Skeleton Anatomy—Vertebrae, Sternum, Ribs, and Appendicular Skeleton (Study Notes)

Vertebral Column: Overview

• Axial skeleton central axis of the body; includes spine, sternum, ribs. Topics covered include vertebrae types, sternum, ribs, and then appendicular skeleton (limbs and girdles).
• There are three major regions of vertebrae discussed here: cervical, thoracic, lumbar, plus the fused sacrum and coccyx. Counts mentioned: $7$ cervical vertebrae, $12$ thoracic vertebrae, $5$ lumbar vertebrae. The sacrum is formed by fusion of five vertebrae, and the coccyx is formed by fusion of $3-5$ vertebrae.
• Intervertebral foramina are not present on a single vertebra; they form when two vertebrae are joined, and spinal nerves exit through them. The vertebral foramen houses the spinal cord.
• Spinal cord features and clinical relevance: nerves branch off at intervertebral foramina and can be pinched if intervertebral spaces narrow (e.g., due to discs, facets, etc.).
• General vertebral features to identify: body (the large circular part), vertebral arch, vertebral foramen (triangular hole), spinous process (projection at the back you can feel along the spine), and transverse processes (projections to the sides).

Cervical Vertebrae (C1–C7)

• Number: $7$ total.
• Key features common to cervical vertebrae:
  • Transverse foramina (holes in the transverse processes) present on all cervical vertebrae; these transmit the vertebral arteries in life.
  • Spinous processes are often bifid (split like a fork) in many cervical vertebrae; not universal, and C1 (atlas) lacks a bifid process.
• Atlas (C1): also called the atlas; unique in that it has no body and no distinct spinous process. Its job is to cradle the skull via the occipital condyles to allow yes-motion (nodding).
  • Superior articular surfaces sit on the occipital condyles; this supports nodding of the head.
• Axis (C2): also called the axis; supports rotation of the head around the dens (odontoid process).
  • The dens (odontoid process) acts as the pivot; C1 rotates around the dens.
• Practical identifications on C1–C2: presence of transverse foramina indicates cervical; absence/presence of bifid spinous process is a clue; atlas lacks a body and a typical spinous process; axis has a distinctive dens that serves as a pivot.

Thoracic Vertebrae (T1–T12)

• Number: $12$ total.
• Features unique to thoracic vertebrae:
  • Facets and demifacets on the body for articulation with ribs (true articulation points for ribs). There are also facets on the transverse processes for rib articulation.
  • Spinous processes point downward compared to cervical vertebrae.
• Ribs: 12 pairs articulate with thoracic vertebrae; this is the vertebral backbone for rib articulation.
• Special thoracic features:
  • Facets (full facets) and demifacets (partial facets; demi = half) for rib articulation.
  • The presence of demifacets is due to head of a rib attaching to two adjacent vertebral bodies (superior demi-facet on one vertebra and inferior demi-facet on the vertebra above).
  • The transverse costal facets on the transverse processes articulate with the tubercles of the ribs.
• Spinous process orientation and overall size are intermediate between cervical and lumbar vertebrae.

Lumbar Vertebrae (L1–L5)

• Number: $5$ total.
• Characteristics:
  • Easiest to identify by size and shape; body is large and bulky; spinous processes are tall and hatchet-shaped when viewed laterally.
  • Remember: lumbar vertebrae are the largest vertebrae, built to bear most of the body's weight.

Sacrum and Coccyx

• Sacrum: formed by the fusion of $5$ sacral vertebrae; the major part of the sacrum sits above the coccyx.
  • Sacral foramina (posterior and anterior) allow passage of nerves; sacral crests are the fused spinous processes along the midline.
  • Auricular surfaces (on the lateral sides) articulate with the ilia of the pelvis to form the sacroiliac joints.
  • The cauda equina is the bundle of nerve roots that continues below the spinal cord into the lower vertebral canal.
• Coccyx: the tailbone; formed by fusion of $3-5$ coccygeal vertebrae; little or no distinctive features on a single coccyx; can be availed or inverted in some people.
• Evolutionary note (from the lecture): coccyx/coccygeal tail remnants reflect ancestral tail structures; some people are born with a vestigial tail which is typically removed with minimal scarring.

Sternum and Ribs

Sternum (Breastbone)

• Three main sections: manubrium (top), body (middle), xiphoid process (tip).
• Notable landmarks on the manubrium:
  • Jugular notch (sternal notch) in the center at the base of the neck.
  • Clavicular notches on either side where the clavicles fuse with the sternum.
• The xiphoid process begins as cartilage and ossifies with age; it can sometimes evert in some individuals.
• There is not a lot to identify on the sternum beyond these landmarks; most rib attachments occur via costal cartilage to the sternum.

Ribs and Costal Cartilage

• Ribs are categorized into three groups:
  • True ribs: $7$ pairs; each rib attaches directly to the sternum via its own costal cartilage (bone → cartilage → bone path).
  • False ribs: $3$ pairs; connect to the sternum via cartilage that fuses with the cartilage of the rib above (bone → cartilage → cartilage → bone).
  • Floating ribs: $2$ pairs; have no anterior attachment to the sternum; they attach to the thoracic vertebrae in the back.
• Costal cartilage is hyaline cartilage and retains its structure in the skeleton; it is highlighted as an important cartilage type in the ribs.
• Rib anatomy to identify on a rib:
  • Head: articulates with the body of a vertebra (often with two facets for two adjacent vertebral bodies).
  • Neck: the small segment between head and tubercle.
  • Tubercle: articulates with the transverse process of the corresponding thoracic vertebra.
  • Costal groove: runs along the inferior border of the rib; houses nerves and vessels; it helps determine orientation (inferior portion).
  • Angle: a noticeable bend in the rib, indicating the curvature.
• Sternum–rib articulation: True ribs have direct bone-to-cartilage-to-bone connections to the sternum; false ribs join via cartilage to the cartilage of the rib above; floating ribs do not connect to the sternum.
• Practical notes about orientation (left vs right ribs):
  • The posterior part is near the vertebrae; the anterior part with the costal cartilage attaches toward the sternum.
  • To determine left vs right, imagine placing the rib on your body and use the costal groove (inferior edge) and the direction of the head (toward the vertebral column) to orient it; the rib that would sit on your left side will be the left rib when placed correctly on your body.
• Xiphoid process caveat: it starts as cartilage and ossifies; can sometimes be everted; when it protrudes, chest compressions can cause injury; normally not removed unless necessary for safety.
• Practical exam note: you will not be asked to identify a rib as true/false/floating by appearance alone without a sternum; you will identify type based on sternum attachment.

Rib Anatomy Summary (posterior view focus)

• Posterior portion faces the vertebral column; anterior portion attaches toward the sternum via costal cartilage.
• Head of rib articulates with the body of a vertebra; tubercle articulates with the transverse process of the same-numbered thoracic vertebra.
• The costal groove is along the inferior edge, guiding orientation and indicating muscle attachment points.

Appendicular Skeleton: Overview

• The appendicular skeleton includes bones of the limbs and girdles (pectoral and pelvic girdles).
• The next sections focus on the upper limb: clavicle, scapula, humerus, radius, ulna, carpal bones, metacarpals, and phalanges. The patterns in the upper limb largely repeat in the lower limb with different names, making upper and lower appendicular skeletons comparable.

Clavicle (Collarbone)

• Structure: a slender, S-shaped bone connecting the sternum to the scapula.
• Three key features to identify:
  • Sternal end: the medial, bulbous end that articulates with the sternum.
  • Acromial end: the lateral, flatter end that articulates with the acromion of the scapula.
  • Tubercle (and sometimes a coracoid tubercle on some diagrams): a projection along the inferior or anterior surface that serves as a muscle attachment site.
• Orientation tips: the sternal end is medial (toward the sternum); the acromial end is lateral (toward the shoulder). The clavicle often has a gentle S-curve, and the curvature helps indicate orientation.
• This is the anterior brace that helps suspend the upper limb; damage to this region can predispose to shoulder dislocations due to relatively small bony support where the clavicle meets the shoulder girdle.

Scapula (Shoulder Blade)

• Structure and surface features:
  • Spine of the scapula: a prominent ridge running diagonally across the posterior surface.
  • Acromion: the extension of the spine at the lateral end; articulates with the clavicle at the acromioclavicular joint.
  • Coracoid process: a hook-like projection on the anterior aspect; serves as an attachment for several muscles and ligaments; orientation is anterior.
  • Glenoid cavity (glenoid fossa): shallow socket on the lateral side where the head of the humerus sits to form the shoulder joint (glenohumeral joint).
  • Supraspinatus fossa (posterior above spine) and Infraspinatus fossa (posterior below spine): muscle attachment sites.
  • Subscapular fossa (anterior surface): muscle attachment site.
  • Inferior angle: the bottom point of the scapula.
• Left vs right orientation: both coracoid and acromion point laterally (toward the outside); spine points posteriorly; inferiors point downward; you can orient by placing the scapula against your back and noting which side it fits more naturally with trapezius muscle.
• Practical notes: scapulae sit against the back of the rib cage; the clavicle articulates with the scapula at the acromion, forming the shoulder girdle connection to the axial skeleton.

Humerus (Upper Arm Bone)

• Orientation: anatomical position (head up, shaft vertical when arm is at side) with the shoulder at the proximal end and elbow at the distal end.
• Proximal end features:
  • Head: articulates with the glenoid cavity of the scapula (glenohumeral joint).
  • Anatomical neck: the slight constriction just distal to the head, giving the appearance of a “rim.”
  • Surgical neck: the more distal constriction below the tubercles; a common site of fractures.
  • Greater tubercle and Lesser tubercle: two prominent bumps near the proximal end that serve as major muscle attachments.
  • Intertubercular groove (bicipital groove): a groove between the tubercles for the tendon of the biceps brachii.
• Shaft features:
  • Deltoid tuberosity: a raised area on the lateral side where the deltoid muscle attaches.
• Distal end features:
  • Capitulum (lateral): rounded eminence that articulates with the head of the radius.
  • Trochlea (medial): spool-shaped structure that articulates with the trochlear/sulcus of the ulna.
  • Medial and Lateral epicondyles: prominent bony projections above the condyles; attachment points for forearm ligaments and muscles (medial epicondyle is particularly relevant to the ulnar collateral ligament and Tommy John surgery).
  • Olecranon fossa: posterior depression that accommodates the olecranon of the ulna when the elbow is extended.
  • Coronoid fossa: anterior depression that accepts the coronoid process of the ulna when the elbow flexes.
• Left vs right orientation: identify by the relative positions of the capitulum (lateral) and trochlea (medial) and by epicondyles; the elbow joint lies at the distal end with the olecranon process of the ulna fitting into the olecranon fossa of the humerus during extension.

Radius and Ulna (Forearm Bones)

• General rule for anatomical position: radius is lateral; ulna is medial.
• Hints to determine left vs right:
  • In anatomical position, the radius lies on the thumb side; the ulna on the pinky side.
  • When hands are pronated or supinated, bones can cross, so use a consistent reference (anatomical position) for left/right.
• Radius (lateral bone in anatomical position):
  • Proximal end features: radial head and radial tuberosity (a bump for ligament attachment to the ulna).
  • Shaft: relatively smooth; the deltoid tuberosity is on the humerus, not radius.
  • Distal end features: styloid process on the lateral/dorsal side; the distal end has a smooth surface that meets the carpal bones in the wrist.
• Ulna (medial bone in anatomical position):
  • Proximal end features: olecranon process (the prominent point of the elbow) and coronoid process; the trochlear notch (semilunar notch) sits between them and articulates with the trochlea of the humerus.
  • Proximal side also has the radial notch on the lateral side where the head of the radius sits when the arm is in certain positions.
  • Distal end features: ulnar styloid process on the medial/posterior side.
• Proximal ulna anatomy cues (for quick ID):
  • Olecranon process sits in the posterior direction at the elbow; coronoid process sits anteriorly when the elbow is flexed; trochlear notch sits between them to articulate with the humerus.
• Radius anatomy cues (for quick ID):
  • Radius is the bone that has the radial head articulating with the capitulum of the humerus; the proximal radial tuberosity is a prominent bump for biceps attachment; the distal end has a pronounced styloid process on the radial/thumb side.
• Pac-Man mnemonic for ulna/radius orientation (from the instructor): Pac-Man has a mouth notch (radial notch on ulna) that faces laterally; this helps you determine which bone is ulna and which is radius when you visualize the hand.
• Summary for forearm bones: radius and ulna articulate with each other at the proximal and distal radioulnar joints; the radius articulates with the humerus at the elbow (via the capitulum) and the ulna articulates with the humerus at the elbow (via the trochlea).

Carpal Bones (Wrist)

• There are eight carpal bones in the two rows:
  • Proximal row (starting from the anatomical position’s ulnar side toward the thumb): Pisiform (on the pinky side), Triquetrum, Lunate, Scaphoid.
  • Distal row (from the ulnar to radial side): Hamate, Capitate, Trapezoid, Trapezium.
• Mnemonic used in lecture: “Pity the Little Student. He Can’t Take Tests.”
  • Mapping: Pisiform, Triquetrum, Lunate, Scaphoid, Hamate, Capitate, Trapezoid, Trapezium.
• Notable features and tactile landmarks:
  • Hamate has a hook (hamulus) that is palpable on the palm’s base; some people have it shaved to avoid catching ligaments/tendons.
  • The trapezium articulates with the thumb at the carpometacarpal joint (important for thumb movements).
• The carpals form the wrist, providing multiple small joints that contribute to the complex motions of the hand.

Metacarpals (Hand Bones)

• Number: $5$ metacarpals, numbered from the thumb side to the pinky side as 1–5.
• The thumb is Metacarpal I; the pinky is Metacarpal V.
• Each metacarpal articulates with a corresponding carpal bone proximally and with a proximal phalanx distally.

Phalanges (Fingers)

• Each finger (digits 2–5) has three phalanges: proximal, middle, distal.
• The thumb (digit 1) has only two phalanges: proximal and distal (no middle phalanx).
• Naming scheme (as presented in the lecture):
  • Proximal phalanges, middle phalanges (where they exist), distal phalanges for each digit 1–5, with the thumb lacking a middle phalanx.
• The hand’s phalanges are named numerically by digit and bone segment (e.g., proximal phalanx of digit 3, distal phalanx of digit 1, etc.).

Practical tips, orientation, and exam strategy

• Left vs right orientation: when identifying bones, rely on how the bone would sit on the body, not how it appears in a mirror. For example:
  • A rib’s orientation is determined by its posterior articulation with the spine and the direction of the costal groove on the inferior edge.
  • For the humerus, the capitulum is on the lateral side and articulates with the radius; the trochlea is medial and articulates with the ulna.
  • For the scapula, acromion and coracoid project laterally; the spine points posteriorly; the inferior angle points inferiorly.
  • For the clavicle, the sternal end (medial) is bulkier; the acromial end (lateral) is flatter.
• Exam handling tips:
  • If the exam involves moving bones, do so conservatively; arrows may detach and locating the correct position becomes harder if bones are displaced.
  • Do not over-rely on the mirror image when determining left vs right; imagine placing the bone on your own body to decide orientation.
• Practical anatomy connections:
  • The shoulder girdle (clavicle + scapula) holds the arm to the axial skeleton, with minimal bony stability and substantial muscular support, which explains common shoulder dislocations.
  • The rib cage attaches to the sternum via costal cartilage; true ribs attach directly to sternum, false ribs via cartilage above, and floating ribs lack anterior attachment, contributing to thoracic flexibility and protection of thoracic organs.
  • The sternum’s landmarks (jugular notch, clavicular notches, xiphoid process) are essential references for locating rib attachments and for chest procedures.
• Connections to prior lectures and practical relevance:
  • Atlas (C1) and Axis (C2) discussed as specialized cervical vertebrae enabling cranial movement; atlas articulates with occipital condyles, axis provides the dens pivot for rotation.
  • The spinal cord ends near the lumbar region with the cauda equina; nerves pass through intervertebral foramina to reach the rest of the body.
  • The sacrum’s auricular surfaces create the sacroiliac joints with the hip bones, forming the pelvic girdle’s connection to the spine.
• Notable clinical and practical implications mentioned in the lecture:
  • Thoracic vertebrae possess facets and demifacets for rib articulation (true joints for the ribs) and intervertebral joints that can influence movement and vulnerability to injury around the rib cage.
  • The humerus’ surgical neck is a common fracture site affecting arm function; the deltoid tuberosity indicates the deltoid muscle’s attachment and shoulder abduction mechanics.
  • The ulnar collateral ligament attaches near the medial epicondyle; injuries like Tommy John surgery involve this region.
  • The coxa-iliac joints (not described in detail here) relate to sacrum’s auricular surfaces, illustrating how spine and pelvis interface in load transfer.

Quick reference counts and key terms (summary)

• Vertebrae counts: $7$ cervical, $12$ thoracic, $5$ lumbar; sacrum = fusion of $5$; coccyx = fusion of $3-5$.
• Ribs: $12$ pairs total; true ribs $7$; false ribs $3$; floating ribs $2$.
• Sternum landmarks: jugular notch; clavicular notches (two); manubrium; body; xiphoid process.
• Shoulder girdle: clavicle (sternal end, acromial end);
  scapula (spine, acromion, coracoid, glenoid cavity, supraspinatus/infraspinatus/subscapular fossae, inferior angle).
• Arm and forearm: humerus (head, anatomical neck, surgical neck, greater/lesser tubercles, intertubercular groove, deltoid tuberosity, capitulum, trochlea, epicondyles, olecranon and coronoid fossae), radius (proximal tuberosity, distal styloid process, ulnar notch), ulna (olecranon process, coronoid process, trochlear notch, radial notch, distal styloid process).
• Hand bones: carpals (Pisiform, Triquetrum, Lunate, Scaphoid, Hamate, Capitate, Trapezoid, Trapezium), metacarpals (I–V from thumb to pinky), phalanges (proximal, middle, distal; thumb lacks a middle phalanx).
• Mnemonics: for carpals, “Pity the Little Student. He Can’t Take Tests” mapping to Pisiform, Triquetrum, Lunate, Scaphoid, Hamate, Capitate, Trapezoid, Trapezium.