STRUCTURE OF BONES 21 
STRUCTURE OF BONES?" 
Bones consist chiefly of bone tissue, but considered as organs they present 
also an enveloping membrane, termed the periosteum, the marrow, vessels, and 
nerves. 
The architecture of bone can be studied best by means of longitudinal and 
transverse sections of specimens which have been macerated so as to remove most 
of 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 (Cellule 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 pneumatic 
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 
Fie. 1.—SaGITraL SECTION OF LARGE 
3 MeraTarRsaL Bone oFr Horse 
bones are not separated by spongy bone, but fuse dane: 
with each other; in some cases of this kind the bone S.c., Compact substance; S.s., 
is so thin as to be translucent, or may undergo spongy substance; C.m., medullary 
AS ay : | ee Ms : | | ae , . cavity; F.n., nutrient foramen Note 
absorption, producing an actual deficiency. he jaseater Hhidlovees of Theilcomioack 
The flat bones of the cranial vault and sides substance of the anterior part of the 
are composed of an outer layer of ordinary compact — “B*“*: 
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 diploé. 
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 
1 Only the gross structure is discussed here. For the microscopic structure reference is to 
be made to histological works. 
