When human infants are born, their bodies contain as many as 300 to 350 bones but, later on, as the developmental processes proceed there also occurs the fusion of these bones. Consequently, as a child reaches the age of 9, only 206 bones are left behind, and this number remains intact throughout the periods of youth, adulthood and old age. As is the matter with any other organ system in your body, the human endoskeleton does have a number of different constituents which are in the first place classified on the basis of their position and structural or functional peculiarity in the body. In this way, as an advanced level learner, you can say that axial and appendicular are the two major divisions which do have extensions or further subdivisions as described below:

Axial Human Skeleton

As the very name suggests, you can immediately realize that the constituent parts of this division are positioned along the axial plane of the body and include vertebral column, cranial structure and the rib fence.

Vertebral Column

Extending from the pelvic province to the cervical sector, this is the lower most part of your skeletal subdivision and is formed by the joining of smaller compact units closely placed and interlocked together to give you the best possible level of support and posture you want. Interestingly, it is the characteristic construction which renders the cognomen "vertebrates" to the living creatures which possess it, while those devoid of it have the sobriquet "invertebrates". The spine or backbone in Home sapiens is subdivided into as many as thirty-three (33) individual units or vertebrae out of which two dozen (24) are the articulating ones while the remaining nine are relatively fused, thus giving rise to the sacrum and the coccyx.

Rib Cage

As we started from the lower position and are moving upwards, the second major division of your axial skeleton is that of the rib cage which, as the very name suggests, is a hollow cage-like bony and cartilaginous construction found in your chest region. Owing to its strategic positioning in the thoracic region, the learners and instructors alike prefer to call it the thoracic cage. This core portion of your human skeleton is surrounding the thoracic cavity and supports the pectoral girdle. If you want to have how-know about its further subdivision, the anatomists are there to guide you who distinguish its constituent parts into a dozen thoracic vertebrae, two dozen ribs, sternum and the costal cartilaginous structures.

The Skull

You shouldn't have any doubt about the established doctrine that human skull is one of the hardest and most important bony configurations in the body as it surrounds and safeguards the master/chief organ brain. On the basis of embryological origin, you can easily systematize the skull of an adult human fellow into two distinguishable categorical parts, which are named as neurocranium and viscerocranium. Here the former is assigned the responsibility of shielding and looking after the brainy contents including the brain stem, whereas the latter is concerned with the formation of facial skeleton to configure and maintain the shape of the face.

Appendicular Skeleton

As you have already been briefly informed, it is the second principal division of the human skeleton which is attached, at various points, to the axial skeleton to form the accessory or supporting structures. With the overall composition of over hundred (approximately 126) bones in your body, for the sake of convenience, it is broadly grouped into six constituent classes, namely, the pectoral girdles, the pelvis, arms & forearms, hands, thighs & legs, feet and ankles.

The Pectoral Girdles

Also termed as shoulder girdles, these are two in number (corresponding to the number of shoulders you have) and serve as connecting structures between the upper limbs and the axial skeleton on each side of your bilaterally symmetrical body. Regarding configuration, each of these girdles consists of two structural components, called scapula and clavicle, which on one hand are connected with the proximal end of the humerus and, on the other hand, make junction with the axial skeleton. As a result of this specific arrangement of bones, an articulatory junction is formed which you term as sternoclavicular articulation.

The Pelvis

The two bones, connected to the left and right of hip, are collectively known as the pelvis, which is an important part of the Homo sapiens' skeletal system found between the thighs and the abdominal region. The skeleton, embedded in this region, is sometimes also pronounced as the pelvic skeleton or even the bony pelvic and consists of the pelvic cavity, floor and the perineum (located below the pelvic floor). It is noteworthy that your pelvic girdle is stronger than the shoulder girdle and it supports your body while you are sitting or standing.

Arms and Forearms

Arms in your body start from the shoulder joint and end at the elbow joint, where the anterior half, forearm, starts and extends further outward. The terminal or posterior most part of your arm is known as hand, which is often listed as a separate skeletal entity. Excluding hand, both of your arms and forearms are collectively made up of as many as 6 bone units, namely, radius (2), ulna (2) and humerus (2). It will be just surprising to know that most of the physical activities you do in your routine life are accomplished by the hands, forearms, and arms.

Hands

Forming the posterior most part of your arms, hands are distinguished from other skeletal structures in the body from the presence of fingers and palms and contain as many as 54 bones in total. Out of these 54 bones, sixteen (16) are the carpals, ten (10) metacarpals, along with ten (10) proximal, eight (8) intermediate and ten (10) distal phalanges.

Thighs & Legs

Both thighs and legs, in combination, form one of the six structural and functional units of the appendicular skeleton in your body, where you will find 8 bones with the left and right pairs of fibula, tibia, patella and femur. The bony formations, found in the legs and thighs, are categorized among the strongest bones in the body as they can perform hectic tasks and are often used in vigorous activities.

Feet and Ankles

These form highly important parts of your appendicular skeleton, virtually remaining in contact with the ground and erecting you straight upward, thus forming an angle of 90o with the earth. The left and right feet and ankles are comprised of about 52 bones, out of which 14 are the tarsals while the remaining 38 are grouped in the separate categories of metatarsals, distal phalanges, intermediate phalanges and proximal phalanges.