Gray's Anatomy

1. The Skeletal System: Foundation of the Human Form

In the construction of the human body, it would appear essential, in the first place, to provide some dense and solid texture capable of forming a framework for the support and attachment of the softer parts of the frame, and of forming cavities for the protection of the more important vital organs; and such a structure we find provided in the various bones, which form what is called the Skeleton.

Framework and Protection. The skeletal system, composed of bones, provides the body's structural framework, enabling movement and protecting vital organs. Bones are classified into long, short, flat, and irregular types, each suited to specific functions. Long bones, like those in the limbs, act as levers for locomotion. Flat bones, such as those in the skull, offer broad protection. Short bones, found in the wrists and ankles, provide stability and limited motion. Irregular bones, like the vertebrae, have complex shapes that serve multiple purposes.

Bone Composition and Development. Bone consists of organic (animal) and inorganic (earthy) materials, providing both flexibility and strength. The organic component, primarily collagen, gives bone its elasticity, while the inorganic component, mainly calcium phosphate, provides hardness and rigidity. Bone development, or ossification, begins early in embryonic life, with cartilage gradually replaced by bone tissue. This process continues throughout childhood and adolescence, with bones growing in length and thickness until adulthood.

Microscopic Structure and Vessels. Microscopic examination reveals a complex structure of Haversian canals, lamellae, lacunae, and canaliculi. Haversian canals contain blood vessels that nourish the bone tissue. The periosteum, a fibrous membrane covering the bone surface, is essential for bone growth and repair. Marrow, found within the medullary canal of long bones and the spaces of cancellous bone, plays a crucial role in blood cell production.

2. Vertebral Column: The Body's Central Support

The Spine is a flexuous column, formed of a series of bones called Vertebræ.

Structure and Function. The vertebral column, or spine, is a flexible column of 33 vertebrae, providing central support and protecting the spinal cord. The vertebrae are divided into cervical, thoracic, lumbar, sacral, and coccygeal regions, each with distinct characteristics. The spine's curves enhance its strength and flexibility, allowing for a wide range of motion while maintaining balance.

Vertebral Anatomy. Each vertebra consists of a body, arch, pedicles, laminae, and processes (spinous, transverse, and articular). The body supports weight, while the arch protects the spinal cord. The processes serve as attachment points for muscles and ligaments. Intervertebral discs, composed of fibrocartilage, cushion the vertebrae and allow for movement.

Regional Variations and Development. Cervical vertebrae are characterized by their small size, bifid spinous processes, and transverse foramina. Thoracic vertebrae have facets for rib articulation. Lumbar vertebrae are the largest and strongest, adapted for weight-bearing. The sacrum and coccyx are fused vertebrae, providing stability to the pelvis. Vertebrae develop from multiple ossification centers, with fusion occurring throughout childhood and adolescence.

3. The Skull: Protecting the Seat of Consciousness

The Skull, or superior expansion of the vertebral column, is composed of four vertebræ, the elementary parts of which are specially modified in form and size, and almost immoveably connected, for the reception of the brain, and special organs of the senses.

Cranial and Facial Bones. The skull, composed of cranial and facial bones, protects the brain and houses sensory organs. The cranium consists of the occipital, parietal, frontal, temporal, sphenoid, and ethmoid bones. The face is formed by the nasal, maxillary, lacrimal, zygomatic, palatine, inferior turbinate, and mandible bones. Sutures, or immovable joints, connect the cranial bones, providing stability and protection.

Key Cranial Bones. The occipital bone forms the posterior and inferior part of the cranium, featuring the foramen magnum for spinal cord passage. The parietal bones form the sides and roof of the cranium. The frontal bone forms the forehead and upper part of the orbits. The temporal bones house the inner ear and contribute to the lateral skull. The sphenoid bone, a complex structure, articulates with all other cranial bones. The ethmoid bone forms part of the nasal cavity and orbits.

Facial Bone Structure. The facial bones provide structure for the face and support sensory organs. The maxillae form the upper jaw, while the mandible forms the lower jaw. The nasal bones form the bridge of the nose. The zygomatic bones create the cheekbones. The lacrimal bones house the tear ducts. The palatine bones form the posterior part of the hard palate. The inferior turbinates are located within the nasal cavity. The vomer forms part of the nasal septum.

4. Thorax: Shielding Vital Organs

The Thorax, or chest, is an osseo-cartilaginous cage, intended to contain and protect the principal organs of respiration and circulation.

Ribs, Sternum, and Vertebrae. The thorax, or chest, is a bony cage protecting vital organs, composed of the sternum, ribs, costal cartilages, and thoracic vertebrae. The ribs, 12 pairs in total, articulate with the vertebrae posteriorly and connect to the sternum anteriorly via costal cartilages. The sternum, located in the midline of the anterior chest, consists of the manubrium, body, and xiphoid process.

Rib Classification and Structure. Ribs are classified as true (vertebrosternal), false (vertebrocostal), and floating (vertebral), based on their anterior attachments. True ribs (1-7) connect directly to the sternum via costal cartilages. False ribs (8-10) connect to the sternum indirectly, via the costal cartilage of the rib above. Floating ribs (11-12) have no anterior attachment.

Sternum and Costal Cartilages. The sternum provides anterior protection for the heart and lungs. Costal cartilages, composed of hyaline cartilage, connect the ribs to the sternum, providing flexibility and elasticity to the thoracic cage. The thoracic vertebrae articulate with the ribs posteriorly, forming the posterior boundary of the thorax.

5. Joints: Enabling Movement and Flexibility

In the construction of the human body, it would appear essential, in the first place, to provide some dense and solid texture capable of forming a framework for the support and attachment of the softer parts of the frame, and of forming cavities for the protection of the more important vital organs; and such a structure we find provided in the various bones, which form what is called the Skeleton.

Synarthrosis, Amphiarthrosis, and Diarthrosis. Joints, or articulations, connect bones, allowing for varying degrees of movement. Joints are classified into synarthrosis (immovable), amphiarthrosis (slightly movable), and diarthrosis (freely movable). Synarthrotic joints, such as sutures in the skull, provide stability and protection. Amphiarthrotic joints, like intervertebral discs, allow limited movement and shock absorption. Diarthrotic joints, such as the hip and knee, enable a wide range of motion.

Diarthrotic Joint Components. Diarthrotic joints feature articular cartilage, a smooth, protective layer covering bone ends. Ligaments, composed of fibrous tissue, connect bones and provide stability. Synovial membrane lines the joint capsule, secreting synovial fluid for lubrication. Menisci, or fibrocartilaginous discs, enhance joint congruity and shock absorption in some joints, such as the knee.

Joint Movements. Joint movements include gliding, angular (flexion, extension, abduction, adduction), circumduction, and rotation. Gliding movements occur between flat surfaces. Angular movements change the angle between bones. Circumduction combines flexion, extension, abduction, and adduction. Rotation involves movement around a longitudinal axis.

6. Muscles and Fasciæ: The Engines of Motion

In the construction of the human body, it would appear essential, in the first place, to provide some dense and solid texture capable of forming a framework for the support and attachment of the softer parts of the frame, and of forming cavities for the protection of the more important vital organs; and such a structure we find provided in the various bones, which form what is called the Skeleton.

Voluntary and Involuntary Muscles. Muscles, composed of contractile fibers, generate movement. Voluntary muscles, or striated muscles, are controlled by conscious effort, while involuntary muscles, or smooth muscles, function autonomously. Voluntary muscles are responsible for skeletal movement, while involuntary muscles control organ function.

Muscle Structure and Attachments. Voluntary muscles consist of bundles of fibers, each containing myofibrils composed of sarcomeres. Tendons, composed of fibrous tissue, connect muscles to bones. Aponeuroses, broad sheets of connective tissue, also attach muscles to bones or other structures.

Fascia and Muscle Function. Fascia, a fibrous connective tissue, surrounds and supports muscles, providing structure and facilitating movement. Superficial fascia lies beneath the skin, while deep fascia invests muscles and forms intermuscular septa. Fascia helps to distribute forces, reduce friction, and compartmentalize muscle groups.

7. Head and Face Muscles: Expression and Function

The Muscles of the Head and Face consist of ten groups, arranged according to the region in which they are situated.

Facial Expression. The muscles of the head and face are responsible for facial expressions, speech, and mastication. These muscles are grouped into cranial, auricular, palpebral, orbital, nasal, maxillary, temporomandibular, and pterygomaxillary regions. The occipitofrontalis muscle raises the eyebrows and wrinkles the forehead.

Eye and Nose Muscles. The orbicularis oculi muscle closes the eyelids, while the levator palpebrae superioris elevates the upper eyelid. The corrugator supercilii draws the eyebrows together, creating a furrowed brow. Nasal muscles, including the compressor naris and depressor alae nasi, control nostril shape and size.

Mouth and Jaw Muscles. The orbicularis oris muscle encircles the mouth, enabling lip closure and pursing. The buccinator muscle compresses the cheeks, aiding in chewing. The masseter and temporalis muscles elevate the mandible, facilitating chewing. The pterygoid muscles assist in jaw movement, including protrusion and lateral excursion.

8. Neck Muscles: Support and Mobility

Muscles and Fasciæ of the Neck. The muscles of the Neck may be arranged into groups, corresponding with the region in which they are situated.

Superficial and Infrahyoid Muscles. The muscles of the neck support the head, facilitate swallowing, and control vocalization. The platysma, a superficial muscle, tenses the skin of the neck. The sternocleidomastoid muscle flexes and rotates the head. The infrahyoid muscles (sternohyoid, sternothyroid, thyrohyoid, and omohyoid) depress the hyoid bone and larynx.

Suprahyoid and Lingual Muscles. The suprahyoid muscles (digastric, stylohyoid, mylohyoid, and geniohyoid) elevate the hyoid bone and depress the mandible. The lingual muscles (genioglossus, hyoglossus, styloglossus, and palatoglossus) control tongue movement for speech and swallowing.

Pharyngeal and Vertebral Muscles. The pharyngeal muscles (constrictor inferior, medius, and superior, stylopharyngeus, and palatopharyngeus) constrict the pharynx during swallowing. The vertebral muscles (rectus capitis anterior and lateralis, longus capitis, and scalenes) flex, extend, and rotate the head and neck.

9. Trunk Muscles: Posture, Breathing, and Protection

Muscles and Fasciæ of the Trunk. The Muscles of the Trunk may be subdivided into four groups:—1. Muscles of the Back. 2. Muscles of the Abdomen. 3. Muscles of the Thorax. 4. Muscles of the Perinæum.

Back Muscles. The muscles of the back maintain posture, control spinal movement, and connect the upper and lower extremities to the trunk. The trapezius elevates, depresses, retracts, and rotates the scapula. The latissimus dorsi extends, adducts, and internally rotates the arm. The erector spinae group (spinalis, longissimus, and iliocostalis) extends and laterally flexes the vertebral column.

Abdominal Muscles. The abdominal muscles compress the abdominal viscera, flex the trunk, and assist in respiration. The external oblique, internal oblique, and transversus abdominis muscles form the lateral abdominal wall. The rectus abdominis flexes the trunk and compresses the abdomen.

Thoracic Muscles. The thoracic muscles control rib movement during respiration. The external intercostals elevate the ribs during inspiration, while the internal intercostals depress the ribs during expiration. The diaphragm, a large dome-shaped muscle, separates the thoracic and abdominal cavities and is the primary muscle of respiration.

10. Upper Extremity Muscles: Precision and Power

Muscles and Fasciæ of the Upper Extremity. The Muscles of the Upper Extremity are divisible into groups, corresponding with the different regions of the limb.

Shoulder Muscles. The muscles of the shoulder control arm movement and stabilize the shoulder joint. The deltoid abducts, flexes, and extends the arm. The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) stabilize the shoulder joint and control rotation.

Arm Muscles. The muscles of the arm flex and extend the elbow joint. The biceps brachii flexes the elbow and supinates the forearm. The brachialis flexes the elbow. The triceps brachii extends the elbow.

Forearm and Hand Muscles. The muscles of the forearm control wrist and finger movement. The flexor carpi ulnaris and radialis flex the wrist. The extensor carpi ulnaris and radialis extend the wrist. The flexor digitorum superficialis and profundus flex the fingers. The extensor digitorum extends the fingers. The muscles of the hand control fine motor movements of the fingers and thumb.

11. Lower Extremity Muscles: Locomotion and Stability

Muscles and Fasciæ of the Lower Extremity. The Muscles of the Lower Extremity are subdivided into groups, corresponding with the different regions of the limb.

Hip Muscles. The muscles of the hip control thigh movement and stabilize the hip joint. The gluteus maximus extends and laterally rotates the thigh. The gluteus medius and minimus abduct and medially rotate the thigh. The iliopsoas flexes the hip. The adductor muscles (longus, brevis, and magnus) adduct the thigh.

Thigh Muscles. The muscles of the thigh flex the knee and extend the hip. The quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius) extends the knee. The hamstrings (biceps femoris, semitendinosus, and semimembranosus) flex the knee and extend the hip.

Leg and Foot Muscles. The muscles of the leg control ankle and toe movement. The gastrocnemius and soleus plantarflex the ankle. The tibialis anterior dorsiflexes the ankle. The peroneus longus and brevis evert the foot. The flexor digitorum longus and hallucis longus flex the toes. The extensor digitorum longus and hallucis longus extend the toes. The intrinsic foot muscles support the arches of the foot and control toe movement.

Last updated: February 22, 2025