28 MAMMALIAN ANATOMY 



vessels and fibres will be seen entering the bone. At points where 

 the muscles and ligaments are attached it cannot be easily removed ; 

 it is bound closely to the underlying roughened surface. 



Internal Structure. The fresh bone appears to be solid because the 

 medullary cavity is filled with marrow. The two kinds of marrow 

 should be observed, the yellow marrow filling the medullary cavity 

 proper, and the red marrow filling the spongy tissue in the ends of 

 the bone. 



STRUCTURE OF BONE. 



Bone is composed almost entirely of one kind of connective tissue, 

 the osseous tissue, which, in common with all tissues, is made up of a 

 matrix and protoplasmic cells. The matrix is impregnated with lime 

 salts, which give hardness and rigidity to the bone. These salts are 

 mainly calcium phosphate and calcium carbonate, and they form two- 

 thirds of the weio-ht of the bone. Thev may be removed by acids, 



O */ V V 



leaving only the animal matter, or ossein, which resembles cartilage in 

 some respects, and forms the other third of the bone's weight. After 

 treatment with acid the bones become pliable, although remaining 

 tough and elastic. The animal matter, on the other hand, can be 

 removed from a bone by burning it ; there are then left only the bone 

 salts, which retain the original shape and size of the bone, but are 

 friable and crumble away if handled roughly. When bones are burnt 

 without free access of air, the animal matter is charred, producing 

 animal charcoal. 



Bone tissue is traversed by vascular canals, which are small in the 

 denser tissue, but become irregular spaces in the spongy parts. 



The protoplasmic bone-cells are of microscopic size and of irregular 

 shape. They communicate by long branches with one another and 

 with the vascular canals, and in this way form a close living network 

 throughout the harder matrix. 



The compact tissue is exceedingly dense, and to the naked eye 

 appears to be solid. 



The cancellous or spongy tissue exhibits small cavities or spaces 

 which in the recent state are filled with marrow. The bony plates 

 which bound these spaces and form the walls of the meshes do not 

 pass indiscriminately in all directions, but have a definite arrangement, 

 which varies with the nature of the bone and with the purposes for 



