The framework of the body is built upon a series of bones, supplemented in certain regions by cartilage; the bony part of the framework constitutes the skeleton.
The skeletal system serves several functions, among them are:
1. Protection and support:
The ribs protect the organs of the thorax and the skull protects the brain. The legs support the weight of the entire body. The vertebrae also support the upper body.
Most skeletal muscles attach to the bones of the skeletal system and use them as leverage points for movement of the body.
3. Production of blood cells:
The bone marrow produces blood cells in a process known as hematopoiesis.
"Yellow bone marrow" or adipose tissue stores fat in the medullary cavity of long bones. Bones can also be broken down to release inorganic calcium and phosphorus stored in the non cellular matrix of the bone.
In the skeleton of the adult there are generally 206 distinct bones:
a. Axial Skeleton:
- Vertebral column: 26
- Skull: 22
- Hyoid bone: 1
- Ribs and sternum: 25
- Auditory ossicles: 6
b. Appendicular Skeleton:
- Upper extremities: 64
- Lower extremities: 62
Parts of the human body:
- Chest (Thorax)
- Stomach (Abdomen)
- Hip (Pelvis)
4. Upper Extremities:
- Arm and Forearm
5. Lower Extremities:
Types of Bones
Bones are divisible into four classes: Long, Short, Flat, and Irregular.
1. Long Bones are found in the limbs and function as levers, they are longer than they are wide.
2. Short Bones transfer forces of movement and are cube shaped as in the carpus and tarsus.
3. Flat Bones are used for either extensive protection or the provision of broad surfaces for muscular attachment. The bones expand into broad, flat plates, as in the skull and the scapula.
4. Irregular Bones have peculiar forms, cannot be grouped under the preceding heads, and are used for muscle attachement and articulation. Some irregular bones include the vertebræ, sacrum and coccyx.
Long bones consists of a body or shaft and two extremities. The body, or diaphysis is cylindrical, with a central cavity termed the medullary cavity. The wall consists of dense, compact tissue of considerable thickness in the middle part of the body, but becoming thinner toward the extremities. Within the medullary cavity is adipose tissue or "yellow bone marrow".
The extremities are refered to as the epiphysis. Within the epiphysis is the "spongy bone" also known as "red bone marrow". It is within this marrow that red blood cells are produced at an average rate of 2.5 million per second. Running horizontally across the spongy bone of this region is the Epiphyseal line which is a region of cell growth responsible for lateral bone growth during youth, when growth is complete this line calcifies and becomes known as the epiphyseal plate.
Nutrient foramen run through the compact bone and allow the passage of nutrients in and out of the bone. There is a thin outer layer of connective tissue called the PERIOSTEUM which is highly vascular and allows for muscle and tendon attachment, it is bound to the bone itself by PERFORATING FIBERS which are composed of collagen.
This layer does not cover the articulating regions of the bone. The bones belonging to this class include: the clavicle, humerus, radius, ulna, femur, tibia, fibula, metacarpals, metatarsals, and phalanges.
Short bones are generally equal in length, width, and thickness. They are found in the wrists and ankles. Aside from points of insertion and vascular areas, short bones are almost completely covered by articular surfaces.
These bones are composed of two thin layers of compact tissue enclosing between them a variable quantity of spongy bone. They generally offer protection, as is the case with the bones of the cranium and with the ribs and sternum. The flat bones are: the occipital, parietal, frontal, nasal, lacrimal, vomer, scapula, os coxæ (hip bone), sternum, ribs, and, according to some, the patella.
The irregular bones are such as, from their peculiar form, cannot be grouped under the preceding heads. They consist of cancellous tissue enclosed within a thin layer of compact bone. The irregular bones are: the vertebræ, sacrum, coccyx, temporal, sphenoid, ethmoid, zygomatic, maxilla, mandible, palatine, inferior nasal concha, and hyoid.
Surfaces of Bones
If the surface of a bone be examined, certain eminences and depressions are seen. These eminences and depressions are of two kinds: articular and non-articular.
Well-marked examples of articular eminences are found in the heads of the humerus and femur; and of articular depressions in the glenoid cavity of the scapula, and the acetabulum of the hip bone.
Non-articular eminences are designated according to their form. Thus, a broad, rough, uneven elevation is called a tuberosity, protuberance, or process, a small, rough prominence, a tubercle; a sharp, slender pointed eminence, a spine; a narrow, rough elevation, running some way along the surface, a ridge, crest, or line. Non-articular depressions are also of variable form, and are described as fossæ, pits, depressions, grooves, furrows, fissures, notches, etc. These non-articular eminences and depressions serve to increase the extent of surface for the attachment of ligaments and muscles, and are usually well-marked in proportion to the muscularity of the subject. A short perforation is called a foramen, a longer passage a canal.
Structural classification of Joints
(according to how the bones are connected to each other)
There are three structural classifications of joints:
1. Fibrous joint - joined by fibrous connective tissue
2. Cartilaginous joint - joined by cartilage
3. Synovial joint - not directly joined
- Fibrous joints are connected by dense connective tissue, consisting mainly of collagen.
- Types of Fibrous joints: Sutures, Syndesmosis, and Gomphosis.
These joints are also called "fixed" or "immoveable" joints, because they do not move. These joints have no joint cavity and are connected via fibrous connective tissue. The skull bones are connected by fibrous joints.
a. Sutures are found between bones of the skull. In fetal skulls the sutures are wide to allow slight movement during birth. They later become rigid (synarthrodial).
b. Syndesmosis are found between long bones of the body, such as the radius and ulna in forearm and the fibula and tibia in leg. Unlike other fibrous joints, syndesmoses are moveable (amphiarthrodial), albeit not to such degree as synovial joints.
c.Gomphosis is a joint between the root of a tooth and the sockets in the maxilla or mandible.
- Cartilaginous joints are connected entirely by cartilage (fibrocartilage or hyaline). Cartilaginous joints allow more movement between bones than a fibrous joint but less than the highly mobile synovial joint.
- An example would be the joint between the manubrium and the sternum. Cartilaginous joints also forms the growth regions of immature long bones and the intervertebral discs of the spinal column.
- Cartilaginous joints can be divided into 2 types: Primary cartilaginous joints and Secondary cartilaginous joints.
a. Primary cartilaginous joints
Known as "synchondroses". Bones are connected by hyaline cartilage or fibrocartilage, sometimes occurring between ossification centers. This cartilage may ossify with age.
Examples in humans are the "growth plates" between ossification centers in long bones. These joints here allow for only a little movement, such as in the spine or ribs.
b. Secondary cartilaginous joints
Known as "symphyses". Fibrocartilaginous joints, usually occurring in the midline.
Examples in human anatomy would be the manubriosternal joint (between the manubrium and the sternum), intervertebral discs, and the pubic symphysis.
Articulating bones at a symphysis are covered with hyaline cartilage and have a thick, fairly compressible pad of fibrocartilage between them.
- A Synovial joint, also known as a diarthrosis, is the most common and most movable type of joint in the body of a mammal. As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones.
- Structural and functional differences distinguish synovial joints from cartilaginous joints (synchondroses and symphyses) and fibrous joints (sutures, gomphoses, and syndesmoses). The main structural differences between synovial and fibrous joints is the existence of capsules surrounding the articulating surfaces of a synovial joint and the presence of lubricating synovial fluid within that capsule (synovial cavity).
- There are seven types of synovial joints. Some are relatively immobile, but are more stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury. In ascending order of mobility, they are:
a. Gliding joints (or planar joints): These joints allow only gliding or sliding movements. Example: The carpals of the wrist, acromioclavicular joint
b. Hinge joints: These joints act like a door hinge, allowing flexion and extension in just one plane. Example: The elbow (between the humerus and the ulna)
c. Pivot joints: One bone rotates about another. Example: Atlanto-axial joint, proximal radioulnar joint, and distal radioulnar joint
d. Condyloid joints (or ellipsoidal joints): A condyloid joint is where two bones fit together with an odd shape (e.g. an ellipse), and one bone is concave, the other convex. Some classifications make a distinction between condyloid and ellipsoid joints. Example: The wrist joint (radiocarpal joint)
e. Saddle joints: Saddle joints, which resemble a saddle, permit the same movements as the condyloid joints. Example: Carpometacarpal or Trapeziometacarpal Joint of thumb (between the metacarpal and carpal - the trapezium ) , sternoclavicular joint
f. Ball and socket joints: These allow a wide range of movement. Example: The shoulder(glenohumeral), and hip joints
g. Compound joints: Condylar joint(condyles of femur join with condyles of tibia) and Saddle joint(lower end of femur joins with patela). Example: The knee joint