158 COMPARATIVE ANATOMY 



organic part while various salts of calcium, mostly the phosphate and 

 carbonate, are the most important inorganic ingredients. The stratified 

 structure of the matrix is primarily due to the arrangement of collagenous 

 fibers in the organic basis of the matrix. The subsequent depositing of 

 calcareous substance in relation to the stratified fibrous material produces 

 the laminated structure of the finished bone. 



The development of bone has been briefly described (pages 103-106) 

 and the distinction between cartilage bone and dermal or membrane bone 

 has been made. Differing in mode of development, bones of the two kinds, 

 when fully developed, are histologically ahke. Just as the perichondrium 

 adds cartilage at the surface of an already-formed mass, so the periosteum 

 builds lamellae at the surface of a bone. But bone, because of the rigidity 

 of its calcified matrix, is incapable of such interstitial growth as occurs in 

 cartilage, unless it be at stages of bone formation so early that the matrix 

 is not yet fully calcified. A further difference between cartilage and bone 

 lies in the fact that the cartilage cell produces matrix in all directions and 

 thus surrounds itself by its own product, whereas the osteoblast produces 

 matrix only at such part of its surface as is adjacent to the already-formed 

 bone. A layer of bone cells building up lamella upon lamella of bone may 

 be likened to a group of masons laying course upon course of stone at the 

 unfinished top of a wall. But, in the case of the bone, every now and then 

 one of the masons, an osteoblast, is left behind and buried between succes- 

 sive courses of the wall, remaining there in his Httle lacuna as a permanent 

 bone cell. 



Bone once formed may be broken down or "resorbed." By this 

 means the marrow cavity of a long bone is enlarged as the bone grows. 

 The nature of the resorption process is not definitely known, although it 

 has been attributed to action of cells called osteoclasts (Fig. 77). 



Adipose Tissue 



Adipose tissue or "fat" consists of cells each of which contains a 

 globule or vacuole of oil so large that the cytoplasm appears as merely 

 an exceedingly thin layer surrounding the vacuole (Fig. 121). The flat 

 nucleus Kes in the peripheral layer of cytoplasm. The irregular polyhedral 

 form of the cells is doubtless the result of their mutual pressures. As seen 

 in sections, fat looks very much Hke notochord tissue, the individual cell 

 of each being greatly distended by its fluid vacuole. 



Fat develops in richly vascular regions, as along the course of large 

 blood vessels and in the subcutaneous connective tissue. 



Blood 



Blood is the circulating medium which provides for the metaboUc 

 needs of tissue cells which are remote from the source of food and oxygen 



