Exhaustive Notes on Supporting Tissues in Animals and the Mammalian Skeletal System
Supporting Tissues in Animals and the Skeletal System
The skeleton is defined as the bony framework of the body designed to provide support, shape, and protection for organisms. There are three primary types of skeletons across different species. The first is the hydrostatic or fluid skeleton, which is possessed by soft-bodied animals that utilize fluid pressure to maintain support. Examples include worms, sea anemones, slugs, earthworms, and caterpillars. The second is the exoskeleton, which is located on the external part of the body in certain animals. Most invertebrates have a cuticle composed of chitin as their exoskeleton; this is found in crabs, centipedes, millipedes, cockroaches, spiders, prawns, and various insects. The third type is the endoskeleton, which is found inside the body of animals. This internal framework is characteristic of fish, lizards, toads, birds, and mammals.
Skeletal materials are found in three specific forms: cuticle, bone, and cartilage. The cuticle is an exoskeleton located on the outer body parts, consisting of the protein chitin and a thin waterproof layer of wax. Animals with a cuticle can only grow through a process called moulting or ecdysis, where the organism sheds its old skeleton to be covered by a new one. Examples include insects, scorpions, and prawns. Bone is a hard tissue structure and is the major component of the vertebral column. It is composed of living cells, protein fibers, and non-living components, primarily such minerals as calcium, phosphorous, and calcium carbonate. Cartilage is a soft connective tissue found at the ends of bones. It acts as a shock absorber and prevents the surface of articulating bones from being worn down by friction during movement. Cartilage is made up of living cells, protein fibers, and carbohydrates. In vertebrates, the endoskeleton is comprised of both cartilage and bone.
Comparison and Functions of the mammalian Skeleton
There are distinct differences between bone and cartilage. Bone consists of both living and non-living components, contains mineral salts, and is a hard, rigid substance that is generally inflexible in adults. Bone can never be replaced by cartilage. In contrast, cartilage is made up of mainly living cells and lacks mineral salts. It remains a flexible tissue in both adults and young organisms, is a soft substance, and in many cases, can be replaced by bone as an organism matures.
The mammalian skeleton serves several vital functions. Protection is a primary role, as the rib cage protects the heart and lungs, the cranium or skull protects the brain, the vertebrae protect the spinal cord, and the pelvic girdle protects the female reproductive organs. Support is provided by the backbone or vertebrae and the pelvic girdle. The axial skeleton and girdles define the shape of the body. Movement is facilitated when muscles attached to the skeleton contract, specifically in the limb girdles and the fore and hind limbs. Long bones, through their marrow, are responsible for the manufacture of blood cells, including both white and red blood cells. The bone matrix serves as a site for mineral storage, specifically for salts like calcium and phosphate. Furthermore, the skeleton assists in breathing movements via the ribs and provides attachment points for muscles throughout the vertebral column, limb girdles, and limb bones.
Structural Divisions of the Mammalian Skeleton
The mammalian skeleton is categorized into two major parts: the axial skeleton and the appendicular skeleton. The axial skeleton comprises the skull, the vertebral column (or backbone), the ribs, and the sternum (breast bones). The appendicular skeleton includes the limb girdles—consisting of the pectotal1 and pelvic girdles—and the limbs themselves, which are categorized as forelimbs and hind limbs.
The mammalian skull is constructed from several flat bones joined by specific joints known as SUCTURES. It is divided into three major parts: the cranium (brain box) which holds the brain, the facial skeleton which supports the nose, eyes, and cheek muscles, and the jaws, which include the upper jaw (maxilla) and the lower jaw (mandible) containing the teeth. The skull protects the brain and vital organs like the eyes and nose, defines the head's shape, and bears the teeth necessary for grinding food.
The Vertebral Column and Comparison of Vertebrae
The vertebral column forms the backbone and protects the spinal cord. In humans, the body contains bones in total, and the vertebral column consists of individual bones called vertebrae (singular: vertebra), which are separated by cartilaginous ligaments called inter vertebral discs. The count of vertebrae varies by species across five regions: cervical (neck), thoracic (chest), lumbar (upper abdomen), sacral (lower abdomen/pelvis), and caudal (tail).
In the cervical region, rats, rabbits, and humans all possess vertebrae. In the thoracic region, rats have , while rabbits and humans have . In the lumbar region, rats have , rabbits have , and humans have . In the sacral region, rats have , rabbits have to , and humans have fused vertebrae. In the caudal region, rats have to , rabbits have , and humans have (fused to form the coccyx). The total number of vertebrae is approximately to for rats, to for rabbits, and exactly for humans.
A typical vertebra has several common features regardless of its specific function. The centrum is the solid body piece of the bone. The neural canal is the passage through which the spinal cord runs. The neural arch forms an arch over the centrum and surrounds the neural canal. The neural spine is a projection for muscle attachment. Transverse processes are projections from the sides of the neural arch for the attachment of muscles and ligaments. A facet is a small, smooth area that acts as a point of contact with another bone; in vertebrae, these are zygapophyses. They are divided into pre-zygapophyses (facing inwards and upwards) and post-zygapophyses (facing outwards and downwards).
Specific Features and Functions of Vertebrae
The first cervical vertebra is the ATLAS, and the second is the AXIS. The ATLAS is distinguished by a large neural canal, flat and broad transverse processes, and a short or absent neural spine. It has a vertebrateral canal for blood vessels and two large smooth facets for skull attachment, which allows for nodding movement. It lacks zygapophyses and a centrum. The AXIS has a broad, flat centrum and a signature elongated odontoid process that fits into the neural canal of the atlas, allowing for head rotation. It also features a large flattened neural spine and small transverse processes.
Normal cervical vertebrae (third to seventh) possess a short neural spine, a small centrum, a large neural canal, and a vertebrateral canal. Each transverse process is divided into two, and cervical ribs are present. These support the neck. Thoracic vertebrae feature a long, prominent neural spine directed backward and short transverse processes. They have large cylindrical centra and demifacets for rib attachment. They aid in muscle attachment and assist in breathing. Lumbar vertebrae have large, thick centra and flattened transverse processes. They possess well-developed zygapophyses and extra-paired projections called anapophysis and metapophysis for abdominal muscle attachment. Sacral vertebrae have a large centrum and a narrow neural canal; the first sacral vertebra's large transverse process attaches to the pelvic girdle for support and rigidity. Caudal vertebrae eventually lose the neural spine, canal, and transverse processes; in humans, these four bones fuse into the coccyx.
Appendicular Skeleton: Girdles and Limbs
The pectoral girdle, found at the shoulder, is made of two halves held by muscles. Each half consists of three bones: the scapula (shoulder blade), clavicle (collar bone), and coracoid. The scapula is a flat, triangular bone with a glenoid cavity at its apex where the humerus fits to form a ball and socket joint. It features a ridge called the scapula spine ending in the acromion and metacromion projections. The clavicle is a small rod attached to the acromion.
The pelvic girdle is located at the waist and consists of two halves joined at the pubis symphysis. Each half is composed of the ilium, ischium, and pubis bones. A deep depression called the acetabulum serves as the socket for the femur head, forming the hip joint. The obturator foramen is a hole for the passage of nerves, muscles, and blood vessels.
The fore limb includes the humerus (upper arm), and the radius and ulna (lower arm). The ulna is longer and forms the olecranon process. These are followed by eight carpals (wrist), metacarpals, and phalanges. The hind limb includes the femur (upper leg), and the tibia and fibula (lower leg). The femur is the longest and strongest bone in the body, articulating with the acetabulum. The tibia is longer than the fibula. These are followed by six ankle bones, metatarsals, and phalanges.
Joints and the Mechanism of Movement
A joint is where two or more bones meet. Immovable or fixed joints, such as the sutures in the skull or the joints in the pelvic girdle, connect bones with ligaments so no movement is possible. Movable joints allow bones to move over one another. These include ball and socket joints (allowing degree movement, e.g., shoulder and hip), hinge joints (allowing degree movement in one direction, e.g., elbow and knee), gliding or sliding joints (bones sliding over each other, e.g., wrists and ankles), and pivot joints (allowing rotation or nodding, e.g., atlas and axis).
A joint's structure includes ligaments to hold bones together and prevent dislocation, and tendons, which are connective tissues that attach muscles to bones. Articular cartilage lines bone ends to reduce friction and absorb shock. The synovial membrane secretes synovial fluid into the capsule (the sac containing the fluid). This fluid lubricates the joint and reduces both shock and friction.
Movement at the elbow joint is achieved by the antagonistic action of biceps and triceps muscles. The biceps are attached to the scapular and radius, while the triceps are attached to the scapular and ulna. To bend the forearm, the biceps muscle contracts while the triceps muscle relaxes. To straighten the forearm, the triceps muscle contracts while the biceps muscle relaxes.
Questions & Discussion
Name three types of skeletons found in animals and in each case name one animal which possesses it. Response: The three types are hydrostatic skeleton (e.g., earthworm), exoskeleton (e.g., cockroach), and endoskeleton (e.g., man/mammals).
List five functions of the mammalian skeleton and name a particular part of the skeleton which performs each of these functions. Response: Functions include protection (e.g., cranium protects the brain), support (e.g., vertebral column supports the body), movement (e.g., limb bones and girdles), blood cell manufacture (e.g., marrow of long bones like the femur), and mineral storage (e.g., bone matrix).