Skeleton The fi rst appearance of the skeletal elements arises during the second month of embryonic life when connective tissue and cartilage precursors to the bones arise. Slowly through fetal life,
childhood, puberty, and the teenage years the bones mature into their adult forms. This developmental process combines more than 500 bone-forming centers into the fi nal 206 bones of the skeleton. This page, the facing page, and the page that follows depict changes in the skeleton from a newborn to an adult.
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Newborn skeleton
Anterior view
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Cranial bones |
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Vertebral column |
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Ribs |
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Clavicle |
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Scapula |
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Humerus |
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Ulna |
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Radius |
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Carpals |
10Metacarpals
11Phalanges
12Os coxae
13Femur
14Patella
15Tibia
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17Tarsals
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Newborn skeleton
Posterior view
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Adult skeleton
Anterior view
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Adult skeleton
Posterior view
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Cranial bones |
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Ulna |
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Vertebral column |
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Radius |
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Ribs |
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Carpals |
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Clavicle |
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Metacarpals |
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Scapula |
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Phalanges |
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Humerus |
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Newborn skeleton
Lateral view
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15Tibia
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Adult skeleton
Lateral view
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5 Axial Skeleton
The axial skeleton, comprised of the skull, vertebral column, ribs, and sternum, forms the central axis of the body. This sturdy central core is the most primitive portion of the vertebrate skeletal system. It evolved as the initial skeleton of the first vertebrate animals, to which the limb bones (the subject of the next chapter) were much later additions. The majority of the axial skeleton’s bony elements, from the bones at the base of the skull through all the
vertebrae and ribs, form as serial homologues from the segmental embryonic somites. Because of this shared developmental similarity each body segment, from the base of the skull to the end of the coccyx, has the same basic skeletal design. This is clearly evident in the structure of the vertebrae and ribs. As you study these skeletal elements in the photos of this chapter, notice their similarities.
The elements of the axial skeleton have many functional roles in the body. Both the cranial skeleton and the vertebral column form a strong protective case around the delicate tissues of the central nervous system. Additionally, the cranium fixes in space important nervous structures, such as the internal ear and eye, both of which would not function properly in an unstable environment. The cranium also plays an important role in the acquisition and processing of food, respiratory gases,
and sensory input such as sound. In addition to protecting the spinal cord, the vertebrae form a strong, flexible rod. This strong, flexible column not only forms the central support axis of the body from which the limbs are suspended, but is also capable of a varied range of joint movements that are essential to our daily functions.
Find more information about the axial skeleton in
R E A L A N AT O M Y
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