# 7.1 HUMAN SKELETON The human skeletal system, comprised of *bones* and *cartilage*, serves as the body's primary framework. It provides support for soft tissues and enables movement through the interaction of muscles and joints. The study of bones and cartilage is known as ***osteology***. ## Structure of Bone A long bone is a complex organ made of various tissues. ### Gross Anatomy - ***Epiphysis:*** The broad, terminal ends of the bone. Contains most of the spongy bone. - ***Diaphysis (Shaft):*** The long, middle section of the bone, containing a central cavity filled with *yellow bone marrow*. ### Coverings and Linings - ***Periosteum:*** A fibrous connective tissue membrane covering the outer surface of the bone. - ***Endosteum:*** A membrane lining the inner cavity of the bone. ### Types of Bone Tissue - ***Compact Bone:*** The dense, hard outer layer of the bone, found peripherally under the endosteum. - ***Spongy Bone:*** The inner, porous bone mass, whose spaces contain *red bone marrow*. ## Cells Associated with Bone Bone is a dynamic tissue with three main cell types derived from osteogenic cells. - ***Osteoblasts:*** These are *bone-forming cells*. They synthesize and secrete the unmineralized ground substance of the bone matrix. - ***Osteocytes:*** Mature bone cells that form when osteoblasts become trapped in the matrix they have secreted. They maintain healthy bone tissue, regulate mineral content, and control calcium release into the blood. - ***Osteoclasts:*** These are *bone-destroying cells*. They perform *bone resorption*-the process of breaking down bone tissue to release calcium and phosphate into the blood. This is crucial for bone growth, repair, and remodeling. ## Structure of Cartilage Cartilage is a flexible connective tissue that is not as strong as bone. ### Composition The matrix is gel-like and contains numerous *collagenous* and *elastic fibres*, making it flexible. ### Key Structures - ***Perichondrium:*** A dense layer of collagen fibres covering the cartilage. - ***Lacunae:*** Small cavities within the matrix that contain cartilage cells. - ***Chondrocytes:*** The living cells of cartilage, located within the lacunae. ### Vascularity Cartilage is *avascular* (lacks blood vessels). Chondrocytes receive nutrients via diffusion, which results in very slow healing. ### Types of Cartilage - ***Hyaline Cartilage:*** Found at the ends of long bones, in the nose, larynx, and trachea. - ***Fibrocartilage:*** Contains thick rows of collagenous fibres. Found in intervertebral disks and the knee joint. - ***Elastic Cartilage:*** Highly flexible. Found in the ear flaps and epiglottis. ## Comparison between Bone and Cartilage | Feature | Bone | Cartilage | | :--- | :--- | :--- | | **Collagen** | Densely packed | Loosely packed | | **Cell Types** | Osteoblasts, osteocytes, osteoclasts | Chondrocytes | | **Blood Vessels** | Present (vascular) | Absent (avascular) | | **Minerals** | Deposits minerals such as calcium, carbonates, phosphates, etc. | No deposition of minerals | | **External Covering** | Covered by periosteum | Covered by perichondrium | ## Axial and Appendicular Skeleton The adult human skeleton consists of 206 bones, divided into two main parts: ### Axial Skeleton Consists of 80 bones including the skull (cranium and facial bones), vertebral column, ribs, and sternum. ### Appendicular Skeleton Consists of 126 bones including the pectoral girdle (scapula and clavicle), pelvic girdle, and the bones of the upper and lower limbs.