Axial Skeleton

The axial skeleton, which forms the longitudinal axis of the body. axial skeleton diagram The skeleton is divided into 2 parts, the axial and appendicular skeletons. The axial skeleton, which forms the longitudinal axis of the body. It can be divided into three parts: the skull, the vertebral column, and the bony thorax.

Skull The skull is formed by two sets of bones. Cranium: encloses and protects the brain. Facial bones: hold the eyes in place and allow facial muscle to show our expressions. All bones but are joined together by sutures, which are interlocking , immovable joints. Only the mandible is attached to the rest of the skull by a freely movable joint.

Cranium The boxlike cranium is composed of eight flat bones. Except for the parietal and temporal, they are all single bones. Frontal Bone: forms the forehead, the bony projections under the eyebrows, and the superior part on the eye’s orbit

Parietal Bones: form most of the superior and lateral walls of the cranium. They meet in the midline of the skull at the sagittal suture and form the coronal suture where they meet the frontal bone.

Temporal Bones: lie inferior to the parietal bones, they join them at the squamous sutures. Several important bone markings appear on the temporal bone. These markings include: external acoustic meatus, styloid process, zygomatic process, mastoid process, jugular foramen, internal acoustic meatus and carotid canal.

Occipital Bone : the most posterior bone of the cranium Occipital Bone : the most posterior bone of the cranium. It forms the floor and back wall of the skull. The occipital bone joins the parietal bones anteriorly at the lambdoid suture. At the base of the occipital bone there is a large opening called the foramen magnum which allows the spinal cord to connect with the brain. Lateral to the foramen magnum are the rocker like occipital condyles which rest on the first vertebra of the spinal column.

Sphenoid Bone: Butterfly shaped bone spans the width of the skull and forms part of the floor of the cranial cavity. Midline of the sphenoid is a small depression, sella turcia which hold the pituitary gland in place. The foramen ovale allows fibers of the cranial nerve V . Part of the sphenoid forms the eye orbits and has 2 important openings, superior orbital fissure and sphenoid sinuses

Ethmoid Bone is a very irregularly shaped bone and lies anterior to sphenoid . It forms the roof of the nasal cavity and part if the medial walls of the orbits. Other structures: crista galli, cribriform plates and superior and middle nasal conchae

Cranium

Facial Bones Fourteen facial bones compose the face. Twelve bones are paired: only the mandible and vomer are single

Maxillae: the two maxillae or maxillary bones , fuse to form the upper jaw. All facial bones except the mandible join the maxillae; thus they are the main, or “keystone.” bones of the face. Extensions, processes and passages: alveolar margin, palatine processes, sinuses, paranasal sinuses.

Palatine bones: paired bones that lie posterior to the palatine processes of the maxillae. The form the posterior part of the hard palate. Failure for these processes to fuse medially results in cleft palate.

Zygomatic bone: commonly referred to as the cheek bones. They form a good size portion of the lateral walls of the orbits.

Lacrimal bones; fingernail – size bones forming the medial wall of each orbit. Each Lacrimal bone has a groove that serves as a passageway for tears.

Nasal Bones Small rectangular bones forming the bridge of the nose . The lower part of the of the skeleton of the nose is cartilage.

Vomer Bone The single bone in the median line of the nasal cavity is the vomer. Vomer means plow which refers to the shape. The vomer forms most of the nasal septum.

Inferior Nasal Conchae The inferior nasal conchae are thin, curved bones projecting from the lateral walls of the nasal cavity.

Mandible Mandible, lower jaw, is the largest and strongest bone of the face. It joins the temporal bones on each side of the face, forming the only freely moveable joints in the skull.

Hyoid Bone Hyoid bone is not really part of the skull. The hyoid bone is closely related to the mandible and temporal bones. The hyoid bone is the only bone that does note articulate directly with any other bone. It is suspended in the mid - neck region about 2cm above the larynx where it is anchored by ligaments to the styloid process of the temporal bone. Hyoid bone is horseshoe shape body with 2 horns or cornua. The cornua sever as movable base for the tongue

Fetal Skull The fetal skull is different from adult skull in many ways. The adult skull is only 1/8 the total body length . The Fetal or newborn skull is 1/4 as long as the its body. When a baby is born the skeleton is still unfinished. Some hyaline cartilage still remains to be ossified. In the newborn the skull also has fibrous regions that have yet to be converted to bone. This fibrous membranes connecting the cranial bones are called fontanels or “soft spots” There are two fontanels the anterior fontanel (largest and diamond shaped) and the posterior fontanel (smaller and triangle shape)

Vertebral Column Serving as axial support of the body is the vertebral column which extends from the skull, which it supports, to the pelvis, where it transmits the weight of the body to the lower limbs. The spine consist of 26 irregular shaped bones connected and reinforces by ligaments in such a way that is flexible.

Running through the central cavity of the vertebral column is the delicate spinal cord, which it surrounds and protects. Before birth, the spine consists if 33 separate bones called vertebrae. After birth 9 of these vertebrae fuse together to form 2 composite bones the sacrum and the coccyx

The remaining 24 single bones are divided the following way: Breakfast = 7 cervical bones. Lunch = 12 Thoracic bones. Dinner = 5 lumbar bones

Each single vertebral bone is separated by pads of flexible fibrocartilage called intervertebral discs which cushions the vertebra and absorb shock which allows for spinal flexibility.

Abnormal Spinal Curvatures These disorders all deal with the way in which the spine curves. Kyphosis: hunchback, the spine curves out causing the head to drop forward. Lordosis: swayback, the spine curves in making the buttock look prominent. Scoliosis: crooked back, where the spine forms an “S” or “C”

Vertebrae All vertebrae have similar structural pattern. Body or Centrum: disclike, weight-bearing part of the vertebra facing anteriorly in the vertebral column.

Vertebral arch: arch formed from the joining of all posterior extensions, the laminae and pedicles, from the vertebral body.

Vertebral foramen: canal through which the spinal cord passes. Transverse processes; two lateral projections from the vertebral arch. Spinous Processes: a single projection arising from the posterior aspect of the vertebral arch (actually fused laminae)

Superior and Inferior Articular Processes: Paired projections lateral to the vertebral foramen, allowing a vertebra to form joints with adjacent vertebra.

Cervical Vertebrae The seven cervical vertebrae form the neck region of the spine. The first two vertebrae(atlas and axis) are different because they perform functions not shared by the other cervical vertebrae. The Atlas has no body. The superior surfaces of its transverse processes contain large depressions that receive the occipital condyles of the skull. This allows you to nod “yes”. The Axis (C2) acts as a pivot for the rotation of the atlas and the skull above. It has a large upright process, the dens or odontoid process, which acts as the pivot point . The joint between the C1 and C2 allows you to rotate your head from side to side to indicate “no”

Thoracic Vertebrae The twelve thoracic vertebrae are larger than the cervical vertebrae. The body is somewhat heart shaped and has two costal facets or articulating surfaces on each side. These costal facets receive the heads of the ribs

Lumbar Vertebrae The five lumbar vertebrae have massive blocklike bodies. Their short, hatchet – shaped spinous process make them look like a moose head from the lateral aspect. Since most stress on the vertebral column occurs in the lumbar region, these are the sturdiest vertebrae.

Sacrum The sacrum is formed by the fusion of five vertebrae. It articulates with L5 and the coccyx. The “wing like” alae articulate laterally with the hip forming the sacroiliac joints. The sacrum forms the posterior wall of the pelvis .

Coccyx The coccyx (tailbone) is formed by the fusion of three to five irregularly shaped vertebrae. In humans it is the remnant of the tail that other vertebrates have.

Bony Thorax The sternum, ribs and thoracic vertebrae make up the bony thorax. The bony thorax is often called the thoracic cage because it forms a protective, cone-shaped cage of slender bones around the organs of thoracic cavity (heart lungs and major blood vessels)

Sternum The Sternum or breastbone is a typical flat bone and the result of the fusion of three bones; manubrium body, xiphoid process. The sternum has three important bony landmarks the jugular notch, the sternal angle and the xiphisternal joint.