STRUCTURE OF BONES 



21 



STRUCTURE OF BONES' 

 Bones consist chiefly of bone tissue, but considered as organs they present 

 ne^eT ^'"''"^"P"'^' "^^"^^rane, termed the periosteum, the marrow, vessels, and 



ir.J^'' ^""^i*""*''"! °^ ^°''' "^"^ ^"^ '*^^^'^ ^'^^^ by "^eans of longitudinal and 

 transverse sections of specimens which have been macerated so as to remove most 

 ol the organic matter. These show that the bone 

 consists of an external shell of dense compact sub- 

 stance, within which is the more loosely arranged 

 spongy substance. In typical long bones the shaft 

 IS hollowed to form the medullary cavity (Cavum 

 medullare). 



The compact substance (Substantia compacta) 

 differs greatly in thickness in various situations, in 

 conformity with the stresses and strains to which 

 the bone is subjected. In the long bones it is 

 thickest in or near the middle part of the shaft and 

 thins out toward the extremities. On the latter the 

 layer is very thin, and is especially dense and smooth 

 on joint surfaces. 



The spongy substance (Substantia spongiosa) 

 consists of delicate bony plates and spicules which 

 run in various directions and intercross. These are 

 definitely arranged with regard to mechanical re- 

 quirements, so that systems of pressure and tension 

 plates can be recognized, in conformity with the 

 lines of pressure and the pull of tendons and liga- 

 ments respectively. The intervals between the 

 plates are occupied by marrow, and are termed 

 marrow spaces (Cellulae medullares). The spongy 

 substance forms the bulk of short bones and of the 

 extremities of long bones; in the latter it is not con- 

 fined to the ends, but extends a variable distance 

 along the shaft also. Some bones contain air-spaces 

 within the compact substance instead of spongy 

 bone and marrow, and hence are called pnetunatic 

 bones (Ossa pneumatica) . These cavities are termed 

 sinuses, and are lined with mucous membrane; 

 they communicate indirectly with the external air. 

 In certain situations the two compact layers of flat 

 bones are not separated by spongy bone, but fuse 

 with each other; in some cases of this kind the bone 

 is so thin as to be translucent, or may undergo 

 absorption, producing an actual deficiency. 



The flat bones of the cranial vault and sides 

 are composed of an outer layer of ordinary compact 

 substance, the lamina externa, an inner layer of 



very dense bone, the lamina interna or tabula vitrea, and between these a variable 

 amount of spongy bone, here termed diploe. 



The periosteum is the membrane which invests the outer surface of bone, 

 except where it is covered with cartilage. It consists of an outer protective fibrous 



Fig. 1. — Sagittal Section of Large 

 Metatarsal Bone of Horse 

 (Right). 



S.c, Compact substance: S.s., 

 spongy substance: Cm., medullary 

 cavity: -P.n., nutrient foramen. Note 

 the greater thickness of the compact 

 substance of the anterior part of the 

 shaft. 



1 Only the gross structure is discussed here, 

 be made to histological works. 



For the microscopic structure reference is to 



