Five sacral vertebrae. (Broad, flat, and fused)

Four coccigeal vertebrae. (Small and fused)

The structure of the vertebra:

Each vertebra consists of an anterior thick part called the centrum, attached to it laterally two transverse processes and posteriorly a bony process directed downwards called neural spine.

2. The skull (Cranium):

It is a bony case that consists of:

1. Posterior part (cerebral part): That consists of 8 bones connected together at their periphery through sutures and its posterior contains a big foramen through which the spinal cord is connected to the brain.

2. Anterior part (Facial part):

That includes facial bones, the two jaws and the positions of sense organs (ears, eyes, and nose).

 

3. The thoracic cage:

A case, slightly conical in shape, connected posteriorly to the 12 thoracic vertebrae, and anteriorly to the sternum (a flat bone pointed at its lower part which is cartilaginous). The thoracic cage consists of 12 pairs of ribs, all of them are connected to the vertebral column posteriorly but only 10 pairs are connected to the sternum. The remaining 2 pairs are called floating ribs, which are short and do not reach the sternum. The rib is a curved bone attached posteriorly to the centrum of a vertebra and its transverse process. During inspiration, the ribs move anteriorly and laterally to increase the volume of the thoracic cavity and vice versa during expiration.

Second – The Appendicular Skeleton:

1. The pectoral girdle and the fore limbs:

The pectoral girdle consists of two identical halves. Each half consists of scapula which is a triangular bone, its inner end is broad and the outer end is pointed and has a process attached to the clavicle, which is a thin bone. At the outer end of the scapula, there is a glenoid cavity through which articulation with the fore limb takes place forming the shoulder joint.

Each fore limb consists of:

a. Upper arm supported by one bone called humerus.

b. Lower arm supported by two bones which are radius and ulna. The upper part of the ulna has a depression inside which the inner projection of the humerus fits. The radius is smaller in size and can rotate around the fixed ulna.

c. The wrist consists of 8 bones called carpals present in 2 rows, their upper ends are attached to the lower part of the radius, while their lower ends are attached to the bones of the hand (palm).

d. The palm which consists of long thin bones called metacarpals followed by five digits each is made of 3 bones called phalanges except the thumb which consists of 2 phalanges only.

2. The pelvic girdle and the hind limbs:

The pelvic girdle consists of two identical halves fused at the ventral side at a region called the pubic sumphysis. Each half consists of a dorsal bone called ilium which is attached anteriorly and ventrally to a bone called pubis and attached posterioly and ventrally to a bone called ischium. At the position of attachment of ilium and ischium bones there is a deep depression called acetabulum into which the head of the thigh bone fits forming an articulation joints.

Each hind limb consists of:

a. Thigh bone which is supported by a bone called femur.

b. Shank, supported by 2 bones: an inner tibia, and an outer fibula. At the lower end of the femur there are two processes that articulate with the shank at the knee joint. In front of the knee joint there is a small bone called the patella.

c. The ankle consists of 7 bones called tarsals and the largest of which is that at the back which forms the heel.

d. The foot is supported by 5 bones called metatarsals which are long and thin and end with 5 toes each is supported by 3 phalanges except the big toe, which is supported by 2 phalanges only.

Movement in Living Organisms.