13.1 The Skeleton
Key Takeaways
An earthworm has a hydrostatic skeleton: muscles squeeze incompressible fluid in a body cavity, and the fluid transmits the force.
An arthropod exoskeleton is a hard outer covering that must be molted; it does not grow continuously.
A vertebrate endoskeleton is made of bone and cartilage and grows with the body.
The human axial skeleton is the skull, vertebral column, and rib cage; the appendicular skeleton is the limbs and girdles.
Bone is living tissue with osteocytes, a calcium phosphate matrix, and marrow that forms blood cells; antagonistic muscles pull and do not push.
13.1 The Skeleton
An animal body needs support that holds its shape, gives muscles a place to pull, and shields soft organs. Three plans solve that problem: a hydrostatic skeleton, an exoskeleton, and an endoskeleton. For each plan, name an animal and state how growth works. Human bone needs the most detail, because bone is living tissue, stores calcium, and helps form blood cells.
Three support plans
Hydrostatic skeleton
A hydrostatic skeleton supports the body with fluid under pressure inside a cavity. The earthworm is the animal to picture. Septa divide its fluid-filled coelom into segments, and the fluid in a segment cannot be compressed. Circular muscles squeeze the segment so it becomes longer and thinner. Longitudinal muscles shorten the segment and make it thicker. Because the fluid volume stays nearly constant, a squeeze in one direction forces a change in another. That transmitted pressure is how the worm crawls. There is no shell and no bone.
Exoskeleton
An exoskeleton is a hard covering on the outside of the body. Arthropods, including insects, spiders, and crustaceans, build one. The cuticle contains the carbohydrate chitin, often stiffened with proteins and, in many crustaceans, mineral salts. Muscles attach on the inner face, so the shell is both armor and a set of levers. The covering does not enlarge smoothly while the animal feeds and grows. The animal must molt, a process also called ecdysis. It splits and sheds the old cuticle, expands by taking up fluid or air while the new cuticle is still soft, and then hardens the replacement.
Endoskeleton
An endoskeleton is an internal framework. In vertebrates it is built from bone and cartilage. Cartilage is firm connective tissue that bends more than mineralized bone. It supports the nose and the outer ear, caps the ends of many bones, and forms much of the skeleton in the embryo. Long bones lengthen at cartilage growth plates, and bone later replaces most of that cartilage. Because the skeleton sits inside the soft tissues, it can thicken and lengthen without being shed. A vertebrate endoskeleton grows with the body.
| Feature | Hydrostatic skeleton | Exoskeleton | Endoskeleton |
|---|---|---|---|
| Where the support is | Pressurized fluid in a body cavity | Hard covering outside the body | Bone and cartilage inside the body |
| Animal to picture | Earthworm | Insect or other arthropod | Human or other vertebrate |
| Growth | The body wall and fluid cavity enlarge | The animal must molt; the shell does not grow continuously | Bone and cartilage grow with the body |
| How force is applied | Muscles squeeze incompressible fluid | Muscles pull on the inner face of the shell | Muscles pull on bones across joints |
Bone is living tissue
Bone is not leftover stone from a dead animal. It is connective tissue that stays alive and keeps changing. Osteocytes are the mature bone cells. They sit in small spaces in the matrix and help maintain the tissue around them. Osteoblasts deposit new matrix, and osteoclasts break old matrix down. Remodeling continues through life as load, growth, and blood chemistry change.
The hardness of the matrix comes largely from calcium phosphate deposited on a protein framework made mostly of collagen. Mineral resists compression. Collagen resists stretch and twisting. If the mineral is removed, a bone becomes flexible. If the protein is destroyed, the remaining mineral is brittle. Both parts belong in a correct picture of bone. The tissue is a composite, not a chunk of pure crystal.
Spaces inside many bones hold marrow. Red marrow forms blood cells, including red cells, white cells, and platelets. Yellow marrow stores fat and can shift toward blood-cell production when demand is high. The skeleton is therefore linked to circulation and to defense, not only to posture.
Calcium storage
Bone is also a reservoir for mineral homeostasis. Calcium ions in the blood are required for nerve signaling, muscle contraction, and clotting. When blood calcium falls, calcium can be withdrawn from the matrix. When blood calcium rises, more calcium can be deposited. The skeletal fact to keep is the storage role itself. Dead rock cannot be laid down and pulled back on a physiological schedule. Living bone can.
Axial and appendicular regions
The human skeleton
The axial skeleton is the central axis of the body: the skull, the vertebral column, and the rib cage. The skull protects the brain and frames the face. The vertebrae protect the spinal cord and support the trunk. The ribs and sternum protect the heart and lungs and move during breathing.
The appendicular skeleton is the equipment used to interact with the surroundings: the limbs and the girdles that attach them to the axis. The pectoral girdle anchors the upper limbs. The pelvic girdle anchors the lower limbs. Bones of the arms, hands, thighs, legs, and feet are appendicular.
Joints and antagonistic muscles
Joints are the places where bones meet. Some joints allow a wide range of motion, such as the shoulder. Others allow almost none, such as the sutures between skull bones. At a movable joint, skeletal muscle produces movement by shortening. Muscle fibers pull on a bone through a tendon. They do not lengthen themselves in order to push the bone away.
Antagonistic pairs supply the return stroke. One muscle pulls a bone in one direction, and a partner muscle pulls it back. The biceps brachii pulls the forearm so the elbow flexes. The triceps brachii pulls the forearm the other way so the elbow extends. When one of the pair contracts, the partner is stretched. Ligaments hold bone to bone at the joint. The muscles pull in opposite directions and never push the joint open.
Warning
Bone is living tissue with osteocytes, a calcium phosphate matrix, and marrow that forms blood cells. It is not dead rock. An exoskeleton does not grow continuously the way bone does; an arthropod must molt.
An earthworm lengthens a body segment as it crawls. Which description of that support system is accurate?
Muscles squeeze fluid inside a body cavity, and the fluid transmits the force because it cannot be compressed.
Osteocytes remodel a calcium phosphate shell each time the worm moves forward.
Internal bones and cartilage grow with the worm and act as levers in each segment.
The worm sheds a chitinous outer shell, then expands before a new shell hardens.
A growing insect has reached a size its hard outer covering will not allow. What must happen before the body can expand further?
The insect replaces the shell with marrow and switches to a fluid-filled skeleton.
Antagonistic muscles push the cuticle outward so the shell grows in place without a molt.
Internal bones lengthen at cartilage growth plates while the outer covering stretches continuously.
The insect molts, shedding the old exoskeleton and expanding before the new covering hardens.
Which statement correctly describes the human skeleton?
The skull and rib cage form the appendicular skeleton, and skeletal muscles push bones apart.
Joints block every movement, and antagonistic muscles move bones by pushing on tendons.
The skull, vertebral column, and rib cage form the axial skeleton, and living bone stores calcium phosphate while marrow forms blood cells.
The limbs and girdles form the axial skeleton, and bone is dead mineral with no cells.
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