The Appendicular Skeleton

The Appendicular Skeleton

• The appendicular skeleton is primarily involved in body movements.
• It includes the bones of the upper and lower limbs, and the girdles that attach them to the axial skeleton.

Pectoral (Shoulder) Girdle

• The two pectoral (shoulder) girdles include a clavicle and a scapula.
• The clavicle (collarbone) is S-shaped.
  • The medial (sternal) end articulates with the manubrium of the sternum.
  • The lateral (acromial) end articulates with the acromion of the scapula.
• The scapula is a flat bone located in the superior part of the posterior thorax between the second and seventh ribs.
  • Its glenoid cavity is the attachment point for the humerus.

Upper Limb (Extremity)

• The humerus, or arm bone, articulates with the scapula proximally (its rounded head fits into the glenoid cavity).
• It articulates with the radius and ulna distally (the trochlea articulates with the ulna and the capitulum with the radius).
• The ulna and radius are the two bones of the forearm.
  • The olecranon and coronoid process at the proximal end of the ulna form the trochlear notch which wraps around the trochlea of the humerus making up the elbow joint.
  • The radius is located on the lateral (thumb) side of the forearm.
  • The articulation of its head with the capitulum of the humerus and with the ulna allow the forearm to rotate.
• The carpal bones are 8 small bones connected to each other by ligaments and they are arranged in two rows of four bones each.
  • The proximal row (scaphoid, lunate, triquetrum, pisiform) articulates with the distal radius and ulna.
  • The distal row (trapezium, trapezoid, capitate, hamate) articulates with the metacarpals.
• The five metacarpals make up the palm and back of the hand.
  • Numbered I through V (or 1–5) starting with the thumb.
  • Bases articulate with the distal carpals while their heads articulate with the proximal phalanges.
• The phalanges are the bones of the digits.
  • 14 total.
  • The thumb contains 2 (proximal and distal) while the other 4 fingers contain 3 each (proximal, middle and distal).

Bony Pelvis

• Components: Pelvic (hip) girdle, hip bone, sacrum, coccyx, pubic symphysis.

Pelvic (Hip) Girdle

• The pelvic girdle is made up of two hip bones (os coxa, coxal bones) that articulate with the sacrum posteriorly.
  • Each hip bone is actually made up of three individual bones: ilium, ischium, pubis.
  • The two bones articulate anteriorly at the pubic bones (pubic symphysis).
  • There is a disc of fibrocartilage between the two bones.
• The head of the femur articulates with the acetabulum of the hip bone as a ball and socket joint.
  • The acetabulum is composed of parts of all three of the bones that make up the hip bone.
• The pelvis is divided into a superior and an inferior portion by the pelvic brim which is where the abdomen meets the pelvic cavity.
  • The area of the bony pelvis superior to the pelvic brim is known as the false (greater) pelvis.
  • The area of the bony pelvis inferior to the pelvic brim is known as the true (lesser) pelvis.
• The male and female pelves differ in several ways.
  • The bones of the male pelvis are usually larger and heavier.
  • The bones of the female pelvis are structured to meet the requirements of pregnancy and childbirth.
    • The female pelvis is wider and shallower than that of the male.

Comparison of Female and Male Pelves:

Lower Limb (Extremity)

• The femur is the longest, heaviest, and strongest bone in the body.
  • The proximal end (head) inserts into the acetabulum of the hip bone.
  • The distal end articulates with the tibia and patella.
• The patella is a triangular bone that develops in the quadriceps tendon.
  • Its posterior surface articulates with the femur.
• The lower leg consists of the tibia and the fibula.
  • The tibia’s proximal end articulates with the femur.
  • The tibia’s distal end articulates with the talus bone of the ankle.
  • The tibial tuberosity on the anterior surface is the point of attachment for the patellar ligament.
• The tarsus contains 7 tarsal bones.
  • These are the talus, calcaneus, navicular, 3 cuneiforms and the cuboid.
• The metatarsus is made up of 5 metatarsal bones.
  • Just like the metacarpals, they are numbered I through V (1–5) starting with the big toe.
  • They make up the sole and dorsal surface of the foot.
  • The proximal ends articulate with the 3 cuneiform bones and the cuboid.
  • The distal ends articulate with the proximal phalanges.
• The phalanges are arranged exactly like those of the hand.
  • The big toe has a proximal and distal phalanx and the other toes have a proximal, middle and distal phalanx.
• The foot has two arches that are supported by ligaments and tendons.
  • The purpose of the arches is to allow the foot to support the weight of the body, provide leverage while walking, and distribute the body’s weight over the foot.
  • The two arches are:
    • the longitudinal arch which is made up of a medial and a lateral portion and
    • the transverse arch.

Development of the Skeletal System

• Most skeletal tissue arises from the middle primary germ layer in embryos known as the mesoderm although most of the skull arises from the outer layer called the ectoderm.

• Skull bones develop in 2 ways:

  • The cartilaginous neurocranium (hyaline cartilage) undergoes endochondral ossification.
  • The membranous neurocranium undergoes intramembranous ossification.

• The bones of the face form from the viscerocranium.

  • This is divided into 2 parts:
    • The cartilaginous viscerocranium comes from cartilage of the pharyngeal arches and this forms the ear bones and hyoid bone.
    • The membranous viscerocranium comes from mesenchyme of the first pharyngeal arch, undergoes intramembranous ossification, and forms the facial bones.

• The skeleton of the limb girdles and limbs is derived from mesoderm.

• Between week 4 and week 8 after fertilization, there is an extensive amount of growth and development in the formation of the upper and lower limbs.

The Skeletal System and Homeostasis

• The skeletal system plays an important role in the homeostasis of every system in the body.
• Both directly and indirectly, the skeletal system ensures the proper functioning of these systems.

Contributions of the Skeletal System for All Body Systems:

• Integumentary System
  • Bones provide strong support for overlying muscles and skin
• Muscular System
  • Bones provide attachment points for muscles and leverage for muscles to bring about body movements
  • Contraction of skeletal muscle requires calcium ions
• Nervous System
  • Skull and vertebrae protect brain and spinal cord
  • Normal blood level of calcium is needed for normal functioning of neurons and neuroglia
• Endocrine System
  • Bones store and release calcium, needed during exocytosis of hormone-filled vesicles and for normal actions of many hormones
• Cardiovascular System
  • Red bone marrow carries out hemopoiesis (blood cell formation)
  • Rhythmic beating of the heart requires calcium ions
• Lymphoid (Lymphatic) System and Immunity
  • Red bone marrow produces lymphocytes, white blood cells that are involved in immune responses
• Respiratory System
  • Axial skeleton of thorax protects lungs
  • Rib movements assist in breathing
  • Some muscles used for breathing attach to bones via tendons
• Digestive System
  • Teeth masticate (chew) food
  • Rib cage protects esophagus, stomach, and liver
  • Pelvis protects portions of the intestines
• Urinary System
  • Ribs partially protect kidneys
  • Pelvis protects urinary bladder and urethra
• Genital (Reproductive) Systems
  • Pelvis protects ovaries, uterine tubes, and uterus in females
  • Pelvis protects part of ductus deferens and accessory glands in males
  • Bones are an important source of calcium needed for milk synthesis during lactation