4.3 Bone Classification
The 206 bones that compose the adult skeleton are divided into five categories based on their shapes (Figure 4.16). Their shapes and their functions are related such that each categorical shape of bone has a distinct function.
Long Bones
A long bone is one that is cylindrical in shape, being longer than it is wide. Keep in mind, however, that the term describes the shape of a bone, not its size. Long bones function as levers; they move when muscles contract.
For example
- In the arms (humerus, ulna, radius) and legs (femur, tibia, fibula)
- In the fingers (metacarpals, phalanges) and toes (metatarsals, phalanges)
Short Bones
A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. Short bones provide stability and support as well as some limited motion.
For example
- The carpals of the wrists
- The tarsals of the ankles
Flat Bones
Although a flat bone is typically thin, it is also often curved. Flat bones serve as points of attachment for muscles and often protect internal organs.
For example
- The cranial (skull) bones
- The scapulae (shoulder blades)
- The sternum (breastbone)
- The ribs.
Irregular Bones
An irregular bone is one that does not have any easily characterised shape and therefore does not fit any other classification. These bones tend to have more complex shapes.
For example
- The vertebrae that support the spinal cord and protect it from compressive forces
- Many facial bones, particularly the ones containing sinuses, are classified as irregular bones
Sesamoid Bones
A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form in tendons (the sheaths of tissue that connect bones to muscles) where a great deal of pressure is generated in a joint. The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement across species.
For example
- They are typically found in tendons associated with the feet, hands, and knees
- The patella
Table 4.1. Bone Classification
| Bone classification | Features | Function(s) | Examples |
| Long | Cylinder-like shape, longer than it is wide | Leverage | Femur, tibia, fibula, metatarsals, humerus, ulna, radius, metacarpals, phalanges |
| Short | Cube-like shape, approximately equal in length, width, and thickness | Provides stability, support, while allowing for some motion | Carpals, tarsals |
| Flat | Thin and curved | Points of attachment for muscles; protectors of internal organs | Sternum, ribs, scapulae, cranial bones |
| Irregular | Complex shape | Protect internal organs | Vertebrae, facial bones |
| Sesamoid | Small and round; embedded in tendons | Protect tendons from compressive forces | Patellae |
Section Review
Bones can be classified according to their shapes. Long bones, such as the femur, are longer than they are wide. Short bones, such as the carpals, are approximately equal in length, width, and thickness. Flat bones are thin, but are often curved, such as the ribs. Irregular bones such as those of the face have no characteristic shape. Sesamoid bones, such as the patellae, are small and round and are in tendons.
Review Questions
Critical Thinking Questions
Click the drop down below to review the terms learned from this chapter.
