CHAPTER 7: THE SKELETAL SYSTEM: AXIAL SKELETON



THE AXIAL SKELETONSKULL: The skull is the most complex bony structure of the skeleton. It’s composed of 22 bones (not including the auditory ossicles), and it’s located on the superior end of the vertebral column. The cranial bones of the skull function to enclose and protect the brain, and the facial bones of the skull function to protect the sense organs in the head and to provide supportive entrances to the digestive and respiratory tracts. All of the skull bones function as attachment sites for skeletal muscles that move the head, and the mandible is the only movable bone of the skull.Cranial bones of skull (cranium):Facial bones of skull:Frontal bone (1)Mandible (1)Parietal bone (2)Maxillae (2)Temporal bone (2)Palatine bone (2)Occipital bone (1)Zygomatic bone (2)Sphenoid bone (1)Nasal bone (2)Ethmoid bone (1)Lacrimal bone (2)Vomer bone (1)Inferior nasal conchae (2)Unique features of skull:Sutures: Sutures are immovable joints located between the cranial and facial bones, which function to hold/join these bones together. EX: coronal suture, sagittal suture, lambdoid suture, squamous sutureSutural bones: Sutural bones are unique, irregular bones located at sutures. They vary from person to person, and their significance is unknown.Paranasal sinuses: Paranasal sinuses are small cavities positioned around the nasal cavity, and they are located in the frontal, sphenoid, ethmoid, and maxillary bones. These sinuses are lined with mucosae, and they function to reduce the weight of the skull and to resonate sound as we speak.Fontanelles: Fontanelles are the “soft spots” on a newborn’s skull. They are areas of unossified fibrous connective tissue located between cranial bones, and they function to allow baby’s head to be compressed during childbirth to fit through the vaginal canal and to accommodate brain growth of the baby.Orbits: Orbits are the bony cavities where the eyes are located. They are formed by the articulations of 7 bones, which are the frontal bone, sphenoid bone, ethmoid bone, lacrimal bones, zygomatic bones, palatine bones, maxillae.Foramina: Foramina are openings in the cranial and facial bones that function to allow soft tissues/organs (i.e. blood vessels, lymphatic vessels, nerves) to pass through the bones. EX: foramen magnum, mental foramen (mandible), supraorbital foramina, infraorbital foraminaHYOID BONE: The hyoid bone is located anterior in the neck, inferior to the mandible, and it’s the only bone that does not directly articulate with any other bone of the skeleton. It remains anchored in position to the temporal bones via the stylohyoid ligaments. It functions as an attachment site for tongue and neck muscles, and it protects the soft tissues of the trachea.VERTEBRAL COLUMN: The vertebral column is composed of a series of 26 vertebrae with intervertebral discs between each vertebra. It functions as a strong, flexible, supportive structure, which distributes the weight of the trunk to the lower limbs. It, also, functions to enclose and protect the spinal cord, to support the head, and to serve as an attachment site for the ribs, pelvic girdle, and skeletal muscles of the neck and back.Vertebra/Vertebrae: The vertebral column consists of 33 vertebrae, which are irregular bones connected by the anterior and posterior longitudinal ligaments. These ligaments function to support the vertebral column and to prevent hyperextension (anterior longitudinal ligament) and hyperflexion (posterior longitudinal ligament) of the vertebral column. In addition, the ligamentum flavum connects to adjacent vertebrae posteriorly; it’s composed of strong elastic connective tissue, which allows the vertebral column to stretch and recoil to maintain posture.Intervertebral discs: Intervertebral discs are cushion-like pads located between each vertebra. They function to provide flexibility and shock absorption. Each intervertebral disc is composed of a nucleus pulposus and an anulus fibrosus.Nucleus pulposus: The nucleus pulposus is the central gel-like substance of the intervertebral disc that functions to provide elasticity and compressibility.Anulus fibrosus: The anulus fibrosus is a strong, outer ring of collagen fibers and fibrocartilage, which surrounds the nucleus pulposus. It functions to limit the expansion of the nucleus pulposus when the vertebral column is compressed. Curvature of vertebral column: The vertebral column exhibits two distinct curves. The cervical and lumbar regions bulge anteriorly, and the thoracic and sacral regions bulge posteriorly. These curves function to increase resiliency and flexibility of the vertebral column and prevent the vertebrae from fracturing as the body moves.Regions of vertebral column:Cervical region (7 vertebrae): The cervical region of the vertebral column is the neck region. These vertebrae are the smallest in the vertebral column, and they are named C1 – C7. C1, also known as the atlas, is located inferior to the skull, articulating with the occipital bone at the occipital condyles. C1, or the atlas, functions to allow the head to move superiorly and inferiorly in a “yes” motion. C2, also known as the axis, articulates with C1 and functions to allow the head to rotate from side-to-side in a “no” motion.Thoracic region (12 vertebrae): The thoracic region of the vertebral column is the chest region. These vertebrae are larger than the cervical vertebrae and exhibit long, thin spinous processes that point inferiorly. They are named T1 – T12, and they articulate with the ribs via vertebrocostal joints (i.e. T1 articulates with rib pair 1; T2 articulates with rib pair 2; etc.)Lumbar region (5 vertebrae): The lumbar region of the vertebral column is the lower back region. These vertebrae are the largest and strongest in the vertebral column, and they exhibit short, broad spinous processes, which are well-adapted for skeletal-muscle attachment. They are named L1 – L5.Sacral region (sacrum; 5 vertebrae): The sacral region of the vertebral column functions are a strong foundation of the pelvic girdle. The sacrum appears as an inverted triangular shaped bone. However, this “bone” is, actually, formed by the fusion of five sacral vertebral, which are named S1 – S5. The fusion of these vertebrae begins between 16-18 years of age and is complete by the age of 30. It’s important to note that the sacral structure of the male and female are slightly different. The female sacrum is shorter, broader, and more curved between S2 and S3 than the male sacrum.Coccygeal region (coccyx; 4 vertebrae): The coccygeal region of the vertebral column is the tailbone region. Like the sacrum, the coccyx appears as an inverted triangular shaped bone. However, this “bone” is, actually, formed by the fusion of four coccygeal vertebrae, which are named Co1 – Co 4. The fusion of these vertebrae occurs between the ages of 20 – 30.THORACIC CAGE (BONY THORAX): The thoracic cage is a bony structure composed of the sternum, ribs, and costal cartilage. It functions to protect the organs in the thoracic cavity, to support the pectoral girdle and upper limbs, and to provide attachment sites for the skeletal muscles of the neck, back, chest, and shoulders.Sternum: The sternum is a flat, narrow bone, which is commonly called the “breastbone”. It is composed of the manubrium, body, and xiphoid process.Manubrium (1)Body (1)Xiphoid process (1): The xiphoid process is composed of hyaline cartilage and slowly ossifies as the skeleton ages. By the age of 40, the xiphoid process is completely ossified.Costal cartilage: The costal cartilages are strips of hyaline cartilage that attach the ribs to the sternum. This cartilage functions to provide elasticity and flexibility to the thoracic cage, which helps prevent damage/fracture to the sternum and ribs as we breathe.Ribs (12 pairs): The ribs are flat, narrow bones that function to provide a strong structural support to the thoracic cage. Each rib pair articulates posteriorly to its corresponding thoracic vertebra (i.e. rib pair 1 articulates with T1; rib pair 2 articulates with T2; etc.). Two types of ribs exist: true ribs and false ribs. These rib types are based on rib attachment to the sternum.True (vertebrosternal) ribs: The true ribs are rib pairs 1 – 7. These ribs attach posteriorly to T1 – T7, and they have a direct anterior attachment to the sternum via the costal cartilage.False ribs: The false ribs are rib pairs 8 – 12, and they attach posteriorly to T8 – T12. In addition, they either attach indirectly to the sternum via the costal cartilage, or they do not attach to the sternum at all.Vertebrochondral ribs: The vertebrochondral ribs are rib pairs 8 – 10. These ribs have an indirect attachment to the sternum via the costal cartilage of rib pair 7. The costal cartilage that extends from rib pairs 8 – 10 converge together and attach to the costal cartilage of rib pair 7, which allows these ribs to attach to the sternum.Floating (vertebal) ribs: The floating ribs are rib pairs 11 – 12, and these ribs do not attach to the sternum at all. Their only attachment site is located posteriorly to T11 – T12.THE APPENDICULAR SKELETONPECTORAL (SHOULDER) GIRDLE: Bones of the pectoral girdle function to attach the upper limbs to the axial skeleton and to provide attachment sites for tendons of skeletal muscles that move the upper limbs.Clavicle (collarbone; 2)Scapula (shoulder blade; 2)UPPER LIMB: Bones of the upper limbs function to provide attachment sites for tendons of skeletal muscles that move the arm, forearm, and hands. These bones are remarkably thinner than those that comprise the lower limbs.Humerus (upper arm bone; largest, longest bone of the upper limb; 2)Ulna (medial in the forearm; longer than radius; 2)Radius (lateral in the forearm; 2)Carpals (8 bones of wrist; 2 sets)Proximal row (lateral to medial):Scaphoid, Lunate, Triquetrum, PisiformDistal row (lateral to medial):Trapezium, Trapezoid, Capitate, HamateMetacarpals (5 bones of palm; 2 sets)Numbered laterally to medially: Metacarpal I, Metacarpal II, Metacarpal III, Metacarpal IV, & Metacarpal VPhalanges (bones of fingers; 3 rows: proximal, middle, and distal; 2 sets) Numbered laterally to medially: Phalanx I, Phalanx II, Phalanx III, Phalanx IV, & Phalanx VPELVIC (HIP) GIRDLE: Bones of the pelvic girdle function to attach the lower limbs to the axial skeleton, distribute upper body weight to the lower limbs, and support and protect the visceral organs of the pelvic region.Os coxae (hip bones; 2)Ilium (superior and largest portion of hip bone)Ischium (inferior, posterior portion of hip bone)Pubis (pubic bone, anterior and inferior part of hip bone)Acetabulum (deep fossa; functions as a socket to accept rounded head of femur)Obturator foramenBony pelvis (part of the appendicular skeleton composed bony structures and cartilage)2 Os coxaePubic symphysis (joint between two hip bones; composed of fibrocartilage)Sacrum (sacral region of vertebral column)Coccyx (coccygeal region of vertebral column)LOWER LIMB: Bones of the lower limbs function to carry body weight and are designed to resist the mechanical and physical forces/stresses exerted on them when the body moves. These bones are remarkably thicker and stronger than those that comprise the upper limbs.Femur (thigh bone; longest, heaviest, and strongest bone in the body; 2)Patella (kneecap; 2)Tibia (shin bone; larger, medial, weight-bearing bone of leg; 2)Fibula (parallel and lateral to tibia; smaller and thinner than tibia; 2)Tarsals (7 bones of ankle; 2 sets)Calcaneus (heel)TalusNavicularCuboidMedial cuneiformIntermediate cuneiformLateral cuneiformMetatarsals (5 bones of intermediate region of foot; 2 sets)Numbered medially to laterally: Metatarsal I, Metatarsal II, Metatarsal III, Metatarsal IV, & Metatarsal VPhalanges (bones of toes; 3 rows: proximal, middle, and distal; 2 sets)Numbered medially to laterally: Phalanx I, Phalanx II, Phalanx III, Phalanx IV, & Phalanx V ................
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