204 THE CIVIL ENGINEERS OF THE BODY 



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

 the snrface 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 fill the space. No odd, empty 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, 

 drying, etc. These again affect tension. The result is that the 

 outer layers are flattened and scale-like. 



The form of the inmost layer of cells 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 segmentation and of growth. 



(c) The outmost layer, away from the nourishing fluids of the body, 

 undergoes keratinisation and resists the outwards push of young 

 cells which are thus put under stress. 



II. Connective Tissues. 



To appreciate the significance of the structure of the vegetative 

 tissues, due attention must be directed to their function. These 

 tissues are cell communities with an important but little studied 

 industry. They are the civil engineers of the body. The structures 

 they build are designed to withstand stresses. Before critically 

 examining their handiwork let us study some elementary engineer- 

 ing problems, so that we may the better understand the phenomena 

 of cell structures. 



