FORM AND FUNCTION 211 



The optical properties of glass can be altered by submitting it to 

 pressure. The electrical conductivity of selenium depends on 

 the amount of light falling on it. When more is known of the 

 laws governing matter in the colloidal state, then one may be 

 able to give a clear answer to this problem. 



In certain situations peculiar modifications of fibrous tissue are 

 found : 



(1) Endothelium consists of flattened cells forming a membrane. 

 It differs from pavement epithelium by having the formed material 

 (colloidal exudate) behveen and not in the cells. Such endothelial 

 layers line all the serous cavities of the body and the lymphatics, 

 blood vessels and heart. A structure similar to endothelium may 

 be produced when an aqueous solution of, say, fatty acid is added 

 to a mixture of hydrated colloids of high concentration. Tender 

 such circumstances the pressure of separation deforms the originally 

 spherical globules to form a beaded flattened honeycomb. 



(2) Fat Cefls. The experiments detailed in Chap. IX. throw 

 light upon the appearance of fat in the cells. There is scarcely 

 a tissue or fluid in the body that does not contain fat in amounts 

 in excess of the quantities that can be dispersed in colloid-free 

 water. Finely divided fat in cell protoplasm is comparable to an 

 emulsion. It depends for its permanence on the same factors as 

 maintain fat in a finely divided form in an aqueous dispersant, i.e., 

 mainly on the presence, in the tissues, of hydrophilic colloids. 

 While the fat in the cells is not ordinarily visible or even demon- 

 strable by microchemical methods, when an excessive amount of fat 

 is present it may be seen in the network of areolar fibrous tissue, 

 especially round the smaller blood vessels. Little droplets of oil 

 at first appear and these become larger, run together and coalesce, 

 forming a single large globule, distending the cell and pushing to 

 the sides the protoplasm as a sort of capsule. Reference to the 

 chapter on emulsions will show that when the colloid in an oil- 

 in-colloid ?mulsion decreases in hydrophilic power beyond a certain 

 amount, the nature of the emulsion is changed to colloid-in-oil. 

 This latter emulsion differs from the former not only in the 

 visibility of the fat, but in this respect that the fat may be 

 stained (black) by osmic acid or (orange) by sudan III. (page 105.) 



In starvation the fat gradually disappears from the cell leaving 

 the hydrated colloid, which also in time disappears and the cell 

 resumes its shape. 



Apart from acting as a storehouse of energy, fatty tissue has 

 important mechanical functions. As we shall see later, the layer 

 of subcutaneous fat serves as an extra garment protecting the 

 wearer from the too rapid loss of heat (Chap. XXXII.). Then too, 



14—2 



