The skeletal system includes bones, joints, cartilages, and ligaments. It is subdivided into the axial skeleton which forms the longitudinal axis of the body, and the appendicular skeleton which includes the bones of the limbs and girdles. Bones have several functions including support, protection, movement, storage, and blood cell formation. There are four classifications of bones based on shape: long bones, short bones, flat bones, and irregular bones. The structure of long bones includes a diaphysis, epiphyses, articular cartilage, medullary cavity, and microscopic features like osteons, lacunae, and canaliculi.

1. THE SKELETAL SYSTEM

2.  The word skeleton comes from the Greek word
meaning “dried-up body”. Strong, yet light, it is
perfectly adapted for its functions of body protection
and motion.
 The skeletal system includes joints, cartilages, and
ligaments (fibrous cord that bind the bones together
at joints).
The skeleton is subdivided into two divisions:
 Axial skeleton – the bones that from longitudinal axis
of the body.
 Appendicular skeleton – the bones of the limbs and
girdles.

4. BONES
 are rigid organ that constitute part of
the endoskeleton of vertebrates. They support and protect the
various organs of the body, produce red and white blood
cells and store minerals.
Functions of the Bones
 SUPPORT – Bones, the “steel girders” and “reinforced
concrete” of the body, from the internal frame work that
supports and anchors all soft organs. The bones of the leg act
as pillars to support the body trunk when we stand, and the rib
cage supports the thoracic wall.

5.  PROTECTION – Bones protect soft body organs.
 MOVEMENT – Skeletal muscles, attached to bones by tendons,
use the bones as levers to move the body and its parts.
 STORAGE – Fat is stored in the internal cavities of bones.
Bones itself serve as a storehouse for minerals, the most
important being calcium and phosphorus, although others are
also stored.
 BLOOD CELL FORMATION – Blood cell formation, or
hematopoiesis, occurs within the marrow cavities of certain
bones.

6. CLASSIFICATION OF BONES
There are two basic types of osseous, or bone, tissue:
 COMPACT BONE – is dense and look smooth and
homogeneous.
 SPONGY BONE – is composed of
small needle like pieces of bone
and lots of open space.

7. Bones are classified according to shape into four groups: long,
short, flat, and irregular.
 LONG BONES – are typically longer than they are wide. As a
rule they have a shaft with heads at both ends. Long bones are
mostly compact bone.
 SHORT BONES – are generally cube-shaped
and contain mostly spongy bone.
Sesamoid bones, which form within
tendons, are special type of short bone.
Ex. Patella or kneecap.

8.  FLAT BONES– are thin, flattened, and usually curved. They have
two thin layers of compact bone sandwiching a layer of spongy
bone between them.
 IRREGULAR BONES – Bones that do not fit one of the
preceding categories. The vertebrae, which make up the spinal
column, and the hip bones fall into this group.

9. STRUCTURE OF A LONG BONE
GROSS ANATOMY
 The diaphysis ,make up most of the bone’s length and is
composed of compact bone. It is covered and protected by a
fibrous connective tissue membrane, the periosteum.
 The epiphyses are the ends of the long bone. Each epiphyses
consist of a thin layer of compact bone enclosing an area filled
with spongy bone.
 Articular cartilage, it covers its external surface. Because the
articular cartilage is grassy hyaline cartilage, it provides
smooth, slippery surface that decreases friction at joint
surfaces.
 Epiphyseal line – is a remnant of the epiphyseal plate ( a flat
plate of hyaline cartilage) seen in a young, growing bone.

10.  Yellow marrow or medullary, cavity – the shaft is a primarily a
storage area for dispose (fat) tissue in adults. However, in
infants this area forms blood cells, and red marrow is found
there.

11. Bone markings – surface of bones are not smooth but scarred
with bumps, holes, and ridges. It reveal where muscles,
tendons, and ligaments were attached and where blood
vessels and nerves passed.
TWO CATEGORIES OF BONE MARKINGS
 a.) projections or processes – which grow out from the
bone surface.
 b.) depressions or cavities – which are indentations in
the bone.

14.  Microscopic anatomy
Compact bone
tissue ----->
 The mature bones cells, osteocytes are found in tiny cavities
within the matrix called lacunae. The lacunae are arranged in
concentric circles called lamellae around central (Harversian)
canals.
 Each complex consisting of central canal and matrix ring is
called an osteon, or Haversian system.

15.  Canaliculi (tiny canal) – rediate outward from the central canals
to all lacunae. The canaliculi from a transportation system that
cannecta all the bone cells to the nutrient supply through the
hard bone matrix.
 Perforating (volkmann’s) canals – the communication pathway
from the outside of the bone to its interior (and the central
canals), which
run into the com-
pact bone at right
angles to the
shaft.

16.  Bones is one of the hardest material in the body, and
although relatively light in weight, it has a remarkable
ability to resist tension and other forces acting on it.
The calcium salts deposited in the matrix give bone its
hardness, whereas the organic parts provide for
bone’s flexibility and great tensile strength.