Exoskeleton is the type of skeleton which is found outside or external part of the body of some animals. Most invertebrates do possess cuticle which is composed of chitin. Chitin is a non-living substance commonly found covering the outer part of the body of some animals. Such external skeletal tissue encloses, supports, gives shape, protects and also enable the animals to move.
Examples of organisms with exoskeleton are invertebrates like tapeworms, snails, insects, prawns, crabs, and crayfish. Organisms with this type of skeleton can only grow by a process called moulting or ecdysis. In this process, an organism sheds off its old skeleton, grow and later is covered with a new one.
Endoskeleton is the type of skeleton which is found inside the body of animals. Endoskeleton exists in bony or cartilagenous skeleton of fishes, toad, lizards, birds and mammals. Endoskeleton in vertebrates is made of cartilages and bones. Endoskeleton in mammals are the skull, vertebral column or back-bones, ribs and the bones of the fore and hind limbs.
Bones of Axial and Appendicular Skeleton The skeletal system or bones in mammals e.g and that of the rabbit are grouped into two major parts. These are the axial and appendicular skeleton.
(A) Axial skeleton: This is made up of the skull, vertebral column or backbone, the rib and sternum or breastbone.
(B) Appendicular skeleton: This is made up of the limb girdles (pectoral and pelvic girdles) and the limbs (fore limbs and hind limbs).
The Skull: The mammalian skull is mads up of several flat bones which are joined together by means of joints called sutures.
(i) The cranium or brain box which holds or contains the brain.
(ii) The facial skeleton which supports the nose, eyes and the muscles of the cheek.
(iii) The jaws which are made up of the upper jaw (maxilla) and lower jaw (mandible) which contain the teeth.
Functions of the Skull
(i) It protects the brain.
(ii) It gives shape to the head.
(iii) It protects vital organs in the head, e.g. eyes, nose and ears.
(iv) It bears the teeth which aids grinding of food.
The Vertebral Column
The vertebral column, also known as the backbone or spinal column, is the central supporting structure of the skeleton. The vertebral column forms the backbone of vertebrate animals and protects the spinal cord. It is made up of five groups of bones called the vertebrae (singular: vertebra) in mammals. In rabbit it consists of 46 vertebrae. The vertebrae are held one to the others with strong ligaments having compressible cartilage pads called intervertebral discs between consecutive vertebrae.
In mammals, the five different vertebrae are: (i) Cervical Vertebrae It is found in the neck region
(ii) Thoracic Vertebrae It is found in the chest region
(iii) Lumber Vertebrae – It is found in the upper abdomen
(iv) Sacral Vertebrae – It is found in the lower abdomen
(v) Caudal Vertebrae – It is found in the tail region.
Features of a Typical Vertebra
All Vertebra even though they have different functions have certain features in common.
A typical Vertebra possess the following features:
i) Neural Spine: This projects upwards dorsally for the attachment of muscles.
(ii) Transverse Processes: They project from the sides of each vertebra for the attachment of muscles and ligaments.
(iii) Centrum: It is a solid piece of bone below the neural canal.
(iv) 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 – the pre- zygapophyses facing inwards and upwards and post zygapophyses facing outward and downward.
Cervical Vertebrae
The cervical vertebrae are found in the neck region. In rabbit, they are seven in number. The first cervical vertebra is called the atlas while the second is called the axis. The third to seventh bones are the normal cervical bones.
Characteristics of the Atlas
(i) The atlas vertebra has a large neural canal.
(Ii) It has a flat and broad transverse process.
(iii) The neural spine is very short or absent. (iv) The centrum is absent.
(v) It has a vertebrarterial canal for the passage of blood vessels.
Function of the Atlas
It allows the head to nod on it (i.e. it permits nodding of head) as it fits into the occipital condyles of the skull.
Characteristicss of the Axis
(i) The Axis has a broad and flat centrum
(Ii) It articulates with the atlas through the odontoid process.

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