ADIPOSE TISSUE 173 



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) between 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. Under 

 such circumstances the pressure of separation deforms the 

 originally spherical globules to form a beaded flattened honey- 

 comb. 



(2) Fat cells. The experiments upon emulsoids detailed in 

 Chapter VIII. 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 proto- 

 plasm is comparable to an emulsion. It depends for its perma- 

 nence on the same factors as maintain fat in a finely divided form 

 in an aqueous dispersartt, i.e. mainly on the presence, in the 

 tissues, of hydrophilic colloids. While the fat in the cells is not 

 ordinarily visible or even demonstrable 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 proto- 

 plasm as a sort of capsule. Reference to the chapter on emulsions 

 will show that when the fat in an oil-in-colloid emulsion is increased 

 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. 



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. XXXI.). Then too, 

 fatty fibrous tissue has a considerable amount of resilience, acting 

 as a buffer protecting organs from external violence. 



