TISSUES AND SUPPORTING SYSTEMS Movement is one of the essential features of living things. Cellular movement is observed in one-celled amoebas, ciliates, and flagellates. Flagella whip about to produce cellular motion, while cilia beat synchronously to propel a cell. In animals, movement is essential for locating food, escaping predators, and seeking mates. In many animals, the movement process iscentered in the muscle cell, which contracts and relaxes. The contraction yields great force, which is applied against a surface by means of askeleton. SKELETON Skeleton is the bony framework of the body which provides support, shape and protection of the soft tissues and organs in animals. It enables animals to move from place to place. Forms and Component of Skeleton: The three forms of skeleton are:- · Cuticles: The cuticle is composed of a protein called chitin. Chitin is a non-living substance, hence, arthropods which are insects, crabs, scorpions, prawns etc with this type of skeletal material can only grow by “moulting” or “ecdysis” (process in which an organism sheds off its old skeleton and put on a new one). · Bones: It is the major component of the skeleton. It consists of the living bone cells (osteocytes), protein fibres (collagen) and minerals (inorganic salts) which are mainly calcium phosphate and calcium carbonate. The inorganic makes the bone stronger and more rigid than the cartilage. Examples of organisms which have bones are mainly vertebrates which are the bony fishes, toads, crocodile, snakes, birds and mammals. · Cartilages: Cartilage is a tissue found in the skeleton of complex vertebrae. It consists of living cells (chondroblasts), carbohydrates and protein fibres. It is a firm, flexible material which can support great weight. It also acts as a shock absorber, cushioning the effect of bones moving against bones during movement. Examples of organisms which possess cartilages are cartilaginous fishes like sharks, rays and mammals. There are three types of cartilages which are hyaline cartilage, fibro-cartilage and elastic cartilage. Types of Skeleton: The three main types of skeleton are: · Hydrostatic (Fluid) Skeleton: Many animals have a water-based skeleton, or hydrostatic skeleton. Hydrostatic skeletons do not contain hard structures, such as bone, for muscles to pull against. Rather, the muscles surround a fluid-filled body cavity. In a worm, for example, movement occurs when muscle cells contract, and the contractions squeeze internal fluid (the hydrostatic skeleton) against the skin, causing the worm to stiffen and the body to shorten and widen. The squirming motion of a worm also depends on a hydrostatic skeleton. Examples of organisms with hydrostatic skeleton are worms, sea anemone, slug, earthworm etc. · Exoskeleton: This is the hard cuticle found in arthropods (millipedes, insects, crabs) and other organisms. It performs the function of skeleton which as it lies outside the muscles is called exoskeleton. The exoskeleton is made up of non-living substance called chitin. Organisms with this type of skeleton can only grow by a process called moulting or ecdysis (a process an organism sheds off its old skeleton, grow and later covered with a new one). Examples of organisms with exoskeleton are thearthropods, hydra, snail, e.t.c. · Endoskeleton: The skeletons are found inside the body of organisms. Muscles are attached to these skeletons inside the body of organisms. Endoskeletal animals grow by continuous increase in size and not by ecdysis. Examples of organisms with this type of skeleton are mammals, aves, cartilaginous fishes, bony fishes, reptiles, amphibian e.t.c. Axial and the Appendicular SkeletonFig 13.1 The human skeleton showing the major bones of the body. Axial Skeleton Axial skeleton is made up of the skull, vertebral column (backbone), the ribs and the breastbone (sternum). The skull: The mammalian skull is made up of several flat bones which are joinedtogether by means of joints called sutures. The three (3) major parts of the skull are the brain box or cranium which contains and enclose thebrain the facial skeleton which supports the nose, eyes, e.t.c. and the jaws which are made up of the upper jaw (maxilla) and lower jaw (mandible) which contains the teeth. The skull gives the head its shape. It protects vital organs in the head e.g. eyes, nose & ears. It bears theteeth which aids grinding of food. The vertebral column: It is the central supporting structure of the skeleton. It protects the spinal cord. In humans, there are 33 vertebrae. The vertebrae are held one to the other with strong ligaments called inter-vertebral discs. In mammals, the five (5) different vertebrae are: · Cervical vertebrae- Found in the neck region. · Thoracic vertebrae- Found in the chest region. · Lumber vertebrae- Found in the upper abdomen. · Sacral vertebrae- Found in the lower abdomen. · Caudal vertebrae- Found in the tail region. Features of a Typical Vertebra A typical vertebrae possess the following features, these are: · Neural canal: This is the passage of the spinal cord. · Neural spine: This projects upwards and dorsally for the attachment of muscles. · Transverse processes: – They project from the sides of each vertebrae for the attachment of muscles and ligaments. · Centrum: – It is a solid piece of bone below the neural canal. · Facet: – This is a small, smooth, flat or slight depressed area on a bone that is usually a point of contact with another bone. · Zygapophysis: – These are articular surfaces for the articulation of successive vertebrae. They are grouped into two parts; – Pre-zygapophyses facing inwards and upwards and – Post-zygapophyses facing outward and downward. Cervical Vertebrae The cervical vertebrae are found in the neck region. The first cervical vertebrae is called the atlas, the second is called axis. The 3rd to 7th bones are the normal cervical bones. Characteristics of Atlas · The atlas vertebra has a large neural canal. · It has a flat and broad transverse process. · The neural spine is very short -or absent. · The centrum is absent. · It has a vertebraterial canal for the passage of blood vessels. Function: Atlas allows the head to nod on it, as it fits into the occipital condyles of the skull. Characteristics of the Axis · The axis has a broad and flat centrum. · It articulates with the atlas through the odontoid process. · It has a large, flattened neural spine. · The transverse process is reduced to a pin-like structure. · It has a vertebraterial canal. Function: It allowsthe head to be turned easily i.e. twisting of the head.