Skeletal System PDF

The Skeletal System **Introduction** The skeletal system is composed of bones and cartilage. It serves as the body\'s primary support framework and is designed to facilitate more effective movement by the attached muscles. At its most basic level, the skeleton provides structure to the body and en...

The Skeletal System **Introduction** The skeletal system is composed of bones and cartilage. It serves as the body\'s primary support framework and is designed to facilitate more effective movement by the attached muscles. At its most basic level, the skeleton provides structure to the body and enables movement.  The skeletal system includes over 200 bones, cartilage and ligaments.  The adult skeleton consists of 206 bones:  - 22 in the skull, - 64 in the shoulder girdle and upper limb, 33 in the vertebral column,  - 25 in the thoracic cage, and  - 62 in the pelvic girdle and lower limbs. **What is Bone?** Bone constitutes one-third of connective tissue. It is covered with calcium salts, which make up two-thirds. The inorganic calcium salts make it strong and rigid, allowing it to withstand compressive weight-bearing pressures and impact forces from jumping. Bone is a strong and resilient kind of connective tissue. - It comprises 25% water. - Organic components include osteoid (the carbon-containing element of the matrix) and bone cells (25%). - Inorganic components, predominantly calcium phosphate (50%). The organic connective tissue within it (collagen fibers) makes it robust and resilient (flexible), allowing it to withstand tensile stresses. **Classification of Bones** 1. **By the Shapes** 1. 2. **Short bones:**   3. 4. 5.   **B. Regional Classification** 1. **Axial Region** 2. **Appendicular Region** **Functions of bones ** Bones provide shape, support, and protection for the body. They store minerals and house bone marrow, where blood cells are formed and stored. Key functions include: - The bones protect the internal organs - It stores and releases fats and minerals - The bones produce the blood cells - It facilitates the movement of the body. - The bones provide structural support to the body **Axial Skeleton** The axial skeleton consists of 80 bones located in the body\'s central core. This includes the bones of the skull (cranial and facial bones), ears, neck, vertebral column (vertebrae, sacrum, and coccyx), and ribcage (sternum and ribs). The axial skeleton\'s primary function is to protect vital organs such as the brain, spinal cord, heart, lungs, and other essential organs. **Skull ** The skull sits atop the spine and has two main parts: the cranium and the facial bones. **Cranium** The cranium is composed of several flat and irregular bones that protect the brain. It has a base on which the brain rests and a vault that encloses it. The inner surface of the skull bones is lined with the outer layer of the dura mater, known as the periosteum. In a mature skull, the joints between these bones, called sutures, are immovable and fibrous. The bones contain numerous openings, such as foramina and fissures, allowing nerves, blood vessels, and lymph vessels to pass through. The bones of the cranium consist of:  - 1 frontal bone - 2 parietal bones - 2 temporal bones - 1 occipital bone - 1 sphenoid bone - 1 ethmoid bone. **Frontal bone** The frontal bone in the forehead forms part of the orbital cavities (eye sockets) and the ridges above the eyes. It contains two air-filled sinuses lined with mucous membranes connecting to the nasal cavity. The frontal bone is joined to the parietal bones by the coronal suture and connects to the sphenoid, zygomatic, lacrimal, nasal, and ethmoid bones. Initially, it starts as two parts joined by the frontal suture. The primary function of the frontal bone is to form the smooth curve of the forehead and protect the frontal lobe of the brain. **Parietal bone** These bones form the sides and roof of the skull. They connect at the sagittal suture, to the frontal bone at the coronal suture, to the occipital bone at the lambdoidal suture, and the temporal bones at the squamous sutures. The inner surface is concave, featuring grooves shaped by the brain and blood vessels. **Temporal Bones** The temporal bones form much of the lateral wall and base of the skull. They are bilateral, symmetrical, and irregular, with extensive muscular attachments and joints with the parietal, occipital, sphenoid, and zygomatic bones. Key features include: - **Squamous part:** Articulates with the parietal bone. - **Zygomatic process:** Forms the zygomatic arch. - **Mastoid part:** Contains air sinuses that communicate with the middle ear. - **Petrous portion:** Houses the organs of hearing and balance. The temporal bone connects to the mandible at the temporomandibular joint, the only movable joint in the skull, and has the external auditory meatus behind it. **Occipital bone** The occipital bone, an unpaired bone at the back of the head, forms part of the skull base and houses the cerebellum. It has immovable joints with the parietal, temporal, and sphenoid bones. Its deeply concave inner surface holds the occipital lobes and cerebellum. The occipital bone has two articular condyles that form hinge joints with the atlas, the first vertebra. Between the condyles is the foramen magnum, allowing the spinal cord to pass into the cranial cavity. **Sphenoid bone** The sphenoid bone, also called the \'wasp bone\' because of its shape, is one of the most intricate bones in the human body. It sits in the middle of the skull\'s base and connects with the occipital, temporal, parietal, and frontal bones. In the middle of this bone, there\'s a small, saddle-shaped dip called the hypophyseal fossa (sella turcica) where the pituitary gland rests. The sphenoid bone also has large air sinuses, which are spaces lined with mucous membranes, that connect to the nasal cavity. **Ethmoid Bone** The ethmoid bone is a single, spongy bone in the middle of the skull. It helps form parts of the orbital cavity, the nasal cavity, and the upper part of the nasal septum. It has two projections on each side called the upper and middle conchae, which stick into the nasal cavity. The bone has small air-filled spaces lined with tiny hairs that connect to the nasal cavity. The cribriform plate is a flat part with tiny holes that allow smell nerves to pass from the nose to the brain. There\'s also a thin, vertical piece that makes up the top part of the nasal septum. **Face** The facial skeleton, also known as the viscerocranium, supports the soft tissues of the face. It comprises 14 bones that fuse to form the eye sockets, nasal and oral cavities, and the sinuses. While the frontal bone is usually considered part of the skullcap, it is sometimes included in the facial skeleton. The facial skeleton includes: - **2 Zygomatic Bones:** Also known as the cheekbones. - **1 Maxilla**: Initially formed as two bones. - **2 Nasal Bones:** Small bones forming the bridge of the nose. - **2 Lacrimal Bones:** Tiny bones located in the inner corners of the eye sockets. - **1 Vomer:** A singular bone forming part of the nasal septum. - **2 Palatine Bones:** Located at the back of the oral cavity. - **2 Inferior Conchae:** Thin, scroll-like bones on the sides of the nasal cavity. - **1 Mandible:** The lower jawbone, which originally develops from two separate bones. **Zygomatic or Cheekbones** The zygomatic bones form the cheekbones of the face and connect with the frontal, sphenoid, temporal, and maxilla bones. They create the prominent part of the cheeks and contribute to the floor and lateral walls of the orbital cavities. **Maxilla or Upper Jaw** The maxilla, or upper jaw bone, forms part of the upper jaw and the hard palate. It originates as two bones that fuse before birth. The maxilla constitutes the upper jaw, the front portion of the roof of the mouth, the lateral walls of the nasal cavity, and part of the floor of the eye sockets. The alveolar ridge projects downward, holding the upper teeth. Each side of the maxilla contains a large air sinus, known as the maxillary sinus, lined with ciliated mucous membrane and opening into the nasal cavity. **Nasal bones** These are two small, flat bones that form most of the lateral and superior surfaces of the bridge of the nose. They are two slender bones located at the bridge of the nose. **Lacrimal bones** These two small bones are located posterior and lateral to the nasal bones, forming part of the medial walls of the orbital cavities. Each lacrimal bone has a foramen for the passage of the nasolacrimal duct, which carries tears from the medial canthus of the eye to the nasal cavity. They are the smallest bones of the face and form part of the medial wall of the orbit. **Vomer** The vomer is a thin, flat bone extending upward from the middle of the hard palate, forming the main part of the nasal septum. It articulates superiorly with the perpendicular plate of the ethmoid bone and forms the posterior aspect of the nasal septum. **Palatine bones** These two L-shaped bones have horizontal parts that unite to form the posterior part of the hard palate. The perpendicular parts project upwards to form part of the lateral walls of the nasal cavity. At their upper extremities, they also contribute to the formation of the orbital cavities. **Inferior conchae** Each inferior concha is a scroll-shaped bone that forms part of the lateral wall of the nasal cavity and projects into it below the middle concha. The superior and middle conchae are parts of the ethmoid bone. **Mandible** The mandible is the only movable bone of the skull, starting as two parts that fuse at the midline. Each half has a curved body with an alveolar ridge for the lower teeth and an upward-projecting ramus. The ramus splits into the condylar process, forming the temporomandibular joint with the temporal bone, and the coronoid process for muscle and ligament attachment. The junction where the ramus meets the body is the angle of the jaw. **Hyoid bone** The hyoid bone is a horseshoe-shaped bone located in the soft tissues of the neck, just above the larynx and below the mandible. It does not articulate with any other bone but is connected to the styloid process of the temporal bone by ligaments. It serves as an attachment point for the base of the tongue. **Sinuses** Air-filled sinuses are found in the sphenoid, ethmoid, maxillary, and frontal bones. These sinuses communicate with the nasal cavity and are lined with ciliated mucous membranes. Their functions include: - Giving resonance to the voice - Lightening the bones of the face and cranium makes it easier for the head to balance on top of the vertebral column. **https://teachmeanatomy.info/head/organs/the-nose/paranasal-sinuses/** **Fontanelles of the Skull** At birth, the cranial sutures are not yet fully ossified. Where three or more bones meet, there are membranous areas called fontanelles. The two largest fontanelles are the anterior fontanelle, which ossifies between 12 to 18 months of age, and the posterior fontanelle, which typically ossifies 2 to 3 months after birth. The skull bones remain unfused before birth to allow the baby\'s head to mold during passage through the birth canal. **Vertebral Column** The vertebral column is made up of 24 individual, movable bones, as well as the sacrum, which consists of five fused bones, and the coccyx, made of four fused bones. The 24 movable bones are divided into three sections: 7 cervical, 12 thoracic, and 5 lumbar vertebrae. Although these vertebrae share several common traits, each section has its unique features. **Cervical vertebrae** - The atlas (C1) is a ring of bone with short transverse processes. Its anterior large vertebral foramen houses the odontoid process of the axis, forming joints with the skull\'s occipital condyles for head nodding. - The axis (C2) has a small body and an upward odontoid process that articulates with the atlas, enabling head-turning movements. **Thoracic Vertebrae** Thoracic vertebrae feature bodies and transverse processes with facets for connecting with the ribs. **Lumbar Vertebrae** The lumbar spine comprises five vertebrae (L1 to L5), increasing in size downward. Linked by joints at the back, they facilitate spinal movements and protect the spinal cord and cauda equina within a bony canal, lacking distinctive features. **Sacrum** The sacrum is a triangular bone formed by the fusion of five vertebrae. It articulates with the 5th lumbar vertebra at its base, the ilium on each side to form the sacroiliac joint, and the coccyx at its tip. The promontory protrudes into the pelvic cavity, and the bone features vertebral foramina and nerve passages on each side. **Coccyx** This bone is formed by the fusion of the four terminal vertebrae into a small triangular shape, with its wide base connecting to the end of the sacrum. **Features of Vertebral Column** **Intervertebral Discs** The vertebral column includes intervertebral discs that separate vertebrae. These discs consist of an outer fibrocartilage ring (annulus fibrosus) surrounding a gel-like core (nucleus pulposus). They vary in thickness, thinnest in the cervical region and thicker in the lumbar region. Supported by the posterior longitudinal ligament, they absorb shock and enhance spinal flexibility through their cartilaginous joints. **Intervertebral foramina** Intervertebral foramina are visible between adjacent vertebrae when viewed from the side, formed by contributions from both vertebrae. These openings span the entire length of the vertebral column, allowing passage for spinal nerves, blood vessels, and lymph vessels. **Ligaments of the Vertebral column** Ligaments of the vertebral column support vertebrae and maintain intervertebral disc positioning. The transverse ligament stabilizes the odontoid process of the axis relative to the atlas. The anterior longitudinal ligament runs along the front of the vertebral bodies, while the posterior longitudinal ligament extends inside the vertebral canal along the backs of the vertebrae. The ligamentum flavum connects the laminae of neighboring vertebrae. The ligamentum nuchae and the supraspinous ligament join the spinous processes, spanning from the occiput to the sacrum. **Thoracic cage** The thoracic cage, or rib cage, forms the chest (thorax) region of the body. It consists of 12 pairs of ribs with associated costal cartilages and the sternum. Posteriorly, the ribs are connected to the 12 thoracic vertebrae (T1--T12). The primary function of the thoracic cage is to protect the heart and lungs. **Sternum or Breast bones** The sternum, or breastbone, is a flat bone that can be felt just beneath the skin in the center of the front of the chest. The manubrium, the uppermost section, articulates with the clavicles at the sternoclavicular joints and with the first two pairs of ribs. The body, or middle portion, provides attachment for the ribs. The xiphoid process, the tip of the bone, serves as an attachment point for the diaphragm, the muscles of the anterior abdominal wall, and the linea alba. **Ribs** The ribs form the main structure of the thoracic cage and protect the thoracic organs. The primary function is to support the respiration process. Twelve pairs of ribs make up the bony lateral walls of the thoracic cage and articulate posteriorly with the thoracic vertebrae. The first 10 pairs are attached anteriorly to the sternum by costal cartilages, either directly or indirectly. The last two pairs, known as floating ribs, do not have a connection in the anterior attachment.  **Appendicular Skeleton** The appendicular skeleton is the part of the bony skeleton that includes and supports the limbs. It includes the pectoral girdle and the pelvis, which is connected to the axial skeleton in the middle and is composed of 126 bones. Appendicular bones develop from the cartilage, through the process of endochondral ossification. The appendicular skeleton includes the shoulder girdle and upper limbs, as well as the pelvic girdle and lower limbs. **Shoulder girdle and Upper limb** The shoulder girdle is made up of: - 2 Scapulae bones - 2 Clavicle  bones The bones of each upper limb include: - 2 Humerus  bones - 2 Radius  bones - 2 Ulna bones - 8 Carpal bones - 5 Metacarpal bones - 14 Phalanges bones **Shoulder Girdle** The shoulder girdle, also known as the pectoral girdle, is a vital component of the appendicular skeleton, responsible for connecting the upper limbs to the torso. These bones work together to provide support, stability, and mobility to the arm. Interestingly, in some species with a three-bone shoulder structure, the pectoral girdle still includes the clavicle and scapula, while an additional bone is present. This anatomical design plays a key role in facilitating a wide range of movements, from lifting and rotating to pushing and pulling. In humans, it consists of two main bones: the clavicle (collarbone) and the scapula (shoulder blade). **Clavicle or Collarbone** The clavicle is a long, Sigmoid-shaped bone with a double curve that lies horizontally between the shoulder blade and the sternum. It connects the upper limb to the body\'s main skeleton. The clavicle joins the sternum at the sternoclavicular joint and the scapula at the acromioclavicular joint, serving as the only bony link between the upper limb and the axial skeleton. **Scapula or Shoulder blade** The scapulae are flat, triangular bones that provide attachment sites for various muscles, including the deltoids. Positioned on the back of the chest wall, they lie over the ribs and are separated from them by muscles. The muscles of the back and thorax link the scapulae to the thoracic cage. At the lateral angle, the glenoid cavity, together with the head of the humerus, forms the shoulder joint. The acromion, a spinous process on the posterior surface, extends over the shoulder joint and connects with the clavicle at the acromioclavicular joint. Additionally, the coracoid process, which projects from the upper border, serves as an attachment point for muscles that move the shoulder joint. **The Upper Limb** **The upper limb can be divided into four main groups: the shoulder girdle, arm, forearm, and hand. Unlike the lower limb, which is designed for weight-bearing and locomotion, the primary function of the upper limb is to control the position of the hand in space, enabling the manipulation of objects.** **Humerus** The humerus is a long bone in the upper limb, extending from the shoulder to the elbow.  Proximally, it forms the glenohumeral joint with the glenoid fossa of the scapula. Distally, it forms the elbow joint with the radius and ulna. The head of the humerus connects with the scapula, with the greater and lesser tubercles below it, and the bicipital groove in between, housing a biceps tendon. The distal end articulates with the radius and ulna to form the elbow joint. **Radius** The radius is one of the two bones in the forearm, alongside the ulna. It forms the radio-carpal joint at the wrist and the radio-ulnar joint at the elbow. Similar to the tibia in the lower limb, the radius is situated on the lateral side of the forearm in the anatomical position. The radius is smaller than the ulna and consists of an upper end, a lower end, and a shaft. The proximal radius consists of the radial head, neck, and tuberosity. The cylindrical radial head, covered with hyaline cartilage, articulates with the capitellum of the humerus at the elbow joint. **Ulna** The ulna (plural: ulnae) is one of the two long bones in the forearm, situated medially in the supinated anatomical position. It has a larger proximal end that tapers to a smaller distal end, opposite to the radius. Prominent features of the ulna include**:** - **Proximal:** olecranon, trochlear notch, coronoid process, radial notch (lateral), sublime tubercle (medial) - **Shaft:** ulnar tuberosity - **Distal:** head, styloid process, fovea, groove for extensor carpi ulnaris **Carpal or wrist bone** The carpal bones, also known as the carpus (plural: carpi), consist of eight wrist bones that connect the forearm to the hand. These eight bones are arranged in two rows of four: - **Proximal row:** scaphoid, lunate, triquetral, pisiform - **Distal row:** trapezium, trapezoid, capitate, hamate The carpal bones are tightly interlocked and held by ligaments, allowing limited movement. The proximal row is associated with the wrist joint, while the distal row forms joints with the metacarpal bones. Tendons of forearm muscles cross the wrist and are secured by strong fibrous bands called retinacula. **Metacarpal bones or the bones of the hand** Metacarpal bones, or the bones of the hand, form the palm. These five bones are numbered 1-5, from the thumb inward, in the radio-ulnar direction. The proximal ends (bases) articulate with the carpal bones, while the distal ends (heads) connect with the proximal phalanges. **Phalanges or Finger bones** Phalanges, or finger bones, are fourteen in each hand: three in each finger (proximal, middle, distal) and two in the thumb (proximal, distal). They articulate with the metacarpal bones and each other. Interconnected by interphalangeal joints, they are vascularized by the palmar digital arteries through the nutrient rami. **Pelvic Girdle and Lower Limb Bones** **Pelvic Girdle** The pelvic girdle, or hip girdle, is composed of a single hip bone (coxal bone) that serves as the attachment point for the lower limbs. Each hip bone connects securely to the axial skeleton at the sacrum and converges anteriorly to join with the other. The bony pelvis includes the two hip bones, the sacrum, and the coccyx below it. Unlike the mobile bones of the pectoral girdle, the pelvic bones are strongly united, creating a largely immobile, weight-bearing structure. This stability allows for the efficient transfer of body weight laterally from the vertebral column through the pelvis and into the lower limbs, providing a solid foundation for the upper body. - 2 innominate bones - 1 sacrum bone **Lower Limb** The lower limb is divided into three regions: the thigh, containing the femur; the leg, with the tibia and fibula; and the foot, comprising seven tarsal bones, five metatarsal bones, and 14 phalanges. These 30 bones support movement, weight-bearing, and balance - 1 femur bone - 1 tibia bone  - 1 fibula bone - 1 patella bone - 7 tarsal bones - 5 metatarsal bones - 14 phalanges bones ** Innominate or Hip Bones** Each hip bone is composed of three fused bones: the ilium, ischium, and pubis. The acetabulum, a deep depression on the outer surface, forms the hip joint with the femur\'s head. The ilium is the upper part of the hip bone, featuring the iliac crest and the anterior superior iliac spine. The pubis is the anterior part and connects with the opposite pubis at the symphysis pubis. The ischium forms the inferior and posterior parts of the hip bone. These three parts unite at the acetabulum. **Sacrum** The sacrum is an irregular bone formed by the fusion of five vertebrae located at the base of the spine. Female: shorter, wider, and more curved sacrum. Male: longer, narrower, and straighter sacrum with a more pronounced sacral promontory **Upper Limb** **Femur or Thigh Bone** The femur, the longest and strongest bone in the body, has an almost spherical head that articulates with the acetabulum of the hip bone, forming the hip joint. The head has a small depression for the attachment of the ligament of the head of the femur, which contains a blood vessel supplying the bone. The neck extends outward and slightly downward from the head to the shaft, mostly within the hip joint capsule. The lower third of the femur\'s posterior surface forms a flat triangular area called the popliteal surface. The distal end has two articular condyles that, with the tibia and patella, form the knee joint. **Tibia or Shin Bone** The tibia, positioned medially in the lower leg, features a broad and flat proximal end with two condyles that articulate with the femur, forming the knee joint. The head of the fibula connects with the underside of the lateral condyle, creating the proximal tibiofibular joint. At its distal end, the tibia contributes to the ankle joint by articulating with the talus and fibula. The medial malleolus, a prominent downward projection, is located on the medial side of the ankle joint. **Fibula** The fibula is the long, slender bone situated laterally in the leg. The head, or upper extremity, articulates with the lateral condyle of the tibia, forming the proximal tibiofibular joint. The distal extremity of the fibula articulates with the tibia and projects beyond it to form the lateral malleolus, contributing to the ankle joint. **Patella or Kneecap** The patella is the largest sesamoid bone, roughly triangular, associated with the knee joint. It develops within the tendon of the quadriceps femoris muscle. Its posterior surface articulates with the patellar surface of the femur, while its anterior surface is embedded in the patellar tendon. The center of ossification of the patella has a knotty or tuberculated outline and is composed mainly of dense cancellous tissue. **Tarsal or Ankle Bones** Seven tarsal bones make up the back part of the foot. They include: - 1 talus - 1 calcaneus - 1 navicular - 3 cuneiform bones - 1 cuboid The talus connects with the tibia and fibula to form the ankle joint. The calcaneus is the bone that forms the heel. The other tarsal bones connect and the metatarsal bones in the foot. **Metatarsal Bones of the Foot** The foot contains five metatarsal bones, arranged from the inner side (big toe) outward. These bones form the majority of the top part of the foot. At their proximal ends, they articulate with the tarsal bones, while at their distal ends, they connect with the phalanges. The enlarged distal head of the first metatarsal bone forms the \"ball\" of the foot. **Phalanges of the Toes** The toes contain 14 phalanges arranged similarly to the fingers. There are two phalanges in the great toe (the hallux) and three in each of the other toes. **Arches of the Foot** The bones of the foot are configured in a way that the foot remains a flexible structure, not rigid. This flexibility can be observed by comparing a normal foot with a \'flat\' foot. The bones create a bridge-like arrangement supported by muscles and ligaments, forming four arches: the medial and lateral longitudinal arches and two transverse arches. - ***Medial Longitudinal Arch*** - This is the highest and most prominent of the arches, formed by the calcaneus, talus, navicular, three cuneiforms, and the first three metatarsal bones.  - Only the calcaneus and the distal ends of the metatarsal bones should make contact with the ground. - ***Lateral Longitudinal Arch*** - This arch is less prominent compared to the medial arch. It consists of the calcaneus, cuboid, and the two lateral metatarsal bones. As with the medial arch, only the calcaneus and the metatarsal bones should touch the ground. - ***Transverse Arches*** - These arches span across the foot and are more visible in a skeletal model than in a live foot.  - They are most noticeable at the level of the three cuneiforms and the cuboid bones. **\ **