Axial Skeleton
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12 pairs of ribs, but only 7 are true ribs. The 8th, 9th and 10th are false (vertebrochondral) ribs, and the 11th and 12th float. Every count question in this chapter comes back to these numbers.
The idea
The skeletal system consists of a framework of bones and a few cartilages; it has a significant role in movement shown by the body. Bone has a very hard matrix due to calcium salts, while cartilage has a slightly pliable matrix due to chondroitin salts. In human beings the skeletal system is made up of 206 bones and is grouped into the axial and appendicular skeleton. The axial skeleton comprises 80 bones distributed along the main axis of the body: the skull, vertebral column, sternum and ribs. The skull is composed of two sets of bones — cranial (8 bones forming the protective covering for the brain, the cranium) and facial (14 bones forming the front part of the skull); a single U-shaped hyoid bone is present at the base of the buccal cavity, and each middle ear contains three tiny ear ossicles — malleus, incus and stapes. The skull region articulates with the superior region of the vertebral column with the help of two occipital condyles (dicondylic skull). The vertebral column is formed by 26 serially arranged units called vertebrae and is dorsally placed; it extends from the base of the skull and constitutes the main framework of the trunk. Each vertebra has a central hollow portion (neural canal) through which the spinal cord passes. The first vertebra is the atlas and it articulates with the occipital condyles. The vertebral column is differentiated into cervical (7), thoracic (12), lumbar (5), sacral (1-fused) and coccygeal (1-fused) regions starting from the skull; the number of cervical vertebrae is seven in almost all mammals including humans. The vertebral column protects the spinal cord, supports the head and serves as the point of attachment for the ribs and musculature of the back. The sternum is a flat bone on the ventral midline of the thorax. There are 12 pairs of ribs; each rib is a thin flat bone connected dorsally to the vertebral column and ventrally to the sternum, with two articulation surfaces on its dorsal end (bicephalic). The first seven pairs are true ribs, attached dorsally to the thoracic vertebrae and ventrally to the sternum with the help of hyaline cartilage. The 8th, 9th and 10th pairs do not articulate directly with the sternum but join the seventh rib with the help of hyaline cartilage; these are vertebrochondral (false) ribs. The last two pairs (11th and 12th) are not connected ventrally and are therefore called floating ribs. Thoracic vertebrae, ribs and sternum together form the rib cage.
Why it matters
The axial skeleton protects the brain, spinal cord, heart and lungs while giving the body a central column for posture and muscle attachment.
NEET lens
Numbers: 206 bones; axial 80; cranial 8, facial 14, hyoid 1, ear ossicles 3 pairs (malleus, incus, stapes); dicondylic skull; vertebrae 26 — cervical 7, thoracic 12, lumbar 5, sacral 1 (fused), coccygeal 1 (fused); ribs 12 pairs — true 7, vertebrochondral 3 (8th-10th), floating 2 (11th-12th); ribs are bicephalic. Cartilage lies between adjacent vertebrae (intervertebral discs). Production of erythrocytes and mineral storage are skeletal functions; body heat production is not.
Clinical significance
The intervertebral discs of cartilage between vertebrae cushion the spine; a slipped (herniated) disc can press on spinal nerves and cause severe back and leg pain. Bone marrow in flat bones such as the sternum is a site of blood-cell production, which is why bone-marrow samples are taken from it.
Introduction
The human skeleton has 206 bones, divided into the axial skeleton (80 bones along the body's axis) and the appendicular skeleton (126 bones of girdles and limbs).
Core concept
Bone: hard matrix (calcium salts); cartilage: pliable matrix (chondroitin salts). Axial (80): skull 29 = cranium 8 + face 14 + hyoid 1 + ear ossicles 6 (3 per ear: malleus, incus, stapes); dicondylic (two occipital condyles). Vertebral column: 26 vertebrae — cervical 7, thoracic 12, lumbar 5, sacral 1 (fused), coccygeal 1 (fused); neural canal for spinal cord; first vertebra = atlas, articulates with occipital condyles; 7 cervical vertebrae in almost all mammals. Sternum: flat bone, ventral midline of thorax. Ribs: 12 pairs, thin flat bones, bicephalic; true ribs 1st-7th (to sternum via hyaline cartilage); vertebrochondral/false ribs 8th-10th (join 7th rib via hyaline cartilage); floating ribs 11th-12th (not connected ventrally). Rib cage = thoracic vertebrae + ribs + sternum.
Go deeper
Functions of the skeletal system include support, protection, locomotion (with muscles), storage of minerals (calcium, phosphate) and production of blood cells (red bone marrow). Production of body heat is mainly a muscle function, not a skeletal one.
From Class 10
Class 9-10 introduced bones and cartilage. NEET asks exact counts and how ribs attach.