 certain forces in addition to those acting on the more central 

 cells, (a) It is obvious that the surface tension will be different 

 at that surface where the cell is in contact with a cell differing 

 from itself in structure and condition. These cells are in contact 

 on either side with similar cells, but above, they press against 

 fully grown spherical cells, while below they form interfaces with 

 the structure on which they lie and from which they derive 

 their nourishment, (b) These deeper cells are in process of division, 

 and, therefore, one must take into account the pressures of seg- 

 mentation and of growth (cf. Plant Cells), (c) The outmost 

 layer, away from the nourishing fluids of the body, undergo 

 keratinisation and resist the outwards push of young cells which 

 are thus put under stress. These various factors modify the shape 

 of the deeper layers. 



