The General Structure of Joints. — The bones of the human body are held in movable, immovable, or mixed relations with each other, depending upon the degree of action required in various movements, functions, and positions assumed by the body. The parts entering into the formation of these relations taking place between bones comprise a joint or articulation— they are: bones, ligaments, cartilage, and a synovial membrane.
Bones. — The articular portions of bones are enlarged to form a joint of suitable size, so that muscles passing over the joint can act at a greater angle. The layer of bone beneath the cartilage entering into a joint is a compact articular lamella.
Cartilage. — There are three varieties of cartilage—hyaline, fibrocartilage, and yellow elastic (see page 51 for description of cartilage). The fibrocartilage and hyalin are utilized in the structure of a joint; the former where slight movement and great strength are required—as between vertebrae and the pubic bones of the pelvis; the latter where freedom of movement is essential, and a greater surface for general convenience of mutual connection is demanded, as in the shoulder- and hip-joints, etc.
Ligaments. — The ligaments connecting the immovable joints, such as the bones of the skull, consist of a thin layer of fibrous membrane—called sutural ligaments, and have a layer of cartilage interposed—as the bones of the base of the skull. Some ligaments contain yellow elastic tissue, which is present in the ligamentum subflava—connecting the adjacent arches of the vertebrae in man.
The Synovial Membrane. — This is a thin, delicate serous membrane arranged like a short white tube, attached by its open ends to the margins of the articulatory cartilages and covering the inner surfaces of the various ligaments, so that along with the cartilage it completely encloses the joint cavity. It secretes a viscous, thick fluid like the white of an egg—hence its term synovia—which acts as a lubricant to the joint. The membrane is composed of a layer of endothelial cells resting upon a thin layer of fibro-elastic (sub-endothelial) tissue. There are three varieties of synovial membranes—articular, bursal, and vaginal.
The articular is found in a freely movable joint. It lines the capsule and the non-articular intracapsular portion of the bones which enter into the formation of a joint. Some of these membranes contain fat, acting as a cushion between the articulatory surfaces. The bursse are mucous—between the skin and bone (subcutaneous synovial bursse) ; and synovial—between muscles, tendons, and bone (subtendinous synovial bursse). Vaginal synovial membranes are sheaths for tendons. They prevent friction and with their secretion lubricate the tendons as they move within the sheath in carrying on their action. Some joints have tendons passing through their cavities (the knee-joint has the popliteus muscle and the shoulder-joint the biceps) and they are always enclosed by the synovial membrane within the joint, to prevent friction and facilitate action.
The Classification of Joints. — All joints of the body are classified under three main groups—immovable articulation (synarthrosis), slightly movable or mixed articulation (amphiarthros), movable articulation (diarthrosis).
Synarthrosis or Immovable Joint. — Under this classification are included all the articulations in which the surfaces of the bones are in almost direct contact, being fastened together by an intervening mass of connective tissue, and in which there is no joint cavity and scarcely any motion. Examples: joints between the bones of the skull and face, excepting those of the mandible.
Amphiarthros or Mixed Joint. — In this variety there is only a slight amount of motion. There are two varieties—symphysis, as the symphysis pubes and bodies of vertebrae in which the articulating osseous surfaces are connected by a broad flattened disk of fibrocartilage which is firmly attached to both bases in the articulation. Syndesmosis, in this variety there is slight motion and the bony surfaces are held in relation by an interosseous ligament. Example: inferior tibiofibular articulation (between tibia and fibula).
Diarthrosis or Movable Joint. — Under this variety are included the greater number of the joints of the body, characterized by their freedom of movement. These joints are formed by the bringing together of the articular surfaces of two bones, covered by hyaline cartilage, and held together by ligaments with a lining synovial membrane.
The Kinds of Movement Admitted in Joints. — These are divided into gliding, angular, circumduction, and rotation. They are often combined, and it is seldom that one distinct kind of motion is seen in any certain joint.
Gliding Movement. — This is common to all movable joints, but in the articulations of the wrist and foot it is the only motion permitted. It consists of one surface of a bone gliding over an adjacent bone without any angular or rotatory movement. The sliding of a bone over a wide range of surface, as is seen in the patella (knee-cap) over the condyles of the femur, is called coaptation.
Angular Movement. — This is seen only in the joints of long bones, whereby the angle between the two bones is either increased or diminished. It is expressed in four ways, as follows: bending or flexion—to bend the arm or leg forward or backward, etc.; straightening or extension—to straighten the legs and thighs as in standing, the arms, fingers, etc.; adduction—to move a limb toward the middle line of the body or extremity; abduction—to move it away from the middle line of the body or extremity. When speaking of adduction or abduction of the fingers or toes, the second finger of the hand and second toe are taken as the middle line and not the middle of the body.
Circumduction. — This is the limited degree of motion which takes place between the head of a bone and its articular cavity, when the extremity is swung in such a manner that the sides and extremities of the limb circumscribe a conical space around an imaginary axis, the base of the cone corresponding to the lower extremity of the limb and the apex to the articular cavity. This kind of movement is best seen in the shoulder- and hip-joints.
Rotation. — This is the movement of a bone upon an axis, which is the axis of the pivot on which the bone turns. This form is seen best in the rotation between the atlas and axis when the odontoid process of the axis serves as a pivot around which the atlas turns.
The Apposition of Joint Surfaces. — This is accomplished by (1) atmospheric pressure—as in the hip-joint; (2) synovial fluid; (3) ligaments to a small extent; (4) muscles to the greatest extent. A short muscle may act on more than one joint—the gluteus maximus extends the hip and also the knee through its insertion into the fascia lata.
Note. — The various articulations of the body have not been described, as they are beyond the scope of this book. However, a general understanding of their attachment, position, and the bones they hold together can be obtained from the plates.
Key Takeaways
- Joints are composed of bones, ligaments, cartilage, and synovial membranes.
- There are three main types of joints: immovable (synarthrosis), slightly movable or mixed (amphiarthros), and movable (diarthrosis).
- Synovial fluid lubricates the joint to reduce friction.
Practical Tips
- Understand the different types of cartilage in joints, as they determine the type of movement allowed.
- Recognize that synovial membranes play a crucial role in maintaining the health and function of joints by secreting lubricating fluid.
- Be aware of the importance of ligaments in stabilizing joints to prevent dislocation.
Warnings & Risks
- Ignoring joint injuries can lead to long-term damage, so proper care is essential.
- Overuse or improper use of certain joints (like the knee) can result in chronic pain and reduced mobility.
- Improper alignment during movement can cause excessive wear on cartilage and ligaments.
Modern Application
While this historical text provides a foundational understanding of joint anatomy, modern techniques have improved our ability to diagnose and treat joint issues. However, the principles outlined here—such as recognizing different types of joints and their functions—are still crucial for survival preparedness in situations where medical care may be limited.
Frequently Asked Questions
Q: What are the three main types of joints mentioned in this chapter?
The three main types of joints are immovable (synarthrosis), slightly movable or mixed (amphiarthros), and movable (diarthrosis).
Q: How does synovial fluid function within a joint?
Synovial fluid acts as a lubricant to the joint, reducing friction between the articular surfaces of bones. It is secreted by the synovial membrane and helps maintain the health and function of the joint.
Q: What are some examples of immovable joints?
Examples of immovable joints include those found between the bones of the skull, except for the mandible. These joints have almost direct contact between their surfaces and no joint cavity or motion.