Skeletal System

Axial Skeleton

The Axial Skeleton

• Eighty bones segregated into three regions– Skull– Vertebral column– Bony thorax

Bones of the Axial Skeleton

Figure 7.1

The Skull

• The skull=the cranium and facial bones – Cranium – protects the brain and is the site of

attachment for head and neck muscles• Eight cranial bones – two parietal, two temporal,

frontal, occipital, sphenoid, and ethmoid

– 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

Frontal Bone & Major Markings

Figure 7.2a

Parietal Bones and Major Associated Sutures

• Four sutures mark the articulations of the parietal bones – Coronal suture – between parietal and frontal

bones– Sagittal suture – where right and left parietal

bones meet – Lambdoid suture – between parietal and occipital

bones– Squamosal or squamous suture – between parietal

and temporal bones

Major Sutures

Figure 7.3a

Occipital Bone & Major Markings

Figure 7.2b

Temporal Bones

Figure 7.5

Temporal Bone & Major Markings

Figure 7.3a

(a)

External acoustic meatus

Mastoid process

Styloid process

Zygomatic process

Temporal bone

Sphenoid Bone

• Butterfly-shaped bone that spans the width of the middle cranial fossa (depression)

• Forms the central wedge that articulates with all other cranial bones

Sphenoid Bone

Figure 7.6a

Ethmoid Bone

•Most deep of the skull bones; lies between the sphenoid and nasal bones

• Forms most of the bony area between the nasal cavity and the orbits

Ethmoid Bone

Figure 7.7

Facial Bones

• Fourteen bones of which only the mandible and vomer are unpaired

• The paired bones are – the maxillae, – zygomatics, – nasals, – lacrimals, – palatines, – and inferior conchae

Mandible and Its Markings

Figure 7.8a

Maxillary Bones

• Medially fused bones that make up the upper jaw and the central portion of the facial skeleton

Zygomatic Bones

• Irregularly shaped bones (cheekbones) that form the prominences of the cheeks and the inferolateral margins of the orbits

Anterior Aspects of the Skull

Figure 7.2a(a)

Parietal bone

Nasal bone

Sphenoid bone(greater wing)Temporal bone

Ethmoid bone

Lacrimal bone

Zygomatic bone

Maxilla

Mandible

Mentalforamen

Frontal bone

Glabella

Middle nasal conchaPerpendicular plate

Inferior nasal concha

Vomer bone

Ethmoidbone

Hyoid Bone

• Not actually part of the skull– lies just inferior to the mandible in the anterior

neck• Only bone of the body that does not articulate

directly with another bone

• Attachment point for neck muscles that raise and lower the larynx during swallowing and speech

THORACIC CAGE

Bony Thorax (Thoracic Cage)

• The thoracic cage is composed of– the thoracic vertebrae dorsally, – the ribs laterally, – and the sternum and costal cartilages anteriorly

Bony Thorax (Thoracic Cage)

• 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 7.19a

Sternum (Breastbone)

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

• Anatomical landmarks include– the jugular (suprasternal) notch, – the manubrium, – the sternal angle, – the xiphoid process – 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 (continued)

• Ribs 8-12 (false, or vertebrocondral ribs) attach indirectly to the sternum via costal cartilage

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

Structure of a Typical True Rib

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

Figure 7.20a

Structure of a Typical True Rib

Figure 7.20b

Vertebral Column

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 – 5 fused bones inferior to the lumbar

vertebrae that articulates with the hip bones– Coccyx-4 fused bones that make up the “tailbone”

Vertebral Curvature

• Spine has an sinusoid (or “s”) shape due to curvature– Concave (posteriorly)=cervical and lumbar– Convex (posteriorly)=thoracic and sacral– Allows it to act like a spring when walking upright• Provides flexibility and resiliency

• At birth, spine curved out like 4 legged animals

Vertebral Column

Figure 7.13

Intervertebral Discs

• Act as shock absorper

• Cushion-like pad composed of two parts– Nucleus pulposus:• inner gelatinous center • gives the disc its elasticity and compressibility

– Annulus fibrosus:• surrounds the nucleus pulposus with a collar composed of

collagen and fibrocartilage

Intervertebral Discs

• As one gets older, discs become thinner & more rigid– Causes slight decrease in height– More herniated discs occur– Return to baby curvature of spine (“hump”)

• Herniated Disc (slipped disc):– Nucleus pulposus bulges out past annulus fibrosus– Painful if presses on spinal cord or nerve

Regional Characteristics of Vertebrae

Table 7.2.1

General Structure of Vertebrae

Figure 7.15

Regional Characteristics of Vertebrae

Table 7.2.2

Cervical

• “neck”

• Atlas & Axis (1st 2 vertebra)– Modified to rotate• No intervertebral disc between them

• Atlas “carries” skull (like Atlas of Greek mythology carries the world)– Articulating surfaces allow head to move up and down (“yes”)

• Altas rotates around dens of axis– Rotates head side to side (“no”)

Thoracic Vertebrae

Figure 7.17b

Thoracic and Lumbar Vertebra

• Thoracic:– Attach to ribs– Larger spines for muscle attachments

• Lumbar:– “lower back”– Thicker and well connected for support & stability

Lumbar Vertebrae

Figure 7.17c

Sacrum and Coccyx

• Sacrum:– Bones fuse as develop– Makes up posterior of pelvis

• Coccyx:– “tailbone”– Bones fuse as develop

Sacrum and Coccyx: Anterior View

Figure 7.18a