ChAPTER 8 SKELETAL SYSTEM: AXIAL AND APPENDICULAR SKELETON

Introduction to the Skeletal System and Bone Lab # General Characteristics of the Skeleton - Bone mass: The skeleton accounts for approximately $20\%$ of the total body mass. - Bone longevity: While muscles decompose after death, bones persist due to the presence of the inorganic mineral hydroxyapatite. - Disarticulation: In a living body, bones are articulated; after death, as muscles decompose, the bones become disarticulated. - Number of bones: An adult human has exactly $206$ bones. - Infants and bone count: Babies are born with over $300$ bones. For example, the maxilla and pelvis consist of multiple bones in infancy that fuse into single units in adulthood. # Fundamental Anatomical Components - Ligaments: Connect bone to bone. Mnemonic: The 'L' is for 'Life' (long-term connection of bones). - Tendons: Connect muscle to bone. Mnemonic: The 'T' is for 'Two' different things (muscle and bone). - Example of Tendon: The Achilles tendon connects the gastrocnemius (calf muscle) to the heel. - Example of Ligament: The ACL (anterior cruciate ligament) connects movement within the knee joint from bone to bone. # Division of the Skeletal System - Axial Skeleton: Consists of $80$ bones located along the central axis of the body (straight up and down). - Appendicular Skeleton: Consists of $126$ bones, including the upper and lower appendages and the girdles that connect them to the axial skeleton. # The Axial Skeleton (80 bones) - The axial system is divided into five main sectors: the skull, middle ear bones, hyoid bone, vertebral column, and thoracic cage. - Skull: Contains $22$ bones ($8$ cranial and $14$ facial). - Hyoid Bone: $1$ bone. - Auditory Ossicles (Middle Ear): $6$ bones total ($3$ per side), to be covered later in the semester. - Vertebral Column: $26$ bones in the adult. - Thorax: $1$ sternum and $24$ ribs. # Detailed Anatomy of the Skull - Main divisions: The skull consists of $8$ cranial bones protecting the brain and $14$ facial bones forming the face. ## Cranial Bones (8 Total) - Frontal Bone ($1$): Forms the forehead and the roof of the orbital sockets. - Parietal Bones ($2$): Paired bones forming the superior and lateral portions of the cranium. - Temporal Bones ($2$): Form the sides and base of the cranium; contain the external acoustic meatus (ear canal). - Occipital Bone ($1$): Forms the back and base of the skull. - Sphenoid Bone ($1$): A butterfly or 'bat-shaped' keystone bone that articulates with every other cranial bone. It contains the sella turcica, a depression where the pituitary gland sits. - Ethmoid Bone ($1$): Located between the orbits; forms part of the cranial floor and nasal cavity. Highlights include the crista galli (where the dura mater attaches to stabilize the brain) and the cribriform plate (a perforated area for olfactory nerves/smell). ## Facial Bones (14 Total) - Zygomatic Bones ($2$): The cheekbones. - Nasal Bones ($2$): Form the bridge of the nose. - Lacrimal Bones ($2$): Small bones housing the tear ducts. - Maxillae ($2$): Upper jaw bones that fuse in adulthood; contain alveolar sockets for teeth. Includes the hard palate. - Inferior Nasal Conchae ($2$): Increase the surface area of the nasal cavity. - Palatine Bones ($2$): L-shaped bones forming the back of the hard palate. - Mandible ($1$): Lower jaw/chin; the only movable bone of the skull. Includes the mental foramen. - Vomer ($1$): Plow-shaped bone forming the inferior portion of the nasal septum. ## Sutures of the Skull - Sagittal Suture: Connects the left and right parietal bones down the midline. - Coronal Suture: Connects the frontal bone to the parietal bones (like a crown). - Lambdoid Suture: Connects the occipital bone to the parietal bones at the back of the head. - Squamous Suture: Connects the temporal bone to the parietal bone on the sides of the head. ## Significant Skull Landmarks - Foramen Magnum: The large opening at the base of the occipital bone where the spinal cord connects to the brain. - Occipital Condyles: Smooth markings on either side of the foramen magnum that articulate with the first vertebrae ($C1$) to allow the 'yes' nodding motion. - External Acoustic Meatus: The canal leading to the middle ear. - Mastoid Process: A rounded protrusion on the temporal bone; site of muscle attachment (e.g., sternocleidomastoid). - Styloid Process: A sharp, needle-like protrusion for tongue and facial muscle attachment. # The Hyoid Bone - Description: A U-shaped, free-floating bone in the neck. - Unique Feature: It is the only bone in the body that does not articulate with any other bone. - Function: Serves as a movable base for the tongue and supports swallowing. # The Vertebral Column (26 bones) - Regional Count Mnemonic: 'Breakfast at $7$, Lunch at $12$, Dinner at $5$'. - Cervical Vertebrae: $7$ bones ($C1$ through $C7$) in the neck. - Thoracic Vertebrae: $12$ bones ($T1$ through $T12$) in the mid-back. - Lumbar Vertebrae: $5$ bones ($L1$ through $L5$) in the lower back. - Sacrum: In the adult, $1$ bone (formed by $5$ fused bones in infants). - Coccyx: The tailbone, $1$ bone (formed by $3$ to $4$ fused bones). ## Specialized Vertebrae - Atlas ($C1$): Has no body and no spinous process. It articulates with the occipital condyles to allow the 'yes' movement. - Axis ($C2$): Features a unique protrusion called the dens (odontoid process). The Atlas rotates on the dens to allow the 'no' movement (rotation). - Recognition Tips: Cervical vertebrae have transverse foramina (holes for nerves). Thoracic vertebrae have sharp spinous processes and rib facets. Lumbar vertebrae are massive with blunt spinous processes to support body weight. ## Spinal Curvatures and Conditions - Primary Curvatures: The C-shaped spine present at birth. - Secondary Curvatures: The cervical curve develops at $3$ to $4$ months (holding head up); the lumbar curve develops at $1$ year (walking upright). - Kyphosis: Hunchback; abnormal thoracic curvature. - Scoliosis: Abnormal lateral curvature; often corrected with rods to allow for proper chest expansion/breathing. - Lordosis: Swayback; abnormal lumbar curvature, often associated with pregnancy or a 'potbelly'. - Herniated Disc: Occurs when the annulus fibrosis breaks down, allowing the nucleus pulposus to protrude and press on spinal nerves. # The Thoracic Cage - Sternum: Consists of the manubrium (top), body (middle), and xiphoid process (bottom). - Safety Warning: The xiphoid process ossifies (hardens) at age $40$. Improper CPR compressions can break it, potentially driving it into the heart or lungs. - Ribs ($24$ total): - True Ribs ($1$ through $7$): Connect directly to the sternum. - False Ribs ($8$ through $10$): Connect to the rib above them. - Floating Ribs ($11$ and $12$): Do not connect to the sternum at all. # The Appendicular Skeleton (126 bones) - Pectoral Girdle: Consists of the clavicle (collarbone) and scapula (shoulder blade). - The scapula features the shallow glenoid cavity, allowing for $360$ degree arm rotation. - Pelvic Girdle: Formed by the left and right hip bones (coxal bones). ## The Upper Limb (30 bones per side) - Humerus: Upper arm bone. - Radius: Lateral forearm bone (aligned with the thumb or 'pollex'). - Ulna: Medial forearm bone; includes the olecranon process (elbow). - Carpals ($8$ per wrist): Wrist bones arranged in two rows. Students must know the scaphoid (proximal) and trapezium (distal) near the thumb. - Metacarpals ($5$): Bones of the palm, numbered $1$ to $5$ starting from the thumb. - Phalanges ($14$): Finger bones. Digits $2$ through $5$ have three phalanges (proximal, middle, distal). Digit $1$ (Pollex/Thumb) has only two (proximal and distal). ## The Lower Limb (30 bones per side) - Femur: Thigh bone; the heaviest and strongest bone. - Patella: Kneecap; keeps the femur and tibia aligned. - Tibia: Larger, medial shin bone; the weight-bearing bone of the lower leg. - Fibula: Slender, lateral bone; non-weight-bearing (often used for bone grafts). - Tarsals ($7$ per ankle): Ankle bones. Keywords: Talus (where the tibia sits) and Calcaneus (the heel bone). - Metatarsals ($5$): Numbered $1$ through $5$. Digit $1$ is the Hallux (great toe). - Phalanges ($14$): Toe bones. The Hallux has two; all other toes have three. ## Fractures and Arch Disorders - Colles' Fracture: Fracture of the distal radius/wrist, often from breaking a fall with an outstretched hand. - Potts' Fracture: Fracture involving the distal tibia or fibula at the ankle. - Arches of the Foot: Lateral, medial, and transverse arches distribute weight. - Plantar Fasciitis: Inflammation of the fascia connecting the toes to the heel. # Questions & Discussion - Q: Does the femur articulate with the fibula? - A: No. The knee joint consists of the femur, tibia, and patella. The fibula is lateral and non-weight-bearing. - Q: How can investigators determine the sex of skeletal remains? - A: The pelvis is the primary indicator. Females have a wider pubic arch (>90 degrees/U-shaped) and a rounded pelvic inlet for childbirth. Males have a narrower arch (<90 degrees/V-shaped) and a heart-shaped inlet. - Q: Which foramen is the largest in the body? - A: The obturator foramen in the pelvis, not the foramen magnum in the skull.