ADIPOSE TISSUE 55 



of the tissue not usually denioustrated in preparations i)lays an 

 extremely vital part in metabolism. This type of tissue plays an 

 important role in repair following injuries and infeetions and is 

 closely linked to serological reactions which are occupying those 

 interested in innnunology. 



SEROUS MEMBRANES. 



The peritoneum, pleunie, and pericardium are thin layers of 

 fibroelastic connective tissue covered with mesothelium. The cellu- 

 lar elements are more numerous in these membranes than in the loose 

 fibroelastic connective tissue of other regions. The mesenteries are 

 composed of similar thin membranes of connective tissue with 

 mesothelium on both surfaces. The cells of serous membranes 

 produce a watery, somewhat viscous fluid, which lubricates the 

 surfaces of organs as they move over one another during their 

 physiological activities. Histiocytes as well as white blood cells, 

 especially neutrophils, increase in number in the fluid ground sub- 

 stance of these membranes in inflammation. Fat cells may accu- 

 mulate in certain regions to form considerable masses. 



ADIPOSE TISSUE. 



This is a modification of the loose fibroelastic type in which many 

 of the cells become active in storage of fat. (Fig. 32.) The fat- 

 storing cells are considered by some to be directly derived from 

 fibrocytes but others believe them to be more directly derived 

 from undifl'erentiated mesenchyme cells, called adipoblasts. How- 

 ever derived, these cells occur in large or scattered masses among 

 the other elements of loosely organized fibroelastic tissue. 



During transformation of fibrocytes or adipoblasts into typical 

 fat cells, droplets of fat accumulate within the cytoplasm. The cell 

 becomes distended into a spherical mass with a protoplasmic 

 periphery. When a number of fat cells are formed close together, 

 the spherical form of each is modificfl at points of contact, so that 

 such cells appear as polyhedral bodies resembling a mass of small 

 soap bubbles. In such groups the dift'erent portions of the cell 

 membranes may be seen by changing focus. In a number of loca- 

 tions, fat cells accumulate in small or large masses, as will be 

 observed in sections of the skin, the mesenteries, the supporting 

 tissues of the thymus, kidney, and intestinal tract. Under condi- 



