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## Introduction to the Skeletal System ### Learning Objectives 1. List the components of the skeletal system. 2. Compare and contrast compact and spongy bone. 3. Describe the types and locations of cartilage within the skeletal system. Our skeletal system includes the bones of the skeleton as well...

## Introduction to the Skeletal System ### Learning Objectives 1. List the components of the skeletal system. 2. Compare and contrast compact and spongy bone. 3. Describe the types and locations of cartilage within the skeletal system. Our skeletal system includes the bones of the skeleton as well as cartilage, ligaments, and other connective tissues that stabilize or connect the bones. ### Bones Bones of the skeleton are the primary organs of the skeletal system. They form the rigid framework of the body and perform other functions, described shortly. Two types of bone connective tissue are present in most of the bones of the body: compact bone and spongy bone. * **Compact bone** (also called dense or cortical bone) is a relatively rigid connective bone tissue that appears white, smooth, and solid. It makes up approximately 80% of the total bone mass. * **Spongy bone** (also called cancellous or trabecular bone) is located internal to compact bone, appears porous, and makes up approximately 20% of the total bone mass. ### Cartilage Cartilage is a semirigid connective tissue that is more flexible than bone. Mature cartilage is avascular (lacks a blood supply). Recall from section 5.2d that there are three subtypes of cartilage; the two subtypes associated with the skeletal system are described next (figure 7.1): * **Hyaline cartilage** attaches ribs to the sternum (costal cartilage), covers the ends of some bones (articular cartilage), and is the cartilage within growth plates (epiphyseal plates). Hyaline cartilage also provides a model during development for the formation of the fetal skeleton. * **Fibrocartilage** is a weight-bearing cartilage that withstands compression. It forms the intervertebral discs, the pubic symphysis (cartilage between bones of the pelvis), and the cartilage pads of the knee joints (menisci). The roles of **ligaments** (dense regular connective tissue that anchors bone to bone), **tendons** (dense regular connective tissue that connects muscle to bone), and other connective tissue structures associated with the skeletal system are described in section 9.4a. ### What Did You Learn? 1. Compare the general appearance of compact bone and spongy bone. 2. In what three locations of the body do you find fibrocartilage? ## Bone: The Major Organ of the Skeletal System Our bones - such as the bone of the thigh (femur) or bone of the upper arm (humerus) - are organs. Note that living bone is not white (like the bones you may be seeing in your lab). Rather, living bone may be yellowish in color. In contrast, the bones you see in lab likely were bleached to acquire that whitish appearance. Here we describe the general functions, classification based upon shape, gross anatomy, and histology of bone. ### General Functions Bones perform several basic functions: support and protection, levers for movement, hematopoiesis, and storage of mineral and energy reserves. ### Support and Protection Bones provide structural support and serve as a framework for the entire body. Bones also protect many delicate tissues and organs from injury and trauma. The rib cage shields the heart and lungs; the cranial bones enclose and protect the brain; the vertebrae enclose the spinal cord; and the pelvis cradles urinary and reproductive organs, as well as the terminal end of the gastrointestinal tract. ## Concept Connection Both the muscular system and the nervous system need calcium to function properly. Fortunately, the skeletal system typically houses a sufficient supply of calcium, which may be "tapped" when blood calcium levels are low. ### Figure 7.1 Distribution of Cartilage in the Adult and Juvenile Skeletons. Three types of cartilage are found in the body; the two subtypes associated with the skeletal system (hyaline cartilage and fibrocartilage) are shown here.