Lecture 1: Functions & Macroscopic Anatomy of Bone
The human skeleton is not merely a dry, inert scaffold; it is a dynamic, living organ system. It provides structural support, facilitates movement via leverage, protects vital internal organs, acts as a massive mineral reservoir, and serves as the exclusive site for hematopoiesis (blood cell formation).
Bone is a specialized form of dense connective tissue with a mineralized matrix, providing both rigidity and metabolic flexibility.
Figure 1.1: Histological classification of connective tissues, including bone.
1.1 Division of the Skeleton
The adult human skeleton consists of 206 named bones, structurally divided into two major groups based on their location and primary functional role.
Forms the long central axis of the body. Its primary function is the rigid protection, support, and carrying of other body parts.
- The Skull (Cranium & Facial bones)
- The Vertebral Column
- The Bony Thorax (Ribs & Sternum)
Consists of the bones of the upper and lower limbs and the girdles that attach them to the axial skeleton. Its primary function is locomotion and environmental manipulation.
- Upper & Lower Limbs
- Pectoral (Shoulder) Girdle
- Pelvic (Hip) Girdle
1.2 Classification of Bones by Shape
Bones are classified into four distinct categories based on their gross morphological shape, which intimately dictates their structural function.
Considerably longer than they are wide. Composed of a central shaft (diaphysis) and two distinct, expanded ends (epiphyses). Almost all bones of the limbs (e.g., Femur, Humerus, Phalanges) fall into this category.
Roughly cube-shaped. They contain mostly spongy bone covered by a thin layer of compact bone. Examples include the carpals (wrist) and tarsals (ankle). A special subtype, Sesamoid bones (e.g., the patella), form entirely within tendons.
Thin, flattened, and usually slightly curved. Composed of two parallel layers of compact bone sandwiching a layer of spongy bone (diploe). Examples include the Sternum, Scapulae, Ribs, and most cranial bones.
Bones that exhibit complex, convoluted shapes that do not fit the other categories. Consist of spongy bone enclosed by thin layers of compact bone. Classic examples are the Vertebrae and the Coxal (hip) bones.
1.3 Macroscopic Structure of a Typical Long Bone
Figure 1.1: Detailed macroscopic structure of a long bone, highlighting the proximal epiphysis and articular surfaces.
Forms the long axis of the bone. It is constructed of a thick collar of highly dense Compact Bone that completely surrounds a central cavity. In adults, this central cavity is called the Medullary Cavity and contains fat (Yellow Marrow).
The expanded ends of the bone. The exterior is composed of a thin layer of compact bone, while the interior houses a honeycomb-like network of Spongy (Cancellous) Bone. The joint surfaces of epiphyses are covered with friction-reducing Articular (Hyaline) Cartilage.
The Epiphyseal Line separates the diaphysis and epiphysis. It is a remnant of the Epiphyseal Plate, the disc of hyaline cartilage that actively grows during childhood to lengthen the bone.
1.4 Bone Membranes: Periosteum & Endosteum
A glistening white, double-layered membrane that covers the entire external surface of the bone (except joint surfaces). The outer fibrous layer is dense irregular connective tissue. The inner osteogenic layer contains stem cells (osteoprogenitors), osteoblasts, and osteoclasts. It is richly supplied with nerve fibers and massive blood vessels, which enter the diaphysis via nutrient foramina. It is anchored to the bone by strong collagen tufts called Sharpey's fibers.
A delicate connective tissue membrane covering all internal bone surfaces. It covers the trabeculae of spongy bone and lines the canals passing through compact bone. Like the inner periosteum, it contains osteogenic stem cells.