_S.C._, Compact substance; _S.s._, spongy substance; _C.m._, medullary
cavity; _F.n._, nutrient foramen. Note the greater thickness of the
compact substance of the inner and anterior parts of the shaft.
]
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 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 plates are definitely arranged with regard to mechanical
requirements, so that systems of pressure and tension plates can be
recognized, in conformity with the lines of pressure and the pull of
tendons and ligaments respectively. The intervals (marrow spaces)
between the plates are occupied by marrow. The spongy substance forms
the bulk of short bones and of the extremities of long bones; in the
latter it is not confined to the ends, but extends a variable distance
along the shaft also. Some bones (Ossa pneumatica) contain =air-spaces=
or =sinuses= within the compact substance instead of spongy bone and
marrow. 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 even 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, an inner layer of very dense bone,
the =tabula vitrea=, and between these a variable amount of spongy bone,
here termed =diploë=.
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 layer, and an inner cellular osteogenic layer. During
active growth the osteogenic layer is well developed, but later it
becomes much reduced. The fibrous layer varies much in thickness, being
in general thickest in exposed situations. The adhesion of the
periosteum to the bone also differs greatly in various places; it is
usually very thin and easily detached where it is thickly covered with
muscular tissue which has little or no attachment. The degree of
vascularity conforms to the activity of the periosteum.
The =marrow= (Medulla ossium) occupies the interstices of the spongy
bone and the medullary cavity of the long bones. There are two varieties
in the adult—red and yellow. In the young subject there is only =red
marrow= (Medulla ossium rubra), but later this is replaced in the
medullary cavity by =yellow marrow= (Medulla ossium flava). The red
marrow contains several types of characteristic cells and is a
blood-forming substance, while the yellow is practically ordinary
adipose tissue.
Since yellow marrow is formed by regressive changes in red marrow,
including fatty infiltration and degeneration of the characteristic
cells, we find transitional forms or stages in the process. In aged or
badly nourished subjects the marrow may undergo gelatinous
degeneration, resulting in the formation of gelatinous marrow.
=Vessels and Nerves.=—It is customary to recognize two sets of
=arteries=—the =periosteal= and the =medullary=. The former ramify in
the periosteum and give off innumerable small branches which enter
minute openings (Volkmann’s canals) on the surface and reach the
Haversian canals of the compact substance. Other branches enter the
extremities of the long bones and supply the spongy bone and marrow in
them. In the case of the larger bones—and especially the long bones—the
large =medullary= or =nutrient artery= enters at the so-called =nutrient
foramen= (Foramen nutricium), passes in a canal (Canalis nutricius)
through the compact substance, and ramifies in the marrow; its branches
anastomose with the central branches of the periosteal set. The larger
=veins= of the spongy bone do not, as a rule, accompany the arteries,
but emerge chiefly near the articular surfaces. Within the bone they are
destitute of valves.
The =lymph vessels= form perivascular channels in the periosteum and the
Haversian canals of the compact substance. Lymph-spaces exist at the
periphery of the marrow.
The =nerves= appear to be distributed chiefly to the blood-vessels.
Special nerve-endings (Vater-Pacini corpuscles) in the periosteum are to
be regarded as sensory, and probably are concerned in mediating the
muscle sense (Kopsch).