Anatomy

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Anatomy

Anatomy, a subfield of biology, is the study of the structure of living things. There are three main areas of anatomy: cytology studies the structure of cells; histology examines the structure of tissues; and gross anatomy deals with organs and organ systems. Comparative anatomy, which strives to identify general structural patterns in families of plants and animals, provides the basis for the classification of species. Human anatomy is a crucial element of the modern medical curriculum.

History

Modern anatomy, as a branch of Western science, was founded by the Flemish scientist Andreas Vesalius (1514–1564), who in 1543 published De humani corporis fabrica (Structure of the human body). In addition to correcting numerous misconceptions, Vesalius’s book was the first description of human anatomy that organized the organs into systems. Although initially rejected by many followers of classical anatomical doctrines, Vesalius’s systematic conception of anatomy eventually became the foundation of anatomical research and education throughout the world; anatomists still use his systematic approach.

Human anatomy

Human anatomy divides the body into the following distinct functional systems: cutaneous, muscular, skeletal, circulatory, nervous, digestive, urinary, endocrine, respiratory, and reproductive. This division helps the student understand the organs, their relationships, and the relations of individual organs to the body as a whole.

The cutaneous system consists of the integument— the covering of the body, including the skin, hair, and nails. The skin is the largest organ in the body, and its most important function is to act as a barrier between the body and the outside world. The skin’s minute openings (pores) also provide an outlet for sweat, which regulates the body temperature. Melanin, a dark pigment found in the skin, provides protection from sunburn. The skin also contains oil-producing cells.

The muscles of the muscular system enable the body to move and provide power to the hands and fingers. There are two basic types of muscles. Voluntary (skeletal) muscles enable movements under conscious direction (e.g., to walk, move an arm, or smile). Involuntary (smooth) muscles are not consciously controlled and operate independent of conscious direction. For example, they play an important role in digestion. The third type of muscle, cardiac muscle, is involuntary, but also is striated, as are skeletal muscles. Because cardiac muscle is self-contractile it allows the heart to pump blood throughout the body, without pause, from early in embryogenesis to death.

The skeletal system, or the skeleton, is the general supportive structure of the body. In addition, the skeletal system is the site of many important and complex physiological and immunological processes. The skeletal frame provides the support that muscles need in order to function. Of the 206 bones in the human body, the largest is the femur, or thighbone. The smallest are the tiny ear ossicles, three in each ear, named the hammer (malleus), anvil (incus), and stirrup (stapes). Often included in the skeletal system are the ligaments, which connect bone to bone; the joints, which allow the connected bones to move; and the tendons, which connect muscle to bone.

The circulatory system comprises the heart, arteries, veins, capillaries, blood and blood-forming organs, and the lymphatic subsystem. The four chambers of the heart allow the heart to act as a dual pump to propel blood to the lungs for oxygenation (pulmonary system) and to pump blood throughout the body (systemic circulation). From the heart, the blood circulates through arteries. The blood is distributed through smaller and smaller tubes until it passes into the microscopic capillaries that bathe every cell. The veins collect deoxygenated blood from the capillaries and return it to the heart.

The nervous system consists of the brain, spinal cord, and sensory organs that provide information to them. For example, our eyes, ears, nose, tongue, and skin receive stimuli and send signals that travel both electrically and chemically to the brain. The brain is an intricate system of complicated neurons (nerve cells) that allow us to process sensory information, visceral signals (e.g. regulating breathing, body temperature, etc.), and perform cognitive thought.

The digestive system is essentially a long tube extending from the mouth to the anus. Food entering the mouth is conducted through the stomach, small intestine, and large intestine, where accessory organs contribute digestive juices to break down the food, extracting the molecules that can be used to nourish the body. The unusable parts of the ingested food are expelled through the anus as fecal matter. The salivary glands (in the mouth), the liver, and the pancreas are the primary digestive glands.

The urinary system consists of the kidneys, the bladder, and the connecting tubules. The kidneys filter water and waste products from the blood and pass them into the bladder. At intervals, the bladder is emptied through the urinary tract, ridding the body of waste.

The endocrine system consists of ductless (endocrine) glands that produce hormones that regulate various bodily functions. The pancreas secretes insulin to regulate sugar metabolism, for example. The pituitary gland in the brain is the principal or “master” gland that regulates many other glands and endocrine functions.

The respiratory system includes the lungs, the diaphragm, and the tubes that connect them to the outside atmosphere. Respiration is the process whereby an organism absorbs oxygen from the air and returns carbon dioxide. The diaphragm is the muscle that enables the lungs to work.

Finally, the reproductive system enables sperm and egg to unite and the egg to remain in the uterus or womb to develop into a functional human.

Anatomical nomenclature

Over the centuries, anatomists developed a standard nomenclature, or method of naming anatomical structures. Terms such as “up” or “down” obviously have no meaning unless the orientation of the body is clear. When a body is lying on its back, the thorax and abdomen are at the same level. The upright sense of up and down is lost. Further, because anatomical studies and particularly embryological studies were often carried out in animals, the development of the nomenclature relative to comparative anatomy had an enormous impact on the development of human anatomical nomenclature. There were obvious difficulties in relating terms from quadrupeds (animals that walk on four legs) who have abdominal and thoracic regions at the same level as opposed to human bipeds in whom an upward and downward orientation might seem more obvious.

In order to standardize nomenclature, anatomical terms relate to the standard anatomical position. When the human body is in the standard anatomical position it is upright, erect on two legs, facing frontward, with the arms at the sides each rotated so that the palms of the hands turn forward.

In the standard anatomical position, superior means toward the head or the cranial end of the body. Inferior means toward the feet or the caudal end of the body. The body’s frontal surface is the anterior or ventral surface. Accordingly, the terms “anteriorly” and “ventrally” specify a position closer to—or toward—the front surface of the body. The body’s back surface is the posterior or dorsal surface, and the terms “posteriorly” and “dor-sally” specify a position closer to—or toward—the posterior surface of the body.

The terms superficial and deep relate to the distance from the exterior surface of the body. Cavities such as the thoracic cavity have internal and external regions that correspond to deep and superficial relationships in the midsagittal (vertical midline) plane.

The bones of the skull are fused by sutures that form important anatomical landmarks. Sutures are joints that run jaggedly along the interface between the bones. At birth, the sutures are soft, broad, and cartilaginous. Near the end of puberty or early in adulthood the sutures eventually fuse and become rigid and ossified.

The sagittal suture, which unites the parietal bones of the skull along the body’s midline, is used as a landmark in anatomical nomenclature to establish sagittal planes. The primary sagittal plane runs through the length of the sagittal suture. Planes that are parallel to the sagittal plane, but that are offset from the midsagittal plane are termed parasagittal planes. Sagittal planes run anteriorly and posteriorly and are always at right angles to the coronal planes (perpendicular to the midsagittal plane, divides the body into dorsal and ventral sections). The medial plane or midsagittal plane divides the body vertically into superficially symmetrical right and left halves.

The medial plane also establishes a centerline axis for the body. The terms medial and lateral relate positions relative to the medial axis. If a structure is medial to another structure, the medial structure is closer to the medial or center axis. If a structure is lateral to another structure, the lateral structure is farther away from the medial axis. For example, the lungs are lateral to the heart.

The coronal suture unites the frontal bone with the parietal bones. In anatomical nomenclature, the primary coronal plane designates the plane that runs through the length of the coronal suture. The primary coronal plane is also termed the frontal plane because it divides the body into dorsal and ventral (front and back) halves.

Planes that divide the body into superior and inferior portions, and that are at right angles to both the sagittal and coronal planes, are termed transverse planes. Anatomical planes that are not parallel to sagittal, coronal, or transverse planes are termed oblique planes.

The body is also divided into several regional areas. The most superior area is the cephalic region, which includes the head. The thoracic region is commonly known as the chest region. Although the celiac region more specifically refers to the center of the abdominal region, celiac is sometimes used to designate a wider area of abdominal structures. At the inferior end of the abdominal region lies the pelvic region or pelvis. The posterior or dorsal side of the body has its own special regions, named for the underlying vertebrae. From superior to inferior along the midline of the dorsal surface lie the cervical, thoracic, lumbar and sacral regions. The buttocks are the most prominent feature of the gluteal region.

The term upper limbs or upper extremities refers to the arms and hands; lower limbs or lower extremities refers to the legs and feet.

The proximal end of an extremity is at the junction of the extremity (i.e., arm or leg) with the trunk of the body. The distal end of an extremity is the point on the extremity farthest away from the trunk (e.g., fingers and toes). Accordingly, if a structure is proximate to another structure it is closer to the trunk (e.g., the elbow is proximate to the wrist). If a structure is distal to another, it is farther from the trunk (e.g., the fingers are distal to the wrist).

Structures may also be described as being medial or lateral to the midline axis of each extremity. Within the upper limbs, the terms radial and ulnar may be used synonymouly with lateral and medial. In the lower extremities, the terms fibular and tibial may be used as synonyms for lateral and medial.

Rotations of the extremities may de described as medial rotations (toward the midline) or lateral rotations (away from the midline).

Many structural relationships are described by combined anatomical terms (e.g., the eyes are ante-rio-medial to the ears).

There are also terms of movement that are standardized by anatomical nomenclature. Starting from the anatomical position, abduction indicates the movement of an arm or leg away from the midline or mid-sagittal plane. Adduction indicates movement of an extremity toward the midline.

The opening of the hands into the anatomical position is termed supination of the hands. Rotation so the dorsal side of the hands face forward is termed pronation.

The term flexion means movement toward the flexor or anterior surface. In contrast, extension may be generally regarded as movement toward the extensor or posterior surface. Flexion occurs when the arm brings the hand from the anatomical position toward the shoulder (a curl) or when the arm is raised over the head from the anatomical position. Extension returns the upper arm or lowers it to the anatomical position. Because of the embryological rotation of the lower limbs that rotates the primitive dorsal side to the adult form ventral side, flexion occurs as the thigh is raised anteriorly and superiorly toward the anterior portion of the pelvis. Extension occurs when the thigh is returned to anatomical position. Specifically, due to the embryological rotation, flexion of the lower leg occurs as the foot is raised toward the back of the thigh, and extension of the lower leg occurs with the kicking motion that returns the lower leg to anatomical position.

The term palmar surface (palm side) is applied to the flexion side of the hand. The term plantar surface is applied to the bottom sole of the foot. From the anatomical position, extension occurs when the toes are curled back and the foot arches upward and flexion occurs as the foot is returned to anatomical position.

Rolling motions of the foot are described as inversion (rolling with the big toe initially lifting upward) and eversion (rolling with the big toe initially moving downward).