388 THE SUPPORTING TISSUES. 



The bone-marrow is pervaded by a network of connective tissue, 

 which is partly of the white fibrous variety and partly reticulated. 

 In its meshes we find the cellular elements of the marrow, viz., 

 the so-called myeloplaxes, the juvenile forms of the polynuclcar 

 neutrophilic and eosinophilic leucocytes, viz., the myelocytes,red cor- 

 puscles in various stages of development, and a variable number of. 

 fat cells. These latter are especially numerous in the so-called yellow 

 marrow, where the amount of fat may represent as much as 96 per 

 cent, of the entire substance. It consists of olein, palmitin, and 

 stearin. The red marrow, on the other hand, contains a much smaller 

 amount of fat, and owes its color to large numbers of red corpuscles. 

 It contains albumins, of which one is regarded as a globulin, and is 

 said to coagulate at 50 C. But especially interesting is the pres- 

 ence of peculiar iron compounds which are as yet but little known, 

 but probably belong to the nucleo-albumins and iron-containing 

 albuminates. Their presence is no doubt intimately associated with 

 the formation of red corpuscles. Among the extractives of bone- 

 marrow we notably find lactic acid and hypoxanthin. 



THE TEETH. 



The dentin of the teeth is a peculiarly modified form of bone- 

 tissue, and is likewise composed of an organic matrix, which con- 

 sists of collagen and is impregnated with mineral salts. The latter 

 are here even more abundant than in true bone, and represent 

 from 64 to 68 per cent, of the fresh tissue, while of organic matter 

 we find between 26 and 28 per cent., thus leaving JO per cent, 

 for water. The relative distribution of the individual salts is about 

 the same as in common bone. 



The dentinal tubules, like the lacunae and canaliculi, are appar- 

 ently lined by the same albuminous substance. 



The cement which surrounds the dentin of the root as far as 

 the neck of the tooth consists of true bone. 



The enamel, in accordance with its epithelial origin, contains no 

 collagen. It is very rich in lime salts, and its mineral constituents 

 represent as much as 96 per cent, of the total substance. Its organic 

 components correspond to about 3.6 per cent., but are as yet 

 unknown. Water is practically absent. 



Other tissues which are closely related to bone are ivory, tortoise- 

 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 the invertebrate animals, with the exception of the cephalopoda, 

 and possibly also the branch iopods, collagen is not found. The 



