ADIPOSE TISSUE. 389 



internal supporting structures are here represented by ingrowths of 

 the cuticular formations, which are derived from the epidermal cells, 

 and consist largely of skeletins and hyalogens which have become 

 impregnated with lime salts. Closely related to the latter is chitin, 

 which enters largely into the composition of the outer skeleton 

 of the arthropods and their nerve-sheaths. Associated with it we 

 find the so-called tunicin, which is regarded as a cellulose, and which 

 is found in especial abundance in the mantle of the tunicates. 



ADIPOSE TISSUE. 



Adipose tissue may be regarded as a special form of connective 

 tissue in which the cellular elements enclose globules of fat. When 

 fully developed, the individual cells appear as greatly distended 

 vesicles, which are covered by a cell-membrane. The original proto- 

 plasm has been almost entirely replaced by fat, and occurs merely 

 as a thin layer beneath the membrane. The nucleus also has 

 been displaced to the periphery, and can scarcely be discerned with- 

 out special methods of staining. Such fat-cells usually occur in 

 groups, and are held together by delicate fibres of connective tissue, 

 in the meshes of which a network of blood-capillaries is found 

 surrounding each cell. When large numbers of fat-cells occur, 

 the individual groups are gathered into lobules, and these into 

 lobes. In the living tissue the contained fat exists in a liquid 

 form, but congeals after death, and is then more or less solid accord- 

 ing to the character of the individual fats. Stearin and palmitin 

 then often separate out in crystalline form. Of the chemical com- 

 position of the original cells, before their invasion with fat-globules, 

 nothing definite is known. They contain albumin and are appar- 

 ently rich in water. The cell-membrane is exceedingly resistant to 

 solvents, but is digested by the gastric juice, and possibly consists 

 of an elastin-like substance. 



The relative amount of water and fat which is found in adipose 

 tissue varies primarily with the state of nutrition, and differs in 

 different animals. 



The fats in question are principally the triglycerides of stearic 

 acid, palmitic acid, and oleic acid. Others, such as the glycerides 

 of capronic acid and valerianic acid, are not constant constituents of 

 adipose tissue, but are met with only exceptionally, and always in 

 very small amounts. In man, a comparatively large amount of 

 olein is found, but it is not so abundant as in certain cold-blooded 

 animals, in which it may form the greater portion of the fat. The 

 quantitative relation between the three forms is by no means con- 

 stant in all parts of the body, so that the melting-point of the fats 

 from different regions may be quite different. It differs, moreover, 

 in different animals. This is shown in the following table, which 

 is taken from Gautier : 



