408 THE SUPPORTING TISSUES. 



shell, and the scales of fishes. In the two former the mineral con- 

 stituents predominate over the organic matter, as in bone, while in 

 the latter more organic material is found. Ivory is especially rich 

 in magnesium phosphate, of which it contains about 15.72 per cent. 

 The outer surface of tortoise-shell is covered with a layer of kera- 

 tinized epidermis. In fish-scales mineral material, collagen, and a 

 peculiar albuminoid body, which is termed ichthylepidin, are found. 

 According to Morner, this gives an intense Millon reaction and 

 contains a large amount of loosely combined sulphur. Green and 

 Tower were able to demonstrate its presence in a large number of 

 different fishes. It is apparently found in most of the teleosts, but 

 is absent in the elasmobranchs. 



In the invertebrate animals, with the exception of the cephalopods, 

 and possibly also the branchiopods, collagen is not found The 

 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 

 often separate out in crystalline form. Of the chemical composi- 

 tion 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. 



