Axis vs Appendicular Skeleton

The skeletal system is composed of bones that make up our body. This tower of bones make up the skeleton. The skeleton is divided into the axis/axial skeleton and appendicular skeleton. The axial skeleton is comprised of bones that form the longitudinal axis of the body. The appendicular skeleton, on the other hand, is the bones of the limbs and girdles.

The axial skeleton is further divided into three parts: the skull, the vertebral column, and the bony thorax. The skull is fashioned by two sets of bones; the cranium and the facial bones. The cranium surrounds and safeguards the delicate brain tissue. The facial bones position the eyes in an anterior position and allows us to smile or frown. The mandible (jaw) is the only bone of the skull which is not fused by sutures and is therefore a freely, movable joint.

The cranium, which is boxlike in shape, consists of eight, large, flat bones: the frontal bone forms the forehead, the paired parietal bones build most of the superior and lateral walls of the cranium, the temporal bones lie behind the parietal bones, and the most posterior bone of the cranium is the occipital bone which forms the base and back wall of the skull. The ethmoid bone is very irregularly shaped and lies anterior to the sphenoid. It makes up the roof of the nasal cavity and part of the middle walls of the orbits.

The face is composed of 14 bones. Twelve are paired; only the mandible and vomer are single. Facial bones include the maxillae, palatine bones, zygomatic bones, lacrimal bones, nasal bones, vomer bone, inferior nasal conchae, mandible, and the hyoid bone.

The vertebral column, also known as the spine, is the axial support of the body. It is formed from 26 irregular bones connected and strengthened by ligaments which makes it a flexible, curved structure.

The cervical vertebrae consist of 7 vertebrae named C1 to C7. This forms the neck region. C1 and C2 (axis and atlas) are different from other vertebrae because they execute functions not done by the other cervical vertebrae. Other cervical vertebrae (C3-C7) are the lightest, smallest vertebrae, and usually their spinous processes are short and divided into branches.

Thoracic vertebrae are composed of 12 (T1-T12) vertebrae. These are larger than the cervical vertebrae. The 5 lumbar vertebrae (L1-L5) are the sturdiest among vertebrae since they handle most of the stress on the vertebral column.

The sacrum is composed of five vertebrae fused together. The sacrum forms the posterior wall of the pelvis. The coccyx is the human “tailbone,” a remnant of the tail that other vertebrate animals have. It is formed from the joined three to five tiny, irregularly shaped vertebrae. The bony thorax is composed of the sternum, ribs, and the thoracic vertebrae. It is also called the thoracic cage since it forms a protective cage around the smooth organs.

The appendicular skeleton comprises the bones of the extremities. It is composed of 126 bones of the limbs and the pectoral and pelvic girdles which attach the limbs to the axial skeleton. The pectoral or shoulder girdle is composed of the clavicle and scapula. The clavicle, also termed as the collarbone, is a slender, doubly curved bone. It holds the arm away from the thorax and prevents shoulder dislocation. The scapulae, or shoulder blades/wings, have two important processes–the acromion and the coracoids process. Although it is exceptionally flexible, it is also easily dislocated. The skeletal framework of the upper limb is formed by 30 separate bones. They form the groundwork of the arm, forearm, and hand.

The pelvic girdle is formed by two coxal bones, or ossa coxae, commonly known as hip bones. The most important function of the pelvic girdle is to bear weight; the total weight of our body rests on the pelvis.

Each hip bone is formed by the fusion of three bones: the ilium, ischium, and pubis. The ilium is a large, flaring bone that forms most of the hip bone. The iliac crest, the top edge of the ilium, is an important anatomical landmark that is always kept in mind by those who give injections.

The pubis, or pubic bone, is the most frontal part of the coxal bone. The pubic bones of each hip bone fuse to form the pubis symphysis. The bony pelvis is divided into the false pelvis and the true pelvis.

The bones of the lower limbs carry our body weight when we are standing. Thus, the thigh, leg, and foot are much thicker and stronger than the bones of the upper limbs.

The femur, or thigh bone, is the only bone in the thigh. The leg is formed by two bones; the tibia and fibula. The tibia, or shinbone, is larger and more medial. The fibula lies alongside the tibia, is thin and stick-like. The foot consists of the tarsals, metatarsals, and phalanges. It supports our body weight and serves as a lever that allows us to move forward when we walk or run. The tarsus is composed of seven tarsal bones. The two largest tarsals, the calcaneus and talus, carry most of the body weight. The sole is formed by 5 metatarsals, and 14 phalanges form the toes. Each toe has three phalanges except for the big toe which has only two.

Summary:

1.Axial, or axis, and appendicular skeleton are divisions of the skeletal system.

2.The axial skeleton comprises the bones that form the longitudinal axis of the body. The appendicular skeleton is composed of the bones of the limbs and girdles.

3.The axial skeleton is composed of the skull, the vertebral column, and the bony thorax.

4.The skull is formed by the cranial and facial bones. Eight cranial bones protect the brain: frontal, occipital, ethmoid, and sphenoid bones, and the pairs of parietal and temporal bones. The 14 facial bones are all paired (maxillae, zygomatics, palatines, nasals, lacrimals, and inferior nasal conchae), except for the vomer and mandible. The hyoid bone, not really a skull bone, is supported in the neck by ligaments.

5.The vertebral column is formed from 24 vertebrae, the sacrum, and the coccyx. There are 7 cervical vertebrae, 12 thoracic, and 5 lumbar vertebrae, which have common as well as unique features. Primary spinal curvatures present at birth are the thoracic and sacral curvatures; secondary curvatures (cervical and lumbar) develop after birth.

6.The bony thorax is formed from the sternum and 12 pairs of ribs. All ribs attach posteriorly to the thoracic vertebrae. Anteriorly, the first seven pairs attach directly to the sternum (true ribs); the last five pairs attach directly or not at all (false ribs). The bony thorax encloses the delicate organs.

7.The shoulder girdle, composed of two bones, the scapula and the clavicle, attaches the upper limbs to the axial skeleton.

8.The bones of the upper limbs include the humerus of the arm, the radius, and ulna of the forearm, the carpals, metacarpals, and the phalanges of the hand.

9.The pelvic girdle is formed by the two coxal bones, or hip bones. Each hip bone is the result of the fusion of the ilium, ischium, and pubis bones. The girdle receives the weight of the upper body and transfers it to the lower limbs. The female pelvis is lighter and broader than the male’s; its inlet and outlet are larger for childbearing.

10.The bones of the lower limbs include the femur of the thigh, the tibia and fibula of the leg, and the tarsals, metatarsals, and phalanges of the foot.

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