Lesson 2: Divisions of the Skeletal System and its Major Bones
By the end of this lesson, students will be able to:
- Identify the major components of the skeletal system by division and by anatomical name.
Jump to Multilingual Approach Lesson Slides with translation
Introduction
By the end of this lesson, you will be able to identify the major components of the skeletal system by division and by anatomical name.
The anatomical terms introduced in this lesson are included in the accompanying Skeletal System Word List, organized by major region of the skeleton.
Bone as a Dynamic Tissue
Bones are living, dynamic tissues that vary in size, shape, and strength according to their function. Rather than remaining static throughout life, bone tissue continuously remodels in response to internal factors, such as hormones, and external factors, such as physical activity and mechanical stress.
The surfaces of bones contain bone markings, many of which serve as passageways for nerves, attachment sites for muscles, tendons, and ligaments, or form part of a joint. The size and prominence of these markings often reflect the strength of the attached muscles and the amount of force placed on the bone.
- Condyle: Rounded articular surface. An example is the occipital condyle, highlighted in pink below.
- Foramen: Hole or opening through which blood vessels or nerves pass. An example is the foramen magnum of the occipital bone.
Divisions of the Skeleton
The human skeleton is divided into two major divisions: the axial skeleton and the appendicular skeleton (Figure 2).
The axial skeleton forms the central axis of the body and includes the:
- Skull
- Vertebral column
- Ribs
- Sternum
The appendicular skeleton consists of the appendages and the girdles that attach the limbs to the axial skeleton. It includes the:
- Shoulder (pectoral) girdle
- Upper limbs
- Pelvic girdle
- Lower limbs
Major Bones of the Appendicular Skeleton
The upper limb consists of the:
- Clavicle
- Scapula
- Humerus
- Radius
- Ulna
- Carpals
- Metacarpals
- Phalanges
The lower limb consists of the:
- Pelvic girdle
- Femur
- Patella
- Tibia
- Fibula
- Tarsals
- Metatarsals
- Phalanges
Bones of the Skull
The skull is composed of numerous bones that protect the brain, support the face, and provide attachment sites for muscles.
From an anterior view:
- The frontal bone forms the forehead.
- The zygomatic bones form the cheekbones.
- The maxillae form the upper jaw and contain the upper teeth.
- The nasal bones form the bridge of the nose.
- The lacrimal bones are located along the medial wall of each orbit and contain ducts that drain tears into the nasal cavity.
Within the nasal cavity, the ethmoid bone contributes to the nasal septum through its perpendicular plate and forms the superior and middle nasal conchae. Inferiorly, the vomer completes the bony nasal septum.
The temporal bones form the lateral sides of the skull and contain the external auditory canal. They also contribute to the zygomatic arch and form the temporomandibular joint (TMJ) with the mandible.
The parietal bones form the superior and lateral portions of the cranium, while the occipital bone forms the posterior and inferior skull. The occipital bone contains the foramen magnum, through which the spinal cord passes to connect with the brain.
The roof of the mouth is formed by two bones. The maxillae create the anterior hard palate, while the palatine bones form the posterior portion. The sphenoid bone is a complex bone that contributes to the floor of the cranium, the orbit, the nasal cavity, and portions of the oral cavity.
The mandible forms the lower jaw, contains the lower teeth, and serves as the only movable bone of the skull.
Paranasal Sinuses
The skull contains four paired paranasal sinuses, which are air-filled cavities located within specific bones (Figure 3.2.5).
The paranasal sinuses include the:
- Frontal sinuses
- Ethmoid sinuses
- Sphenoid sinuses
- Maxillary sinuses
These cavities reduce the overall weight of the skull while contributing to voice resonance and to the conditioning of inspired air.
Vertebral Column
The vertebral column supports the head and trunk while protecting the spinal cord (Figure 8). It is composed of:
- 7 cervical vertebrae
- 12 thoracic vertebrae
- 5 lumbar vertebrae
A common mnemonic for remembering these numbers is “Breakfast at 7, lunch at 12, and dinner at 5.”
The vertebral column also contains two fused regions:
- The sacrum, which articulates with the pelvic girdle at the sacroiliac joints
- The coccyx, or tailbone, which consists of several fused vestigial vertebrae
The vertebral column exhibits normal spinal curvatures that help distribute body weight and absorb shock. As the vertebrae descend from the cervical to lumbar regions, they become progressively larger to support increasing body weight.
The first two cervical vertebrae are specialized:
- Atlas (C1) supports the skull.
- Axis (C2) contains the dens, which allows the head to rotate.
Figure 11. Regions of the vertebral column and specialized cervical vertebrae.
Thoracic Cage
The thoracic cage protects the heart and lungs while supporting respiration (Figure 9).
The sternum consists of three regions:
- Manubrium
- Body
- Xiphoid process
There are 12 pairs of ribs, classified as:
- True ribs (1–7): attach directly to the sternum by costal cartilage.
- False ribs (8–10): attach indirectly through shared costal cartilage.
- Floating ribs (11–12): have no anterior attachment to the sternum.
Figure 12. Bones of the thoracic cage.
Appendicular Skeleton
Shoulder Girdle and Upper Limb
The shoulder (pectoral) girdle consists of the clavicle and scapula (Figure 10).
The clavicle forms the sternoclavicular joint medially and the acromioclavicular joint laterally. The scapula includes several important landmarks, including the scapular spine, acromion, and glenoid cavity, where it articulates with the humerus.
The humerus forms the upper arm. Distally, it articulates with the two forearm bones:
- Radius (lateral; thumb side)
- Ulna (medial; little finger side)
The hand consists of:
- Carpals (wrist)
- Metacarpals (palm)
- Phalanges (fingers)
Digits 2–5 each contain three phalanges, whereas the thumb (pollex) contains only two.
Figure 13. Bones of the shoulder girdle and upper limb.
Pelvic Girdle and Lower Limb
The pelvic girdle is composed of three fused bones:
- Ilium
- Ischium
- Pubis
The left and right pubic bones meet anteriorly at the pubic symphysis. In general, the female pelvis is wider than the male pelvis, reflecting adaptations for childbirth.
The femur, the largest and strongest bone in the body, articulates with the pelvis at the acetabulum. Distally, it articulates with the:
- Tibia (medial)
- Fibula (lateral)
A helpful mnemonic is “Fibula = flimsy; Tibia = tough.”
The patella protects the knee joint. The proximal tibia includes the tibial plateau and attachment sites for important stabilizing ligaments, including the anterior and posterior cruciate ligaments (ACL and PCL).
Figure 14. Bones of the pelvic girdle and lower limb.
Ankle and Foot
The foot contains seven tarsal bones, which correspond functionally to the carpals of the wrist (Figure 3.2.10). The largest tarsal, the calcaneus, forms the heel and serves as the attachment site for the Achilles tendon.
Distal to the tarsals are the:
- Metatarsals, which help form the arches of the foot
- Phalanges, which form the toes
Toes 2–5 each contain three phalanges, whereas the hallux (great toe) contains only two.
Figure 15. Bones of the ankle and foot.
Summary
The human skeleton is organized into two major divisions: the axial skeleton, which forms the body’s central framework, and the appendicular skeleton, which consists of the limbs and their supporting girdles. Together, these divisions include more than 200 bones that protect internal organs, provide structural support, facilitate movement, and serve as sites for muscle attachment. Understanding the names, locations, and functions of these bones provides a foundation for studying joints, muscles, and movement in subsequent lessons.
Multilingual Approach Lesson Slides with translations
Use the interactive below to access the multilingual lesson slides.
Lesson Video
Watch this lesson video to hear an A&P instructor walk you through the lesson.
Practice Questions
Use these practice questions to assess your knowledge before you move on to the next section. Click the square icon in the top right corner of the image to enlarge it.
Click on the flashcard links below to practice identifying bones from 3-D images (links open in a new window):
