Chapter 8

• These study notes cover the skeletal system

Overview of the Skeletal System

• The axial skeleton forms the body's central supporting axis.
• The axial skeleton includes the skull, vertebral column, and thoracic cage.
• The appendicular skeleton includes the bones of the limbs and their girdles.
• An adult skeleton typically has 206 bones.
• At birth, there are more bones, and some fuse during childhood.
• Sesamoid bones are bones that form within tendons, like the patella.
• Wormian bones, also known as sutural bones, are extra bones in the skull.
• Bones exhibit various anatomical features called bone markings.

The Skull

• The skull is the skeleton's most complex part.
• It consists of 22 bones.
• Sutures are the immovable joints between the skull bones.
• The largest cavity is the cranial cavity, which encloses the brain.
• Other cavities include the orbits, nasal cavity, oral cavity, paranasal sinuses, and middle-ear cavities.
• Foramina are small holes in bones that allow blood vessels and nerves to pass through.
• The cranium forms the cranial cavity.
• The cranium protects the brain and associated sensory organs and is composed of eight bones.
• Three membranes called meninges separate the brain from the cranial bones.
• The thickest and toughest of the meninges is the dura mater.
• The foramen magnum is a large opening in the cranium where the spinal cord enters.
• The frontal bone extends from the forehead back to the coronal suture.
• The coronal suture crosses the crown of the head and joins the frontal bone to the parietal bones.
• The right and left parietal bones form most of the cranial roof and part of its walls.
• Each parietal bone is bordered by four sutures.
• The sagittal suture lies between the two parietal bones.
• The coronal suture is at the anterior margin.
• The lambdoid suture is at the posterior margin.
• The squamous suture is at the lateral margin.
• The right and left temporal bones form much of the lower wall and part of the floor of the cranial cavity.
• The temporal bone's complex shape can be broken down into four parts.
• The zygomatic process forms part of the zygomatic arch (cheekbone).
• The mandibular fossa is the site at which the mandible articulates with the cranium.
• The tympanic part is a ring of bone that borders the external acoustic meatus (ear canal opening).
• The styloid process, which resembles a stylus (writing implement), is located on its inferior surface.
• The mastoid part is posterior to the tympanic part.
• The mastoid part has a mastoid process, filled with small air sinuses that are susceptible to infection (mastoiditis).
• The occipital bone forms the skull's rear (or occiput) and much of its base.
• The foramen magnum is a large opening in the occipital bone.
• The occipital condyles, smooth knobs, are on either side of the foramen magnum and rest on the vertebral column.
• The sphenoid bone has a complex shape.
• The sphenoid bone includes a thick medial body and outstretched greater and lesser wings.
• The lesser wing contains the optic canal through which the optic nerve and ophthalmic artery pass.
• The superior orbital fissure angles upward on the posterior wall of the orbit and lateral to the optic canal.
• The superior orbital fissure serves as a passage for nerves supplying the eye muscles.
• The sella turcica is a saddle-shaped structure on the superior surface of the sphenoid bone that houses the pituitary gland; the stalk that penetrates this membrane connects to the hypothalamus.
• The ethmoid bone has three major portions.
• It is an anterior bone located between the eyes.
• The ethmoid bone contributes to the medial wall of the orbit, the roof and walls of the nasal cavity, and the nasal septum.
• The perpendicular plate forms the superior two-thirds of the nasal septum.
• The cribriform plate is a horizontal bone perforated with numerous foramina.
• The crista galli is a median blade on the cribriform plate that forms an attachment point for the dura mater.
• A depression is found on each side of the crista; the olfactory fossae rest in these depressions.
• The cribriform foramina allow passage for the olfactory nerves.
• Facial bones have no direct contact with the brain or meninges.
• There are 14 facial bones: the maxillae (2), palatine bones (2), zygomatic bones (2), lacrimal bones (2), nasal bones (2), inferior nasal conchae (2), and the mandible and vomer.
• The maxillae (maxillary bones) are the largest facial bones.
• They form the upper jaw and meet at the median intermaxillary suture.
• Alveolar processes are bony points between teeth.
• The root of each tooth is inserted into a deep socket, also called alveolus.
• If a tooth is lost, the alveolar processes are resorbed and the alveolus fills in with new bone.
• The maxilla forms part of the floor of the orbit.
• The inferior orbital fissure is a gash exhibited by the maxilla that forms part of the floor of the orbit.
• The inferior and superior orbital fissures form a sideways V whose apex lies near the optic canal.
• The palate forms the roof of the mouth and floor of the nasal cavity.
• The palate consists of a bony palate and a fleshy soft palate.
• The palatine processes are horizontal extensions of the maxillae that form most of the bony palate.
• A cleft palate occurs when the palatine processes fail to join during development.
• The palatine bones are located in the posterior nasal cavity.
• Each has an L shape formed by a horizontal and a vertical plate.
• The zygomatic bones form the angles of the cheeks.
• The lacrimal bones form part of the medial wall of each orbit.
• They are the smallest bones of the skull—about the size of the little fingernail.
• The inferior nasal concha is the largest of the 3 nasal conchae and is a seperate bone.
• The other conchae are parts of the ethmoid.
• The vomer forms the inferior half of the nasal septum.
• The mandible is the strongest bone of the skull and the only one that can move.
• Mastication is the act of chewing.
• The mandible has two major parts on each side: the horizontal body and the ramus.
• The body and ramus meet at a corner called the angle.
• The temporomandibular joint (TMJ) is formed where the mandible articulates with the temporal bone.
• Seven bones are closely associated with the skull but not considered part of it.
• These are the three auditory ossicles in each middle-ear cavity and the hyoid bone beneath the chin.
• The bones of the auditory ossicles include the malleus, incus, and stapes.
• The hyoid bone is a slender bone between the chin and larynx (voice box).
• The hyoid bone does not articulate with any other bone.
• The digastric, geniohyoid, mylohyoid, omohyoid, stylohyoid, sternohyoid, thyrohyoid, middle pharyngeal constrictor, and hyoglossus muscles all attach to the hyoid bone.
• The skull bones of an infant are not fused, allowing it to pass through the pelvic outlet during birth.
• Fontanelles are spaces between the unfused cranial bones.
• The bones at these points are joined only by fibrous membranes and are gradually ossified.
• Most fontanelles ossify by 1 year of age.
• The largest one, the anterior fontanelle, is still evident 1½ to 2 years after birth.

The Vertebral Column and Thoracic Cage

• The vertebral column (spine) supports the trunk of the body.
• The vertebral column protects the spinal cord.
• The vertebral column absorbs stress from walking, running, and lifting.
• The vertebral column provides attachment points for the limbs, thoracic cage, and postural muscles.
• It allows for movements of the head and trunk.
• The vertebral column consists of a chain of 33 vertebrae with intervertebral discs of fibrocartilage between most of them.
• The vertebrae are divided into five groups: 7 cervical vertebrae in the neck, 12 thoracic vertebrae in the chest, 5 lumbar vertebrae in the lower back, 5 sacral vertebrae at the base of the spine, and 4 coccygeal vertebrae in the tailbone.
• The vertebral column has an S-shaped curvature, which is not present at birth.
• The four bends are the cervical, thoracic, lumbar, and sacral curvatures.
• The vertebral column in newborns is C-shaped.
• The S-curve develops as a child begins to crawl and walk.
• The S-curve allows for upright posture, shock absorption, and balance.
• The most obvious feature of a vertebra is the body (centrum).
• The body is a weight-bearing, roughly cylindrical mass that faces anteriorly.
• The vertebral foramina form the vertebral canal.
• The spinal cord passes through the vetebral canal.
• The spinous process projects posteriorly and downward.
• The spinous and transverse processes provide attachments for muscles and ligaments.
• The intervertebral foramen is an opening between adjacent vertebrae.
• The intervertebral foramen allows passage for spinal nerves.
• An intervertebral disc consists of a nucleus pulposus (inner gelatinous mass) and anulus fibrosus (outer ring of fibrocartilage).
• The discs bind adjacent vertebrae together, support weight, and absorb shock.
• A herniated disc occurs when the nucleus pulposus protrudes through the anulus fibrosus, pressing on the spinal cord or nerves.
• Vertebrae differ from one another depending on the region of the spine in which they are located; these differences reflect their functional differences.
• The cervical vertebrae (C1–C7) are relatively small and allow for a greater range of head movement.
• Vertebra C1 is called the atlas.
• The atlas articulates with the occipital condyles of the skull.
• The atlanto-occipital joint allows for nodding the head "yes".
• Vertebra C2 is called the axis.
• The atlantoaxial joint, formed between the atlas and axis, allows the head to pivot "no."
• The dens (odontoid process) is a prominent process on the axis.
• A heavy blow to the top of the head could cause the dens to fracture and compress the spinal cord, leading to paralysis or death.
• The articulation between the atlas and the cranium forms the atlanto-occipital joint; this is a condylar joint and allows primarily for flexion and extension (nodding).
• The articulation between the atlas and axis forms the atlantoaxial joint; this is a pivot joint and allows rotation (shaking the head "no").
• The axis is the first vertebra that exhibits a dens (odontoid process).
• All seven cervical vertebrae have a transverse foramen in each transverse process.
• These foramina allow passage for the vertebral arteries and veins.
• No other vertebrae have these foramina.
• The 12 thoracic vertebrae (T1–T12) correspond to the 12 pairs of ribs attached to them.
• Costal facets for rib articulation are four distinguishing features of the throacic vertebrae.
• The five lumbar vertebrae (L1–L5) are distinguished by their thick, blocklike bodies and blunt, squarish spinous processes.
• The sacrum is a bony plate that forms the posterior wall of the pelvic cavity.
• In children, there are five separate sacral vertebrae (S1–S5); these begin to fuse around age 16–18 years and are complete by age 30.
• Four pairs of large anterior sacral (pelvic) foramina allow passage for nerves and blood vessels to and from the pelvic organs.
• On each side of the sacrum, an ear-shaped region called the auricular surface articulates with the ilium of the hip bone to form the sacroiliac joint.
• The coccyx usually consists of four coccygeal vertebrae.
• The coccygeal vertebrae sometimes fuse by the age of 20 or 30.
• The coccyx can be fractured by falling in a sitting position.
• The thoracic cage consists of the thoracic vertebrae, the ribs, and the sternum (breastbone).
• The thoracic cage forms a more or less conical enclosure for the thoracic organs and the mediastinum.
• Its most important role is to protect the thoracic organs, especially the heart and lungs.
• The sternum (breastbone) is a bony plate anterior to the heart, with three regions: the manubrium (superior portion), the body (middle portion), and the xiphoid process (inferior tip).
• Improperly performed chest compressions in cardiopulmonary resuscitation can drive the xiphoid process into the liver and cause a fatal hemorrhage.
• There are 12 pairs of ribs, with no difference in number between the sexes.
• Many ribs are attached at the anterior (distal) end to the sternum.
• The anterior attachment is through a long strip of hyaline cartilage called the costal cartilage.
• True ribs (ribs 1–7) attach directly to the sternum by their own costal cartilage.
• False ribs (ribs 8–12) do not have independent cartilaginous attachments to the sternum.
• Floating ribs (ribs 11–12) do not have any attachment to the sternum.

The Pectoral Girdle and Upper Limb

• The pectoral girdle (shoulder girdle) supports the arm.
• The pectoral girdle consists of two bones: the clavicle (collarbone) and the scapula (shoulder blade).
• The sternoclavicular joint is made up of the sternum and clavicle.
• The acromioclavicular joint is made up of the scapula and clavicle.
• The glenohumeral joint is made up of the scapula and humerus.
• The clavicle exhibits a slight S shape.
• The clavicle has a sternal end (medial) and an acromial end (lateral).
• Fracture of the clavicle is common because it is relatively slender and exposed.
• The scapula is named for its resemblance to a spade or shovel.
• The triangular plate posteriorly overlies ribs 2 through 7.
• The scapula's only direct attachment to the thorax is by muscles.
• The suprascapular notch in the superior border allows passage for nerves.
• The broad anterior surface, also known as the subscapular fossa, is slightly concave and relatively featureless.
• The posterior surface has a transverse ridge called the spine with a deep indentation superior to the spine in the supraspinous fossa, and a broad surface inferior to it in the infraspinous fossa.
• The acromion is a platelike extension of the scapular spine that forms the apex of the shoulder.
• The acromiom articulates with the clavicle.
• The glenoid cavity is a swallow socket that articulates with the head of the humerus and what joint does this form?
• The upper limb is divided into four regions and contains 30 bones per limb.
• The four regions are the brachium (arm), antebrachium (forearm), carpus (wrist), and manus (hand).
• The brachium, or arm proper, extends from shoulder to elbow and contains only on bone, the humerus.
• The antebrachium (forearm) extends from elbow to wrist and contains two bones, the radius (lateral) and ulna (medial).
• The carpus, or wrist consists of eight small bones arraged in 2 rows.
• The manus, or hand, contains 19 bones in two groups.
• The 5 metacarpals in the palm and 14 phalanges in the fingers.
• The humerus has a hemispherical head that articulates with the glenoid cavity of the scapula.
• The surgical neck is the most common fracture site on the humerus.
• The deltoid tuberosity is a rough area on the shaft that is the insertion for the deltoid muscle.
• The distal end of the humerus has two smooth condyles.
• The lateral condyle, the capitulum, is shaped somewhat like a wide tire and articulates with the radius.
• The medial condyle, called the trochlea, is pulleylike and articulates with the ulna.
• Also shows three deep pits, one posterior and two anterior.
• The posterior pit, the olecranon fossa, accommodates the olecranon of the ulna when the arm is extended.
• The anterior medial pit, the coronoid fossa, accommodates the coronoid process of the ulna when the arm is flexed.
• The anterior lateral pit, the radial fossa, is named for the nearby head of the radius.
• The radius has a distinctive discoidal head at its proximal end.
• When the forearm is rotated, the circular superior surface of this disc spins on the capitulum of the humerus, and the edge of the disk spins on the radial notch of the ulna.
• Immediately distal to the head, the radius has a narrow neck and then widens to theradial tuberosity on its medial surface where the biceps brachii inserts.
• The ulna has a deep, C-shaped trochlear notch at its proximal end; this trochlear notch wraps around the trochlea of the humerus.
• The radius and ulna are attached along their shafts by a ligament called the interosseous membrane; this allows for pronation and supination of the forearm.

The Pelvic Girdle and Lower Limb

• The pelvic girdle is composed of two hip bones (ossa coxae or coxal bones).
• Each hip bone is joined to the vertebral column at the sacroiliac joint.
• The two hip bones are joined to each other at the pubic symphysis, which consists of the interpubic disc and the adjacent region of each hip bone.
• The pelvis has the greater (false) pelvis between the flare of the hips and the narrower lesser (true) pelvis below.
• The two pelves are separated by a round margin called the pelvic brim.
• The opening cicumscribed by the brim is called the pelvic inlet; the lower margin is the pelvic outlet.
• The hip bones have three distinctive features.
• The iliac crest is the superior crest of the hip.
• The acetabulum is the hip socket.
• The obturator foramen is a large round-to-triangular hole below the acetabulum
• The adult hip bone is formed by the fusion of three childhood bones: the ilium, ischium, and pubis.
• The ilium, the largest bone, extends from the iliac crest to the center of the acetabulum.
• Below the posterior inferior spine on each side is the greater sciatic notch, named for the sciatic nerve that passes through it.
• Each ilium has an auricular surface that joins with the sacrum to form the sacroiliac joint.
• The ischium is the inferoposterior portion of the hip bone.
• The ischial tuberosity supports the body when sitting
• The pubis (pubic bone) is the most anterior portion of the hip bone and has superior and inferior rami and a triangular body.
• The body if one pubis meets the body of the other at the pubic symphysis.
• The pubis and ischium encircle the obturator foramen.
• The pelvis is sexually dimorphic means that it has different characteristics in females compared to males, reflecting the capacity to give birth in females.
• The lower limb, like the upper limb, is divided into four regions and contains 30 bones per limb.
• The three regions are the femoral region (thigh), the crural region (leg proper), the tarsal region (ankle), and the pedal region (foot).
• The femur is the longest and strongest bone of the body.
• The femur has a hemispherica head that articulates with the acetabulum in a ball-and-socket joint.
• Distal to the head is a constricted neck, the greater and lesser trochanters.
• The medial and lateral epicondyles are the widest points of the femur at the knee.
• These epicondyles and the supracondylar lines are attachments for certain thigh and leg muscles and knee ligaments.
• The patella, or kneecap, is a large sesamoid bone and is cartilaginous at birth and ossifies at 3 to 6 years of afe.
• Tendons connect muscle to bone, whereas ligaments connect bone to bone.
• The tibia is on the medial side of the leg and is the only weigh-bearing bone of the crural region.
• The shaft of the tibia contains a sharply angular anterior border, which can be palpated in the shin.
• The two prominent bony knobs on each side of ankle form the medial and lateral malleoli
• The medial malleolus is part of the tibia.
• The lateral malleolus is part of the fibula.
• The fibula is a slender strut that heslp stabilize the ankle.
• Like the radius and ulna, the tibia and fibula are joined by an interosseous membrane along their shafts.
• The ankle and foot consist of bones in proximal and distal groups similar to those of the wrist, but because of their weight-bearing role, the bones' shapes and arrangement are very different.
• the calcaneus forms the heel.
• The second largest tarsal bine is the talus.