The eye is a complex sensory organ that allows us to see the world around us. The eye works by gathering light from the environment and converting it into electrical signals that can be interpreted by the brain. The basic structure of the eye includes the cornea, iris, pupil, lens, retina, and optic nerve. The cornea is the transparent outer layer of the eye that helps to focus incoming light. The iris is the colored part of the eye that regulates the amount of light that enters the eye through the pupil, which is the black circular opening in the center of the iris. The lens is a clear, flexible structure located behind the iris that helps to focus light onto the retina. The retina is a thin layer of tissue that lines the back of the eye and contains photoreceptor cells called rods and cones. These cells convert light into electrical signals that are sent to the brain via the optic nerve. The rods are responsible for the low light vision and detecting motion, while the cones are resp
Skeletal system
The adult human skeleton is composed of 206 bones and their associated cartilage. The bones are supported by ligaments, tendons, bursae, and muscles. The bones of the body are grouped into two distinct divisions:
Axial skeleton: includes the bones along the long axis of the body. The axial skeleton consists of the vertebral column, bones of the head and bones of the thoracic cage.
Appendicular skeleton: that involves the bones of the shoulder and pelvic girdle, as well as the bones of the upper and lower extremities.
Bones:
Bones are rigid structures made of calcified dense connective tissue. Bone tissue is composed of a mineralized bone matrix that consists of type 1 collagen fibers dispersed throughout the ground substance. The cellular component of the bones is represented by three types of specialized bone cells called osteocytes, osteoblasts, and osteoclasts.
The bones consist of two distinct layers that differ in histological appearance and characteristics;
* Compact (cortical) bone is the outer much denser layer of the bone which gives it its smooth, white, and solid appearance. The outer surface of the compact bone is covered with a layer of dense connective tissue called the periosteum. On its inner surface, the compact bone is covered with endosteum, which is the boundary between the compact and spongy bones.
* Spongy (cancellous) bone is the deep airy layer of the bone. Unlike compact bone, spongy bone is highly vascularized and more metabolically active. It is typically found within the ends of long bones and in the vertebrae. In certain bones, like the hip bone, sternum, or femur, the central part of the spongy bone houses the bone marrow, which is the site of hematopoiesis in the adult.
Types of bones
- Bones can be classified according to their shapes as follows:
* Long bones have a tubular shape, with a longer longitudinal and a shorter transverse diameter. They are composed mostly of compact bone, while the spongy bone and bony marrow fill the ends of the bones. Examples of long bones include the humerus, ulna, tibia, and clavicle.
* Short bones have a roughly cuboid or round shape, and only contain a thin layer of compact bone surrounding the spongy bone. Examples include the tarsal and carpal bones.
* Flat bones are mostly thin, flattened, and usually curved. They contain two parallel layers of compact bones surrounding a layer of spongy bone. Examples include most of the skull bones, scapula, sternum, and sacrum.
* Sesamoid bones are small, rounded unique types of bones that are embedded in muscle tendons where the tendon passes over a joint. The largest sesamoid bone in the body is the patella, but several other smaller sesamoid bones can be found in the hand and foot, usually in close proximity to the joints.
* Irregular bones do not fit into any of the other categories. Generally, irregular bones contain foramina through which soft tissue and neurovascular structures pass. Examples include the vertebrae, hip bone, and some bones of the skull.
* Sulcus – a shallow groove on the bone surface (e.g. radial sulcus of the humerus)
* Condyle – rounded articular area (e.g. lateral condyle of the tibia)
* Epicondyle – eminence superior to a condyle (medial epicondyle of the femur)
* Crest – a ridge of bone (e.g. iliac crest)
* Facet – smooth, flat area, usually covered with cartilage (e.g. articular facet on vertebrae)
* Foramen – passage through a bone (e.g. foramen magnum on the occipital bone)
Cartilage:
=> Cartilage is a flexible connective tissue found in multiple organ systems of the body. Cartilage is composed of specialized cells called chondrocytes, collagen fibers, and abundant ground substance rich in proteoglycan and elastin fibers.
Cartilage is classified into the following types based on its composition:
* Hyaline cartilage is composed of type II collagen and an abundance of ground substance, which gives it a glossy appearance. It is the most abundant type of cartilage found in joints (articular cartilage), as well as the nose, larynx, trachea, and ribs.
* Elastic cartilage is similar to hyaline cartilage but contains more elastic fibers. It is found in structures such as the pinna of the ear, auditory tube, and epiglottis.
* Fibrocartilage is composed of plenty of collagen fibers type I and a smaller amount of ground substance. Examples of fibrocartilage include intervertebral discs, pubic, and other symphyses.
The musculoskeletal system specifically contains articular cartilage, a type of cartilage that lines the articulating surfaces of bones. The articular cartilage provides congruence to the articulating bones and allows them to bear weight and glide over each other with very little friction.
Joints
Each bone of the musculoskeletal system is connected to one or more bones via a joint. Joints provide a fulcrum to the bones, on which they pivot and thereby allow movements of body parts. However, movement is not a necessary attribute of a joint as some joints do not move, such as joints between the bones of the skull. The integrity or stability of a joint is provided by several factors including the bony congruence and structures that cross the joint, such as tendons and ligaments.
Based on the type of tissue that holds the neighboring bones together and the range of motion they exhibit, joints can be classified into the following:
*Synovial joints are freely mobile joints in which the bones are not in direct contact but are separated by a potential space called the synovial cavity. The synovial cavity is lined by a synovial membrane that secretes the synovial fluid which nourishes and lubricates the articulating surfaces in order to reduce friction. The articulating bones in most synovial joints are lined with hyaline cartilage. These joints usually have a wide range of motion, which is defined by the joint capsule, the supporting ligaments, and the muscles that cross the joint. Examples of synovial joints include the knee, shoulder, sternoclavicular, and elbow joints.
*Fibrous joints are the articulations in which the bones are connected by dense fibrous connective tissue. The bones in fibrous joints are firmly held together so that the joint allows negligible movement. Fibrous joints are found between the cranial sutures, the distal tibiofibular, and the cuboid navicular joints.
*Cartilaginous joints are articulations in which the bones are connected by cartilage. The bones have a range of motion between synovial and fibrous joints. Cartilaginous joints are subdivided into synchondrosis (e.g. costochondral joints) and symphysis joints (e.g. pubic symphysis).
=> According to the movements they allow, the synovial joints are further subdivided into:
* Ball and socket joints (e.g. hip joint)
* Condylar joints (e.g. knee joint)
* Hinge joints (e.g. elbow joint)
* Pivot joints (e.g. proximal and distal radioulnar joints)
* Ellipsoid joints (e.g. 2nd – 5th metacarpophalangeal joints)
* Plane joints (e.g. joints between the carpal bones)
Ligaments
- Ligaments are fibrous bands made of dense regular connective tissue which are similar in structure to tendons. Unlike the tendons that connect muscles to bone, the ligaments connect bone to bone. Besides the musculoskeletal system, the ligaments are also found in many other parts of the body, where they usually stabilize and hold internal organs in place and transmit neurovascular structures.
- In the musculoskeletal system, ligaments stabilize the articulating bones and reinforce the joints. Depending on their anatomic position relative to the joint capsule, ligaments are classified into:
* Capsular ligaments are essentially thickenings of the joint capsule that form either elongated bands or triangular structures. These ligaments serve to reinforce the integrity of the joint capsule. An example of the capsular ligament is the iliofemoral ligament of the hip joint.
* Intracapsular ligaments are the ligaments that lie internal to the joint capsule. These ligaments reinforce the connection of the articulating surfaces of the joint, but allow a far wider range of motion than other ligaments. Examples include the anterior and posterior cruciate ligament of the knee joint.
* Extracapsular ligaments are ligaments that lie outside the joint capsule. These ligaments provide the most stability to the articulating bones and are important for preventing dislocations. Extracapsular ligaments can lie in close proximity (e.g. medial collateral ligament of the ankle joint) or a bit further from the joint capsule (vertebral ligaments).
Bursae
Bursae are small sac-like outpouchings of the joint cavity lined by synovial membrane. They are found around the joints, providing cushioning of the associated bones, tendons, and muscles and reducing friction between adjacent structures.
The majority of synovial bursae are located near the large joints of the arms and legs. For example, one of the bursae of the knee joint is the suprapatellar bursa, found superior to the patella, between the femur and the tendon of the quadriceps femoris muscle. The suprapatellar bursa allows for these structures to slide over each other without friction during flexion and extension of the knee joint.
Functions of the skeletal system:
The skeletal system serves a variety of functions. The bones give the shape to the body and provide the site of attachment to muscles, tendons, ligaments and cartilage. These tissues function together as a whole to generate a force that provides the biomechanical basis of movement.
Due to its structural integrity, the skeletal system protects the internal organs, most importantly the brain, which is surrounded by the skull, as well as the heart and lungs, which are protected by the rib cage.
Moreover, the skeletal system serves several metabolic functions. The bones are the storage site of important minerals, most notably calcium and phosphorus. This makes the bones essential for balancing calcium levels in the blood, which is regulated by adjusting the rate of bone resorption.
Lastly, the bone marrow found in spongy bone is the site of hematopoiesis, which is a process of production of new blood cells. Cells that are produced in the bone marrow are red blood cells, platelets, and white blood cells, such as monocytes, granulocytes, and lymphocytes.
Comments
Post a Comment