These pages have been left in this location as a service to the numerous websites around the world which link to this content. The original authors are no longer at the University of Leeds, and the former Centre for Human Biology became the School of Biomedical Sciences which is now part of the Faculty of Biological Sciences.
Introductory Anatomy: Bones
Dr. D.R.Johnson, Centre for Human Biology
Anatomists talk about both bone and bones. The former is a
type of connective tissue made up of cells suspended in a matrix:
the collagenous matrix in bone just happens to be heavily impregnated
with minerals. You will learn about bone cells elsewhere, but
here is a picture of a cast of one, just to prove they exist
. This osteocyte has characteristic long processes which run
through the bone putting it in touch both with other cells and
with blood vessels and nerves. Bones are discrete organs made
up of bone tissue, plus a few other things.
Determination of shape
The shape and structure of bones is governed by many factors,
genetic, metabolic and mechanical. Genetic determination of primary
shape can be demonstrated by organ culture of bone rudiments,
which subsequently grow into recognisable bones, i.e. roughly
the finished shape in all major respects. Fine tuning is by muscular
action. The muscles are active in utero, although it is difficult
to isolate their effect at this stage. After birth, however,
and up to adolescence there is a correlation between activity
and growth. this is seen in reverse if we look at people who
are bedridden, or who have paralyses (such as poliomyelitis).
Origin of bone is again in two main forms. Some bone (in broad
terms almost everything except the top of the skull) is preformed
in cartilage - replacement or endochondral bone. Details will
come in histology lectures. In the skull and one or two other
places, however, bone forms direct in membranous connective tissue
- membrane bone.
Classification of bones
The skeleton is made up of many bones which change in proportion between man and his close relatives but are easily recognisable. The easiest way to classify bones is by shape.
Long bones Typical of limbs, and a good place to start.
They consist of a central, usually hollow, tubular region, the
diaphysis linked to specialised ends (epiphysis) by a junctional
region (metaphysis). Look at the shaft first. Tubular, a bit
like a bicycle frame tube. Galileo was the first to write sensibly
about this, noting that a hollow tube was stronger, weight for
weight than a solid rod, and that the dimensions had to be related
to body weight rather than area: so the bones of an elephant
have to be proportionally broader than those of a man. In some
bones we can see adaptations for specific forces. For example
the wing bones of vultures and other large birds have strengthening
that makes them very like bridges: it is a sobering thought that
the first vulture predates the first girder bridge by some millions
of years. The diaphysis has layers of bone arranged like plywood
for strength. The cavity is filled with bone marrow (red and
active in children, yellow, fatty and inactive in adults). The
shaft walls are made of compact hard bone, and thickest in the
middle where forces are greatest. If these forces are too great
the shaft may fracture. Young bones have less calcium and are
pliable, so fracture raggedly and partially (greenstick): older
bones will fracture transversely or spirally according to force
applied. Fractures usually heal spontaneously, albeit rather
slowly in some cases, but the broken surfaces need to be manipulated
into the right place and may need to be held with casts, pins
Short bones are found in the wrist and ankle, carpals and tarsals respectively. They have no shaft, as they do not increase dramatically in size in one dimension during growth, and tend to be cuboidal in shape. They are rather like a Malteser in construction, with cancellous bone in the centre and a hard outer shell of compact bone.
Flat bones like those of the cranium or the scapula are sandwiches of spongy bone between two layers of compact bone. They are usually curved, so we can refer to an inner and outer table with diploe between them. These diploe, especially in the skull, may become pneumatised, i.e. filled with air. A ring of facial sinuses around the nose may become infected, leading to sinusitis.
Any bones which don't fit these arbitrary categories (bones of the face, vertebrae) are referred to as irregular.
Sesamoid bones are interesting because they occur in tendon, especially where a tendon turns a corner, and is thus exposed to friction. We shall come across these again when we talk about muscles.
Surface markings of bone.
We can often glean clues about what is going on around a bone from its surface. In places, like joint surfaces, the bone will be covered with smooth articular cartilage. This falls off in preparation but leaves the underlying bone smooth too. Bone is constantly growing or being reshaped, and this takes place on the surface. At high magnification we can see, in a dried bone, what it was up to the point of death. This picture shows a hole for a blood vessel, a foramen. Around roughly half its diameter the collagenous bone is rough, the other half smooth. The rough is resorbing bone, being eaten by large osteoclasts which leave pits and the smooth is depositional, bone being formed. This indicates that the foramen was on the move as the bone grew. Other areas also show deposition and resorption: these would be building up and hollowing out respectively. On a macroscopic scale these effects can be seen as points of attachment to the bone - of ligaments, tendons or the fibrous insertions of muscles. All these structures transmit forces, and demand a well organised junction. Any part of this structure which has deposited calcium will appear as a bit of bone. Within the bone we often see rows of trabeculae or thick ropes of collagen, Sharpey's fibres running across the marrow cavity to insert in the cortical bone opposite. Blood vessels and nerves similarly have canals.
The various lumps for fixing things to have different names according to shape, usually derived from a dead language. There are lots of these, but common ones are: