The Appendicular skeleton
The bones of the upper and lower limbs along with
their girdles constitute the appendicular skeleton. The appendicular skeleton
is composed of 126 bones.
The upper limbs are attached to the pectoral
girdles. These are very light and allow the upper limbs a degree of mobility
not seen anywhere else in the body. The girdle is formed of two halves. Each half of the pectoral girdle
(Figure 9.10) consists of a clavicle or collar bone and a scapula.
The scapula is a large, thin, triangular bone situated in the dorsal surface of the ribcage between the second
and seventh ribs. It has a slightly elevated ridge called the spine which
projects as a flat, expanded process called the acromion. The clavicle articulates with this process. Below the
acromion is a depression called the glenoid
cavity which articulates with the head of the humerus to form the shoulder
joint. Each clavicle is a long slender bone with two curvatures which lies
horizontally and connects axial skeleton with appendicular skeleton.
The upper limb consists of 30 separate bones and is
specialized for mobility. The skeleton of the arm, the region between the
shoulder and elbow is the humerus.
The head of humerus articulates with the
The distal end of humerus articulates with the two forearm bones
the radius and ulna. The forearm is the region between the elbow and the wrist. Olecranon process is situated at the
upper end of the ulna which forms the pointed portion of the elbow.The hand
consists of carpals, metacarpals and phalanges.
Carpals,the wrist
bones, 8 in number are arranged in two rows of four each. The anterior surface of the wrist has
tunnel-like appearance, due to the arrangement of carpals with the ligaments.
This tunnel is termed as carpal tunnel.
Metacarpals,the palm
bones are 5 in number and phalanges the
digits bones are 14 in number.
The pelvic girdle is a heavy structure specialized
for weight bearing. It is composed of two hip bones called coxal bones that
secure the lower limbs to the axial skeleton (Figure 9.11). Together, with the
sacrum and coccyx, the hip bones form the basin-like bony pelvis.
Each coxal bone consists of three fused bones, ilium, ischium and pubis. At the point of fusion of ilium, ischium, and pubis a deep
hemispherical socket called the acetabulum is present on the lateral surface of
the pelvis. It receives the head of the femur or thigh bone at the hip joint
and helps in the articulation of the femur.
Ventrally the two halves of the
pelvic girdle meet and form the pubic
symphysis containing fibrous cartilage.
The ilium
is the superior flaring portion of the hip bone. Each ilium forms a secure
joint with the sacrum posteriorly. The ischium
is a curved bar of bone. The V-shaped pubic
bones articulate anteriorly at the
pubic symphysis. The pelvis of male is deep and narrow with larger heavier bones and the female is shallow,
wide and flexible in nature, and this helps during pregnancy which is
influenced by female hormones.
The lower limb consists of 30 bones which carries
the entire weight of the erect body
The bones of the lower limbs are thicker and stronger than the upper
limbs. The three segments of each lower limb are the thigh, the leg or the shank and the foot. The femur is
the single bone of the thigh. It is the largest, longest and strongest bone in
the body. The head of femur articulates with the acetabulum of the pelvis to form the hip joint. Two parallel
bones, the tibia and fibula, form the skeleton of the shank.
A thick, triangular patella forms
the knee cap, which protects the knee joint anteriorly and improves the
leverage of thigh muscles acting across the knee. The foot includes the bones
of ankle, the tarsus, the metatarsus and the phalanges or toe bones. The foot
supports our body weight and acts as
a lever to propel the body forward, while walking and running. The tarsus is made up of seven bones called tarsals. The metatarsus consists of five bones called metatarsals. The
arrangement of the metatarsals is parallel to each other. There are 14 phalanges in the toes which are smaller
than those of the fingers.
A typical long bone has a diaphysis, epiphyses
(singular-epiphysis) and membranes (Figure 9.12). A tubular diaphysis or shaft,
forms the long axis of the bone. It is constructed of a thick collar of compact
bone that surrounds a central medullary cavity or marrow cavity. The epiphyses
are the bone ends. Compact bone forms the exterior of epiphyses and their
interior contains spongy bone with red marrow. The region where the diaphysis
and epiphyses meet is called the metaphysis. The external surface of the entire
bone except the joint surface is covered by a double-layered membrane called
the periosteum. The outer fibrous layer is dense irregular connective tissue.
The inner osteogenic layer consists of osteoblasts (bone- forming cells)
In addition, there are primitive stem cells,
osteogenic cells, that give rise to the osteoblasts. The periosteum is richly
supplied with nerve fibres, lymphatic vessels and blood vessels. Internal bone
surfaces are covered with a delicate connective tissue membrane called the
endosteum. The endosteum covers the trabeculae of spongy bone and lines the
canals that pass through the compact bone. It also contains both osteoblasts
and osteoclasts. Between the epiphysis and diaphysis epiphyseal plate or growth
plate is present.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2024 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.