The Skeletal System: The Appendicular Skeleton

The Appendicular Skeleton

• The appendicular skeleton is essential for movement, support, and interaction with the environment.

• It includes bones of the upper and lower limbs and the girdles that attach them to the axial skeleton. These girdles facilitate a wide range of motion and provide stability.

Pectoral (Shoulder) Girdle

• The pectoral girdle, which connects the upper limbs to the axial skeleton, consists of a clavicle and a scapula.

• The clavicle (collarbone) is S-shaped, providing both flexibility and strength. It acts as a strut to keep the upper limb away from the thorax.

  • The medial (sternal) end articulates with the manubrium of the sternum at the sternoclavicular joint, providing the only bony attachment of the shoulder girdle to the axial skeleton. This joint allows for protraction, retraction, elevation, and depression of the scapula.

  • The lateral (acromial) end articulates with the acromion of the scapula at the acromioclavicular joint. This joint allows for gliding and rotational movements to assist with arm movements.

• The scapula is a flat, triangular bone located in the superior part of the posterior thorax between the second and seventh ribs. It is held in place by muscles and ligaments, allowing for considerable movement.

  • Its glenoid cavity is the attachment point for the head of the humerus, forming the glenohumeral joint (shoulder joint). This is a ball-and-socket joint that allows for a wide range of motion, including flexion, extension, abduction, adduction, rotation, and circumduction.

Upper Limb (Extremity)

• The humerus is the arm bone, extending from the shoulder to the elbow.

  • It articulates with the scapula proximally at the glenohumeral joint (shoulder joint). The rounded head of the humerus fits into the glenoid cavity of the scapula.

  • It articulates with the radius and ulna distally at the elbow joint. The trochlea of the humerus articulates with the trochlear notch of the ulna, and the capitulum of the humerus articulates with the head of the radius.

• The ulna and radius are the two bones of the forearm, located between the elbow and the wrist.

  • 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. This arrangement allows for flexion and extension of the forearm.

  • The radius is located on the lateral (thumb) side of the forearm. It is shorter and smaller than the ulna.

  • The articulation of the head of the radius with the capitulum of the humerus and with the ulna (at the proximal radioulnar joint) allows the forearm to rotate (pronation and supination).

• The carpal bones are 8 small bones connected by ligaments and arranged in two rows of four bones each, forming the wrist (carpus).

  • The proximal row (scaphoid, lunate, triquetrum, pisiform) articulates with the distal radius and ulna, forming the radiocarpal joint (wrist joint). The scaphoid is the most commonly fractured carpal bone.

  • The distal row (trapezium, trapezoid, capitate, hamate) articulates with the metacarpals.

• The five metacarpals make up the palm and back of the hand. They are numbered I through V (or 1–5) starting with the thumb (pollex).

  • Their bases articulate with the distal carpals at the carpometacarpal joints, while their heads articulate with the proximal phalanges at the metacarpophalangeal joints (knuckles).

• The phalanges are the bones of the digits (fingers and thumb), 14 in total.

  • The thumb (pollex) contains 2 phalanges (proximal and distal), while the other 4 fingers contain 3 phalanges each (proximal, middle, and distal).

• Mnemonic for remembering carpal bones: Stop Letting Those People Touch The Cadaver's Hand. (Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate).

Bony Pelvis

• Consists of the pelvic (hip) girdle, sacrum, and coccyx. The bony pelvis provides strong support for the vertebral column and pelvic organs and connects the axial skeleton to the lower limbs.

Pelvic (Hip) Girdle

• The pelvic girdle is made up of two hip bones (os coxa, coxal bones) that articulate with the sacrum posteriorly at the sacroiliac joints.

  • Each hip bone is made up of three individual bones: ilium, ischium, and pubis. These bones fuse together during adolescence.

  • The ilium is the largest of the three bones and forms the superior part of the hip bone. It contributes to the acetabulum and the iliac crest.

  • The ischium forms the posterior and inferior part of the hip bone. It includes the ischial tuberosity, which supports the body when sitting.

  • The pubis forms the anterior and inferior part of the hip bone. It articulates with the other pubic bone at the pubic symphysis.

  • The two hip bones articulate anteriorly at the pubic symphysis, with a disc of fibrocartilage between them. This joint provides limited movement.

• The head of the femur articulates with the acetabulum of the hip bone as a ball and socket joint. The acetabulum is a deep, cup-shaped socket that provides stability to the hip joint.

  • The acetabulum is composed of parts of all three bones that make up the hip bone (ilium, ischium, and pubis).

• The pelvis is divided into a superior and an inferior portion by the pelvic brim (linea terminalis), 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. It supports the abdominal viscera.

• The area of the bony pelvis inferior to the pelvic brim is known as the true (lesser) pelvis. It surrounds the pelvic cavity and contains the pelvic organs.

• Male and female pelves differ in several ways, reflecting differences in function and body size.

  • The bones of the male pelvis are usually larger and heavier, with more prominent bone markings.

  • The bones of the female pelvis are structured to meet the requirements of pregnancy and childbirth. The female pelvis is adapted for childbearing.

  • The female pelvis is wider and shallower than that of the male, with a larger pelvic inlet and outlet to facilitate the passage of the fetus during childbirth.

Lower Limb (Extremity)

• The femur is the longest, heaviest, and strongest bone in the body, extending from the hip to the knee.

  • The proximal end (head) inserts into the acetabulum of the hip bone, forming the hip joint. The head of the femur is connected to the shaft by the neck, which is a common site for fractures in elderly individuals.

  • The distal end articulates with the tibia and patella at the knee joint. The medial and lateral condyles of the femur articulate with the medial and lateral condyles of the tibia.

• The patella (kneecap) is a triangular bone that develops in the quadriceps tendon. It protects the anterior aspect of the knee joint and improves the leverage of the quadriceps muscle.

  • Its posterior surface articulates with the femur.

• The lower leg consists of the tibia and the fibula, located between the knee and the ankle.

  • The tibia (shinbone) is the larger and stronger of the two bones. It is located on the medial side of the lower leg.

  • The tibia’s proximal end articulates with the femur and fibula at the knee joint.

  • The tibia’s distal end articulates with the talus bone of the ankle at the ankle joint.

  • The tibial tuberosity on the anterior surface is the point of attachment for the patellar ligament, which is a continuation of the quadriceps tendon.

• Mnemonic for remembering the location of the tibia and fibula: The fibuLA is LAteral.

• The tarsus contains 7 tarsal bones, which make up the ankle. These bones provide stability and allow for movement of the foot.

  • These are the talus, calcaneus, navicular, 3 cuneiforms (medial, intermediate, and lateral), and the cuboid.

• The metatarsus is made up of 5 metatarsal bones, which form the sole and dorsal surface of the foot. They are numbered I through V (1–5) starting with the big toe (hallux).

  • They make up the sole and dorsal surface of the foot.

  • The proximal ends articulate with the 3 cuneiform bones and the cuboid at the tarsometatarsal joints.

  • The distal ends articulate with the proximal phalanges at the metatarsophalangeal joints.

• The phalanges are arranged exactly like those of the hand. They form the toes.

  • The big toe (hallux) has a proximal and distal phalanx, and the other toes have a proximal, middle, and distal phalanx.

• Mnemonic for tarsals: Tall Centers Never Take Shots From Corners. (Talus, Calcaneus, Navicular, Third cuneiform, Second cuneiform, First cuneiform, Cuboid).

• The foot has two arches that are supported by ligaments and tendons. These arches provide flexibility and shock absorption during weight-bearing activities.

  • 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. The medial longitudinal arch is higher and more prominent than the lateral longitudinal arch.

  • The transverse arch, which runs obliquely from the medial to the lateral side of the foot.

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, in which cartilage is replaced by bone.

  • The membranous neurocranium undergoes intramembranous ossification, in which bone forms directly from mesenchymal tissue.

• 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. This process is regulated by various signaling molecules and transcription factors.

The Skeletal System and Homeostasis

• The skeletal system plays an important role in the homeostasis of every system in the body. It provides support, protection, movement, mineral storage, and blood cell formation.

• Both directly and indirectly, the skeletal system ensures the proper functioning of these systems.

• Contributions of the Skeletal System for All Body Systems:

  • Bones provide support and protection for internal organs, such as the brain, spinal cord, heart, and lungs. The rib cage protects the thoracic organs, and the skull protects the brain.

  • Bones store and release calcium, which is needed for proper functioning of most body tissues. Calcium is essential for nerve impulse transmission, muscle contraction, blood clotting, and enzyme activity.

  • Integumentary System: Bones provide strong support for overlying muscles and skin. The shape of bones determines the contours of the body surface.

  • Muscular System:

  • Bones provide attachment points for muscles and leverage for muscles to bring about body movements. Muscles pull on bones to produce movement at joints.

  • Contraction of skeletal muscle requires calcium ions. Calcium ions trigger the interaction of actin and myosin filaments, leading to muscle contraction.

  • Nervous System:

  • Skull and vertebrae protect brain and spinal cord from injury.

  • Normal blood level of calcium is needed for normal functioning of neurons and neuroglia. Calcium ions are involved in nerve impulse transmission and synaptic transmission.

  • Endocrine System:

  • Bones store and release calcium, needed during exocytosis of hormone-filled vesicles and for normal actions of many hormones. Several hormones, such as parathyroid hormone and calcitonin, regulate calcium balance in the body.

  • Cardiovascular System:

  • Red bone marrow carries out hemopoiesis (blood cell formation). Red blood cells, white blood cells, and platelets are produced in the red bone marrow.

  • Rhythmic beating of the heart requires calcium ions. Calcium ions are involved in the excitation-contraction coupling in cardiac muscle cells.

  • Lymphoid (Lymphatic) System and Immunity:

  • Red bone marrow produces lymphocytes, white blood cells that are involved in immune responses. Lymphocytes recognize and attack foreign invaders, such as bacteria and viruses.

  • Respiratory System:

  • Axial skeleton of thorax protects lungs.

  • Rib movements assist in breathing. Contraction of the diaphragm and intercostal muscles causes the rib cage to expand, allowing air to enter the lungs.

  • Some muscles used for breathing attach to bones via tendons.

  • Digestive System:

  • Teeth masticate (chew) food, breaking it down into smaller pieces that can be more easily digested.

  • 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. Calcium is transferred from the mother's bones