# Divisions of the Human Skeleton The human skeletal system is composed of **206 bones**, which are categorized into two primary divisions: the **Axial Skeleton** and the **Appendicular Skeleton**. ## Axial Skeleton *The axial skeleton forms the central axis of the body.* It consists of the bones of the skull, vertebral column, and rib cage. ### Head Bones (Skull) The head contains a total of **29 bones**, organized into four groups. | **Skull Division** | **Number of Bones** | **Details** | | :----------------- | :------------------ | :---------- | | **Cranial Bones** | 8 | Form the cranium (brain box). Paired: Parietal, Temporal (2 pairs). Unpaired: Frontal, Occipital, Ethmoid, Sphenoid (4). | | **Facial Bones** | 14 | Form the structure of the face. Paired: Lacrimal, Zygomatic, Nasal, Inferior Nasal Concha, Maxilla, Palatine (6 pairs). Unpaired: Mandible (jaw), Vomer (2). | | **Ear Ossicles** | 6 (3 pairs) | Located in the middle ear: Malleus, Incus, Stapes. | | **Hyoid Bone** | 1 | A single U-shaped bone at the base of the skull, below the tongue. *It is unique because it does not articulate with any other bone.* | ### Vertebral Column The vertebral column consists of **33 vertebrae** organized into five regions. It protects the spinal cord and supports the head and body. | **Region** | **Number of Vertebrae** | **Description** | | :-------------------- | :---------------------- | :-------------- | | **Cervical** | 7 | Neck vertebrae. The first is the atlas (C1) and the second is the axis (C2). | | **Thoracic** | 12 | Chest region vertebrae that articulate with the ribs. | | **Lumbar** | 5 | Abdominal region vertebrae. | | **Sacral (Sacrum)** | 5 (fused) | Fused vertebrae that articulate with the hip bones to form the back of the pelvis. | | **Coccygeal (Coccyx)**| 4 (fused) | Fused vertebrae forming the tailbone. | *Note: The sacral and coccygeal vertebrae are sometimes referred to as the pelvic vertebrae.* ### Rib Cage The rib cage consists of **12 pairs of ribs** that articulate posteriorly with the thoracic vertebrae and the sternum (breastbone) anteriorly. It protects the heart and lungs. | **Type of Ribs** | **Pairs** | **Description** | | :--------------- | :---------- | :-------------- | | **True Ribs** | 1-7 | Attach directly to the sternum via their own costal cartilage. | | **False Ribs** | 8-10 | Attach to the sternum indirectly via the costal cartilage of the 7th rib. | | **Floating Ribs**| 11-12 | Do not attach to the sternum at all. | ## Appendicular Skeleton *The appendicular skeleton includes the bones of the limbs (appendages) and the girdles that attach them to the axial skeleton.* It consists of **126 bones**. ### Pectoral Girdle and Upper Limb - **Pectoral Girdle:** Connects the upper limb to the axial skeleton. - **Clavicle** (collar bone): Articulates with the sternum and scapula. - **Scapula** (shoulder blade): A flat, triangular bone. - **Upper Limb (Forelimb):** - **Humerus:** Long bone of the upper arm. - **Radius and Ulna:** Bones of the forearm. The radius is on the thumb side. - **Carpals:** 8 short bones forming the wrist. - **Metacarpals:** 5 bones forming the palm. - **Phalanges:** 14 bones of the fingers (3 in each finger, 2 in the thumb). ### Pelvic Girdle and Lower Limb - **Pelvic Girdle:** Connects the lower limb to the axial skeleton. - Composed of two hip bones (coxa), each formed by the fusion of three bones: **ileum, ischium,** and **pubis**. - The two halves join at the **pubic symphysis**. - The **acetabulum** is the socket that articulates with the head of the femur. - **Lower Limb (Hind Limb):** - **Femur:** Thigh bone; the longest and strongest bone in the body. - **Patella:** Kneecap. - **Tibia and Fibula:** Bones of the lower leg. The tibia (shin bone) is the larger bone. - **Tarsals:** 7 bones forming the ankle. - **Metatarsals:** 5 bones forming the sole of the foot. - **Phalanges:** 14 bones of the toes (3 in each toe, 2 in the big toe). ## Possible Questions and Answers **Q:** What are the two main divisions of the human skeleton? **A:** The axial skeleton (central axis: skull, vertebral column, rib cage) and the appendicular skeleton (limbs and girdles). **Q:** Which bone is unique because it does not articulate with any other bone? **A:** The hyoid bone, located at the base of the skull below the tongue. **Q:** What is the difference between true, false, and floating ribs? **A:** - **True ribs (1-7):** Attach directly to the sternum. - **False ribs (8-10):** Attach indirectly to the sternum via shared cartilage. - **Floating ribs (11-12):** Do not attach to the sternum. **Q:** What three bones fuse to form the coxa (hip bone) of the pelvic girdle? **A:** The ileum, ischium, and pubis. ## Summary - The human skeleton consists of 206 bones, divided into the **axial** and **appendicular** skeletons. - The **axial skeleton** forms the body's central axis and includes the skull, vertebral column, and rib cage. - The **appendicular skeleton** consists of the pectoral and pelvic girdles, and the bones of the upper and lower limbs, facilitating movement. | Skeleton Division | Major Components | Primary Function | | :---------------------- | :---------------------------------------------------------- | :------------------------------------ | | **Axial Skeleton** | Skull (29), Vertebral Column (33), Rib Cage (12 pairs), Sternum | Protection of vital organs, support | | **Appendicular Skeleton** | Pectoral Girdle, Upper Limbs, Pelvic Girdle, Lower Limbs | Movement, manipulation of objects | **Biological Significance:** The skeletal system provides a rigid framework for support, protects internal organs (such as the brain, heart, and lungs), allows for movement through articulation with muscles, stores minerals, and is the site of blood cell formation (hematopoiesis).