SKELETON

At present we are preparing for our classes in Anatomy. As medical students we must have deep knowledge on this subject. I am reading the chapter considering the structure and functions of skeletal system. My fellow-student is making notes of the material on the bone shape and bone ossification. The following material must help us to prepare for our classes in Anatomy.

There are 206 bones in the average adult skeleton, although the actual number varies from person to person and decreases with the age as some bones become fused.

The skeleton is divided into the skull, vertebral (spinal) column, thoracic cage, upper and lower limbs and the girdles that attach the limbs to the body.

The skull is composed of 28 bones. These bones are organized into cranial vault, facial bones, and auditory ossicles.

The vertebral column usually consists of 33-34 bones. They are divided into five regions. There are 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 sacral vertebrae, and 4-5 coccygeal vertebrae. The vertebral column performs some major functions: it supports the weight of the head and trunk, it protects the spinal cord, it provides a site for muscle attachment, and it permits movement of the head and trunk.

The thoracic cage, or rib cage, protects the internal organs within the thorax. It consists of the thoracic vertebrae, the ribs with their associated costal (rib) cartilages, and the sternum (breastbone). Each rib consists of the head, the neck, and the body. Twelve pairs of ribs attach to the thoracic vertebrae. The sternum, or breastbone, is composed of the manubrium, the body, and the xiphoid process.

The human upper limb is capable of a wide range of movements, including lifting, grasping, and touching. The arm (the portion of the upper limb from the shoulder to the elbow) contains only one bone, the humerus. Between the arm and forearm there is a cubital (elbow) joint. The forearm has 2 bones, the ulna on the medial side of the forearm and the radius on the lateral side of the forearm. The wrist is a relatively short region between the forearm and hand and is composed of 8 carpal bones. The hand consists of five metacarpal bones. The pectoral, or shoulder, girdle attaches the upper limb to the body. It consists of two bones: the scapula, or shoulder blade, and the clavicle, or collar bone.

The lower limb is very similar to that of the upper limb, except the pelvic girdle. It is attached much more firmly the body than is the pectoral girdle. The bones in general are thicker, heavier, and longer than those of the upper limb. The pelvic girdle supports the weight of the body and protects internal organs. The male pelvis usually is more massive than the female pelvis as a result of the greater weight and size of the male. Pelvic girdle is formed by the sacrum and paired bones, called the coxae, or hip bones. The thigh contains a single bone, the femur, which has a prominent rounded head. The femur articulates with the coxa, the tibia, and the patella. The knee joint is a joint located between the femur and the tibia. The leg (the portion of the lower limb between the knee and the ankle) consists of the two bones, the tibia and the fibula. The tibia supports most of the weight of the leg. The fibula doesn't articulate with the femur but has a small proximal head where it articulates with the tibia. The ankle consists of seven tarsal bones. The ankle is relatively much larger than the wrist. The foot consists of five metatarsal bones.

Human Skeleton

BONE'S STRUCTURE

The skeletal system consists of bones, cartilages, tendons, and ligaments. Because bone is very rigid, it is well adapted to help maintain the shape of the body and protect internal organs. Cartilages, which are somewhat rigid but more flexible than bones, also provide support. Tendons and ligaments are strong bands of fibrous connective tissue. Tendons attach muscles to the bones, and ligaments attach bones to bones.

Individual bones can be classified according to their shape as long, short, flat, or irregular. Most of the bones of the upper and lower limbs are long bones. These bones are very strong. They are broad at the ends where they join with other bones, and have large surface areas for muscle attachment. Short bones are approximately as broad as they long. They are nearly cube shaped or round. They are in the wrist and ankle. Flat bones have a relatively thin, flattened shape. The examples of flat bones are some skull bones, ribs, pelvic bones, and the breastbone (sternum). Irregular bones are ones such as the vertebrae and facial bones with shapes that do not fit into three categories.

Each long bone consists of three major components: the diaphysis; an epiphysis; and the epiphyseal line (or epiphyseal plate in each growing long bone). The diaphysis is long middle region of a long bone composed of compact bone, which is bone matrix. Each end of a long bone is called an epiphysis. The epiphyses consisting of cancellous bone (sometimes called spongy bone), has many small spaces or cavities within the bone matrix. The outer surface of the epiphyses consists of a layer of compact bone. In addition to the small spaces within cancellous bone and compact bone, some bones contain large cavities. The diaphyses of long bones have a large medullary cavity (some of the skull bones have spaces called sinuses). The medullary cavity and the cavities of the cancellous bone are filled with marrow. Medullary cavity contains yellow bone marrow and spaces in cancellous bone contain red bone marrow. Yellow bone marrow is mostly composed of adipose tissue. Red bone marrow is the site of blood formation. In general, yellow marrow is associated with the long bones of the limbs, and red marrow is associated with the rest of the skeleton. The outer surface of bones consists of a periosteum. The periosteum is a strong, fibrous, vascular membrane that covers the surface of a long bone, except at the ends of the epiphyses. The inner layer consists mostly of a single layer of osteoblasts with a few osteoclasts. Osteoblasts are bone producing cells, and osteoclasts are cells breaking down bone.

Flat bones usually have no diaphyses or epiphyses. They contain an interior framework of cancellous bone sandwiched between two layers of compact bone. Short and irregular bones have a composition similar to the epiphyses of long bones. They have compact bone surfaces surrounding a cancellous bone center with small spaces that usually filled with marrow. Short and irregular bones have no diaphyses. However, certain regions of these bones (e.g., the processes of irregular bones) have epiphyseal growth plates and therefore have small epiphyses.

Ex. 28. Answer the following questions:

1. What does the skeletal system consist of? 2. What is the major function of the bone? 3. What does the cartilage provide? 4. What are ligaments and tendons? 5. What are their functions? 6. What types of bones do you know? 7. What components does every long bone consist of? 8. What is diaphysis composed of? 9. What does the epiphysis consist of? 10. What is compact bone? 11. What is osteoblast? 12. What is osteoclast? 13. What do the flat bones contain? 14. What are short and irregular bones composed of?

Structure of Bone