06 Skeleton: Axial

Human Biology

Classification of Bones

Human body consists in 206 bones. They are divided in two groups:

• Axial skeleton (form the long axis of the body) includes – bones of the skull, vertebral column, and rib cage. Functions: protecting, supporting or carry other body parts.

• Appendicular skeleton – bones of the upper and lower limbs, shoulder, and hip. Function: locomotion and manipulation of our environment

Gross Anatomy of Bones: Bone Textures

Every bone has two textures:

Compact bone (also called cancellous bone) – dense outer layer

Spongy bone - (internal) honeycomb of trabeculae filled with red or yellow bone marrow

Classification of Bones: By Shape

• Long bones – longer than they are wide (e.g., humerus)

• Has a shaft plus two ends

Figure 6.2a

Bones come in many sizes and shapes. For most part , bones are classified by their shape as: long, short, flat and irregular

Ex: All limb bones except patella and wrist and ankle bones are long bone

Structure of Long Bone

Figure 6.3

Structure of Long Bone Long bones consist of a diaphysis and an

epiphysis and membranes.

Diaphysis

Tubular shaft that forms the axis of long bones

Composed of compact bone that surrounds the medullary cavity

Yellow bone marrow (fat) is contained in the medullary cavity

Structure of Long Bone Epiphyses

Expanded ends of long bones

Exterior is compact bone, and the interior is spongy bone

Joint surface is covered with articular (hyaline) cartilage

Epiphyseal line (remnant of the epiphyseal plate) separates the diaphysis from the epiphyses

Classification of Bones: By Shape

Figure 6.2b

• Short bones– Cube-shaped bones of

the wrist and ankle– Sesamoid bones

“shaped like a sesame seed” that form within tendons (e.g., patella). They vary in size and number in different individual. Some sesamoid bones act to alter the direction of pull of a tendon

Classification of Bones: By Shape

• Flat bones – thin, flattened, and a bit curved (e.g., sternum, and most skull bones)

Figure 6.2c

Structure of a Flat Bone

Figure 6.4

Classification of Bones: By Shape

• Irregular bones – bones with complicated shapes (e.g., vertebrae and hip bones)

Figure 6.2d

Function of Bones• Support – form the framework that supports

the body and cradles soft organs. Ex: Lower limb

• Protection – provide a protective case for: the brain (bones of skull), spinal cord (vertebrae), and vital organs (rib cage)

• Movement – provide levers for muscles• Mineral storage – bone is a reservoir for

minerals, especially calcium and phosphorus• Blood cell formation – hematopoiesis occurs

within the marrow cavities of bones

• Tuberosity – rounded projection

• Crest – narrow, prominent ridge of bone

• Trochanter – large, blunt, irregular surface

• Line – narrow ridge of bone

Bone Markings: Projections – Sites of Muscle and Ligament Attachment

• Tubercle – small rounded projection

• Epicondyle – raised area above a condyle

• Spine – sharp, slender projection

• Process – any bony prominence

Bone Markings: Projections – Sites of Muscle and Ligament

Attachment

Chemical Composition of Bone: Organic

• Bone has both organic and inorganic components.

Organic components:1.-Cells:• Osteoblasts – bone-forming cells• Osteocytes – mature bone cells• Osteoclasts – large cells that resorb or break

down bone matrix2.- Osteoid – unmineralized bone matrix

composed of proteoglycans, glycoproteins, and collagen

Chemical Composition of Bone: Inorganic

The balance of the bone tissue (65% by mass) consists of inorganic mineral salts:

Hydroxyapatites, or mineral salts

Mainly calcium phosphates

Responsible for bone hardness and its resistance to compression

The Skeleton

• The skeleton (“dried up body” or mummy)

• It is strong, yet light, and almost perfectly adapted for the protective, locomotor and manipulative functions it perform

• Composed of bones, cartilages, joints and ligaments, accounts for about 20% of body mass.

• The skeleton is divided into Axial and Appendicular

The Axial Skeleton

• This part of skeleton supports the head, neck and trunk and it protects the brain, spinal cord and the organs in the thorax

• Eighty bones segregated into three regions– 1.- Skull– 2.- Vertebral column– 3.- Bony thorax

The Skull• The skull, the body’s most complex bony

structure, is formed by the cranium and facial bones

• Cranium – protects the brain and is the site of attachment for head and neck muscles

• Facial bones– Supply the framework of the face, the sense

organs, and the teeth– Provide openings for the passage of air and

food– Anchor the facial muscles of expression

Anatomy of the Cranium

• The cranium can be divided in :

1.- cranial vault (calvaria)

2.- cranial base (floor)

• Eight cranial bones – two parietal, two temporal, frontal, occipital, sphenoid, and ethmoid

• Cranial bones are thin and remarkably strong for their weight

Frontal Bone• Forms the anterior portion of the cranium

• Articulates posteriorly with the parietal bones via the coronal suture

• The most anterior part of the frontal bone is the vertical frontal squama, commonly called the forehead

• Major markings include the supraorbital margins, the anterior cranial fossa, and the frontal sinuses (internal and lateral to the glabella)

Skull: Anterior View

Figure 7.2a

Skull: Posterior View

Figure 7.2b

Parietal Bones and Major Associated Sutures

• They are curved, rectangular bones. Form most of the superior and lateral aspects of the skull

Figure 7.3a

Occipital Bone and Its Major Markings• It articulates anteriorly

with the paired parietal and temporal bones via the lambdoid and occipitomastoid sutures.

• It also joins with the sphenoid bone in the cranial floor via a plate called the pharingeal tubercule.

• Forms most of skull’s posterior wall and base

• Major markings include the posterior cranial fossa, foramen magnum, occipital condyles, and the hypoglossal canal

Figure 7.2b

Temporal Bones

Figure 7.5

• Between the styloid and mastoid processes exist the stylomastoid foramen that allow cranial nerve VII to leave the skull.

• The mastoid region of the temporal bone exhibits the conspicuous mastoid process, an anchoring site for some neck muscles

Sphenoid Bone

Figure 7.6a, b

Ethmoid Bone

Figure 7.7

Allow the olfatory nerves to pass from the smell receptors in the nasal cavites to the brain

Mandible and Its Markings

Figure 7.8a

Maxillary Bone

Figure 7.8b

The anterior nasal spine allows the infraorbital nerve and artery to reach the face

Vertebral Column & Ribs

Vertebral Column

• Formed from 26 irregular bones (vertebrae) connected in such a way that a flexible curved structure results– Cervical vertebrae – 7 bones of the neck– Thoracic vertebrae – 12 bones of the torso– Lumbar vertebrae – 5 bones of the lower back– Sacrum – bone inferior to the lumbar vertebrae

that articulates with the hip bones

Vertebral Column

Figure 7.13

Vertebral Column: Curvatures

• Posteriorly concave curvatures – cervical and lumbar

• Posteriorly convex curvatures – thoracic and sacral

• Abnormal spine curvatures include scoliosis (abnormal lateral curve), kyphosis (hunchback), and lordosis (swayback)

Vertebral Column: Ligaments

• Anterior and posterior longitudinal ligaments – continuous bands down the front and back of the spine from the neck to the sacrum

• Short ligaments connect adjoining vertebrae together

Vertebral Column: Ligaments

Figure 7.14a

Vertebral Column: Intervertebral Discs

• Cushionlike pad composed of two parts– Nucleus pulposus – inner gelatinous nucleus

that gives the disc its elasticity and compressibility

– Annulus fibrosus – surrounds the nucleus pulposus with a collar composed of collagen and fibrocartilage

Vertebral Column: Intervertebral Discs

Figure 7.14b

General Structure of Vertebrae

• Body or centrum – disc-shaped, weight-bearing region

• Vertebral arch – composed of pedicles and laminae that, along with the centrum, enclose the vertebral foramen

• Vertebral foramina – make up the vertebral canal through which the spinal cord passes

General Structure of Vertebrae

• Spinous processes project posteriorly, and transverse processes project laterally

• Superior and inferior articular processes – protrude superiorly and inferiorly from the pedicle-lamina junctions

• Intervertebral foramina – lateral openings formed from notched areas on the superior and inferior borders of adjacent pedicles

General Structure of Vertebrae

Figure 7.15

Cervical Vertebrae

• Seven vertebrae (C1-C7) are the smallest, lightest vertebrae

• C3-C7 are distinguished with an oval body, short spinous processes, and large, triangular vertebral foramina

• Each transverse process contains a transverse foramen

Cervical Vertebrae

Table 7.2

Cervical Vertebrae: The Atlas (C1)

• The atlas has no body and no spinous process

• It consists of anterior and posterior arches, and two lateral masses

• The superior surfaces of lateral masses articulate with the occipital condyles

Cervical Vertebrae: The Atlas (C1)

Figure 7.16a, b

Cervical Vertebrae: The Axis (C2)

• The axis has a body, spine, and vertebral arches as do other cervical vertebrae

• Unique to the axis is the dens, or odontoid process, which projects superiorly from the body and is cradled in the anterior arch of the atlas

• The dens is a pivot for the rotation of the atlas

Cervical Vertebrae: The Axis (C2)

Figure 7.16c

Cervical Vertebrae: The Atlas (C2)

Figure 7.17a

Thoracic Vertebrae

• There are twelve vertebrae (T1-T12) all of which articulate with ribs

• Major markings include two facets and two demifacets on the heart-shaped body, the circular vertebral foramen, transverse processes, and a long spinous process

• The location of the articulate facets prevents flexion and extension, but allows rotation of this area of the spine

Thoracic Vertebrae

Figure 7.17b

Lumbar Vertebrae

• The five lumbar vertebrae (L1-L5) are located in the small of the back and have an enhanced weight-bearing function

• They have short, thick pedicles and laminae, flat hatchet-shaped spinous processes, and a triangular-shaped vertebral foramen

• Orientation of articular facets locks the lumbar vertebrae together to provide stability

Lumbar Vertebrae

Figure 7.17c

Sacrum

• Sacrum– Consists of five fused vertebrae (S1-S5), which

shape the posterior wall of the pelvis

– It articulates with L5 superiorly, and with the auricular surfaces of the hip bones

– Major markings include the sacral promontory, transverse lines, alae, dorsal sacral foramina, sacral canal, and sacral hiatus

Coccyx

• Coccyx (Tailbone)– The coccyx is made up of four (in some cases

three to five) fused vertebrae that articulate superiorly with the sacrum

Sacrum and Coccyx: Anterior View

Figure 7.18a

Characteristics of Cervical,Thoracic and Lumbar Vertebrae Characteristics Cervical (3-7) Thoracic Lumbar

Body Small, wide side to side

Larger than cervical, heart shaped, bears two costal demifacets

Massive, kidney shaped

Spinous process

Short bifid, projects directly posteriorly

Long, sharp, projects inferiorly

Short, blunt, projects directly posteriorly

Vertebral foramen

Triangular Circular Triangular

Transverse process

Contain foramina Bear facets for ribs (except T11-T12)

Thin and tapered

Sup. and inf. art. process

Sup facet→ sup-posInf facet → inf-ant

Sup facets→ posteriorInf. facets → anterior

Sup facet→ posmedInf. facet → ant-lat

Movements Flex and extension, lateral flex, rotation

Rotation, limited lateral flex (ribs), flex & ext prevented

Flex and ext, some lat flexion, rotation prevented

Homeostatic Imbalance• Herniated (prolapsed) disc.

Severe or sudden physical trauma to the spine, may result in hernation of one or more discs. A herniated disc (slipped disc) usually involves rupture of the annulus fibrosus followed by protrusion of the spongy nucleus pulposus through the annulus (Fig 7.14). If the protrusion presses on the spinal cord or on spinal nerves exiting from the cord, numbness or excruciating pay result.

Treatments: moderated exercise, massage, heat ther. and painkillers if this fail→ surgery

Bony Thorax (Thoracic Cage)• The thoracic cage is composed of the

thoracic vertebrae dorsally, the ribs laterally, and the sternum and costal cartilages anteriorly

• Functions– Forms a protective cage around the heart, lungs,

and great blood vessels

– Supports the shoulder girdles and upper limbs

– Provides attachment for many neck, back, chest, and shoulder muscles

– Uses intercostal muscles to lift and depress the thorax during breathing

Bony Thorax (Thoracic Cage)

Figure 7.19b

Sternum (Breastbone)

• A dagger-shaped, flat bone that lies in the anterior midline of the thorax

• Results from the fusion of three bones – the superior manubrium, the body, and the inferior xiphoid process

• Anatomical landmarks include the jugular (suprasternal) notch, the sternal angle, and the xiphisternal joint

Ribs• There are twelve pair of ribs forming the

flaring sides of the thoracic cage

• All ribs attach posteriorly to the thoracic vertebrae

• The superior 7 pair (true, or vertebrosternal ribs) attach directly to the sternum via costal cartilages

• Ribs 8-10 (false, or vertebrocondral ribs) attach indirectly to the sternum via costal cartilage

• Ribs 11-12 (floating, or vertebral ribs) have no anterior attachment

Ribs

Figure 7.19a

Structure of a Typical True Rib

• Bowed, flat bone consisting of a head, neck, tubercle, and shaft

Figure 7.20