Introduction to Gross Anatomy

Introduction to Gross Anatomy

Terminology

Gross Anatomy, or macroscopic anatomy, may be defined as the study of organs and organ systems comprising the individual. In a typical one-semester course in gross anatomy, a student learns approximately 4000 new terms – the equivalent of the vocabulary required by a one-year college beginning language course!

Mastery of the myriad concepts presented depends on the student’s ability to organize and integrate them. This task is facilitated by an early introduction to the terminology of the field. The definitions contained in this section will continue to be useful to students throughout their careers, as they learn to express themselves in  the common language of health professionals.

Anatomical Position, Planes and Sections, Directional Terms

ANATOMICAL POSITION is defined as the stance in which the patient is facing forward, head erect, feet parallel. The upper limbs are positioned so that the palms face forward.

A SAGITTAL SECTION passes front-to back, through the central axis of the body (midsagittal plane). Planes parallel to the midline are referred to as parasagittal sections. The midsagittal plane divides the body into right and left halves.

A TRANSVERSE section passes horizontally through the body in a plane parallel to the floor. (NOTE: radiological convention dictates that the transverse sections are always viewed as if the examiner were standing at the foot of the table looking towards the head f the patient/cadaver).

A FRONTAL (CORONAL) SECTION passes through the body from front to back in a plane perpendicular to the sagittal plane.

DIRECTIONAL TERMS fall into two categories: descriptive adjectives which indicate relative position, and nouns which define specific movements. All these terms are defined in relation to the patient (cadaver) in anatomical position.

ANTERIOR (VENTRAL): the front surface of the body (as a relative term, in front of)

POSTERIOR (DORSAL): the back surface of the body (as a relative term, in back of).

SUPERIOR (ROSTRAL, CEPHALIC, CRANIAL): toward the head

SUPRA: prefix meaning “above”

INFERIOR (CAUDAL): toward the “tail” region of the trunk; toward the foot (lower limb)

PROXIMAL: toward the central axis of the body. In reference to the limbs, the shoulder is proximal to the elbow; the hip is proximal to the knee.

DISTAL: away from the central axis of the body. The digits (fingers) are distal to the wrist; the ankle is distal to the knee.

MEDIAL: toward the midline of the body. In anatomical position, the little finger is the most medial digit of the hand. (The midline of the body is described as median, indicating its “most central” position.

LATERAL: away from the midline of the body. In anatomical position, the thumb is the most lateral digit of the hand.

SUPERFICIAL: nearer the surface of the body

DEEP: farther from the surface of the body.

FLEXION: a decrease in the angle of a joint; the approximation of two ventral surfaces. An exception is plantarflexion, which refers to the movement produced by contraction of the calf muscles of the leg, and causes an increased angle between leg and foot at the ankle.

ADDUCTION: movement toward the midline

ABDUCTION: movement away from the midline

MEDIAL (INTERNAL) ROTATION: turning toward the midline

LATERAL (EXTERNAL) ROTATION: turning away from the midline

PRONATION: turning the anterior surface (of the forearm) posteriorly; facing the palm of the hand posteriorly

SUPINATION: returning the anterior surface of the forearm to its original (anatomical) position; facing the palm of the hand anteriorly

Tissue Types Encountered in Dissection

Organs (viscera) and organ systems studied in gross anatomy are composed of the four basic tissue types studied in histology (microscopic anatomy). The terminology utilized is often different, however, and may be a source of confusion to the beginning student in gross antomy. This section offers an overview of how the four basic tissues (epithethium, connective tissue, muscle and nerve) are described and identified on the gross (macroscopic) level.

Epithelium

Epithelial tissue is the cellular tissue of the body surface and forms the linings of the viscera and body cavities. It may be derived from any of the three embryonic germ layers: ectoderm, mesoderm, or endoderm. Glands and hair follicles derived from the surface and lining layers are also formed from epithelium.

Surface epithelium is described as cutaneous, and forms the epidermal layer of the skin. Depending on its location, it is a keratinized or non-keratinized stratified squamous epithelium.

The epithelium lining of the luminal (inner) surface of a hollow region or organ is known, together with its underlying loose connective tissue and tin muscular layer, as a MUCOSA, or MUCOUS MEMBRANE.  Mucosal surfaces are covered by non-keratinized stratified squamous epithelium if they are subject to wear and tear (as in the oral cavity), or by a simple columnar or pseudostratified epithelium (as in the lining of the GI tract or uppe portions of the respiratory system). Columnar epithelial cells are usually interspersed with mucus-secreting cells.

Skin and mucosal surfaces are interconnected at specialized epithelial regions known as MUCOCUTANEOUS JUNCTIONS (such as the lips).

Arteries and veins are lined by a non-keratinizing simple squamous epithelial layer known as ENDOTHELIUM. Capillary walls are comprised exclusively of this endothelial layer. In the dural venous sinuses of the cranial cavity, the endothelial lining of the spaces is supported solely by the dense connective tissue of the dura mater.

The blind cavities of the body (pleural in the thorax, pericardial around the heart in the middle mediastinum (a division of the thorax), and peritoneal in the abdomen) are lined by a specialized simple epithelial layer known as MESOTHELIUM. The cells of this layer normally secrete a small amount of serous fluid, which lubricates the surface cells and allows the viscera to glide smoothly over each other. The three serous (fluid-producing) membranes of the body cavities each have two components: a VISCERAL layer, which is continuous with the outer connective tissue covering the organs themselves (SEROSA, or ADVENTITIA), and a PARIETAL layer, which lines the wall of the body cavity. These two layers are continuous.

Connective Tissue

Connective tissue is the supporting tissue of the body. It consists of ground substance, cells and fibrous material such as collagen or elastic fibers. The types of connective tissue are determined by the relative concentration and organization of these elements. Loose connective tissue, of the type found in superficial fascial layers, has a high concentration of uncalcified ground substance (matrix), and a relatively low concentration of fibers, Dense connective tissue, which includes tendons, ligaments and aponeuroses, has a high concentration of oriented collagen fibers. Cartilage is a semirigid, non-vascular form of connective tissue; bone has a calcified matrix. These and othet types of connective tissues are described below.

A word about fascia: the term fascia is often used to refer to the “filler” connective tissue which separates and organizes regions of the body below the skin surface. SUPERFICIAL FASCIA is continuous with the dermis of the skin and fills the space between muscle groups and organs, It is variable thckness, depending on the amount of fat it contains; a thin person has much less interstitial (filler) tissue than an obese one (an example is Camper’s fascia, the most superficial of the fascial layers of the anterior abdominal wall.

DEEP FASCIA or DEEP INVESTING FASCIA as it is known when it covers and separates muscles and muscle groups, protects and compartmentalizes the organ systems and regions of the body. A knowledge of the organization  of fascial planes and the potential spaces between them is important in the understanding of the spread of infection between these planes. Deep fascia is membranous in nature, and contains no fatty tissue. It is far less variable in thickness than superficial fascia, with which it blends and is continuous.

The INTERMUSCULAR SEPTA which define and separate the compartments of the limbs are formed by deep investing fascia. They are formed of dense regular connective tissue, and attach to bone and to the fascia which surrounds the muscles. The INTEROSSEOUS MEMBRANES which interconnect the bones of forearm and leg are similar in composition.

TENDONS and LIGAMENTS are sheets or cords of dense connective tissue. Tendons attach muscle to bone, and ligaments attach one bone to another. A tear of a tendon or ligament heals slowly because of the relative avascularity (lack of blood supply) of dense connective tissue.

An APONEUROSIS is a specialized form of dense regular connective tissue in the shape of a broad, thin sheet. The rectus sheath of the anterior abdominal wall, which encases the rectus abdominus muscle, is formed from the aponeurotic fibers of the lateral abdominal muscles (external oblique, internal oblique and transversus abdominus).

Cartilage is a semihard form of connective tissue characterized by its resiliency. It forms the moveable core of structures such as the external ear and the anterior portion of the nose. It is present on the ends of long bones of the limbs where they articulate with other bones (synovial joints), on the tips of the ribs (costal cartilages) and at the sites where movement takes place.

Types of cartilage include hyaline cartilage (present in most synovial joints), elastic cartilage (epiglottis, external ear) and fibrocartilage (temporomandibular joint, menisci of knee joint).

Bone is the hardest of the connective tissues, characterized by the presence of calcium deposits in its matrix. Unlike dense regular connective tissue and cartilage, bone is vascular. For this reason, fractures often take less time to heal than tears to tendons, ligaments and cartilage.

Bone may be classified as spongy or compact. Spongy bone is present at the ends of long bones, and between the inner and outer layers of compact bone which characterize the flat bones of the skull (frontal, occipital, parietal, squamous temporal bones).

Unlike the static, dead bony specimens we are accustomed to from the lab, living bone is a dynamic tissue, readily responsive to external stimuli. The alveolar processes of mandible and maxilla are quickly resorbed following removal of the dentition (teeth). Infants are born without mastoid processes, and develop them during the first year of life as they contract their sternocleidomastoid muscles in repeated efforts to raise and turn their heads.

Joints (Articulations)

Joints are sites where tow or more bones meet. Some allow no movement (cranial sutures), some allow slight movement (intervertebral joints) and some allow a great range of movement (shoulder joint).

A synovial joint is enclosed within a ligamentous joint capsule which surrounds the joint cavity. The bony surfaces in contact at a synovial joint are usually covered by hyaline cartilage. The joint cavity is lined by a specialized synovial membrane which secretes a small amount of synovial fluid. This type of joint allows free movement and is present between most of the bones of the limbs.

There are six major types of synovial joint:

1. Ball and Socket joints (shoulder, hip) are the most moveable, allowing motion in a variety of planes and axes. They permit flexion, extension, adduction, abduction, medial rotation and lateral rotation; collectively, these movements are known as circumduction

      2.    Condyloid joints (metacarpophalangeal joints) allow flexion, extension, adduction, abduction and circumduction

3.    Hinge joints (elbow, knee) permit flexion and extension only

4. Pivot joints (atlantoaxial joint, radius and capitulum of humerus) allow

Rotation

5. Plane joints (acromioclavicular articulation) permit gliding movements, usually in one plane only.
6. Saddle joints have opposing concave and convex surfaces

Muscle

The muscles of the body are composed of muscle cells, or fibers. There are three types of muscle tissue: skeletal (striated), smooth, and cardiac. Skeletal muscle tissue forms the voluntary muscles of limbs, trunk, head and neck. Smooth muscle is present in the walls of the visceral organs, in blood vessels, in the eye, and in association with hair follicles of the skin. Cardiac muscle is unique to the myocardium of the heart.

Skeletal muscle is innervated by somatic fibers contained in spinal and cranial nerves, and in the case of muscles derived from the pharyngeal arches, by voluntary motor fibers in cranial nerves classified as “special visceral efferent”. Smooth muscle is under the control of the autonomic nervous system. Cardiac muscle contraction is controlled by an intrinsic system, the conducting system of the heart, which is made up of specialized cardiac muscle. The heart rate is influenced by the autonomic nervous system, which can cause it to speed up (sympathetic division) or slow down (parasympathetic division).

In gross anatomy, the muscles which are dissected individually or in groups are skeletal muscles. The following principles apply to skeletal muscle.

Skeletal muscles have at least two points of attachment, usually to bone, cartilage or fascia. In general (especially in the limbs), the more proximal ttachment is called the origin of the muscle; the more distal attachment is called the insertion. The origin and insertion are tendinous in nature (tendons are cords or sheets of dense regular connective tissue). The majority of muscles cross one or more joints and act upon the joints crossed, producing movements such as flexion and extension. In general, if a muscle crosses more than one joint, it exerts a stronger action at the more distal joint.

The action of the muscle is termed CONTRACTION. When a muscle contracts, it pulls the insertion closer to the origin (an exception to this rule is the case of certain muscles such as the infrahyoid (strap) muscles of the neck, in which the origin and insertion are interchangeable, and function can vary accordingly. As a result, a muscle always shortens when it contracts. Elongation of a muscle is relaxation, or STRETCHING, of a muscle, and is accomplished by a muscle or muscles of an opposing group. Therefore, if the biceps brachii, which originates from the scapula and passes through the anterior compartment of the arm to insert on the radius and bicipital aponeurosis of the forearm, contracts, it pulls the forearm closer to the arm, resulting in flexion at the elbow joint. By contrast, extension of the elbow is accomplished by triceps brachii, which occupies the posterior compartment of the arm and inserts on the posterior forearm. Contraction of triceps results in stretching of biceps, and vice-versa.

Using the above principle, one can deduce the function of a muscle or muscle group from knowing the origin and insertion of that muscle.

It is helpful to realize that muscles with similar functions are often grouped together in the same fascial compartment. In the lower limb, where considerable rotation from the embryonic positioning has taken place, extensors are located in the anterior compartment of thigh and leg, whereas in the upper limb, where there has been less change in the embryonic orientation, fleor muscles are found in the anterior compartments of both arm and legs and extensors are posterior in location.

Nerve

Nervous tissue, like muscle, is characterized by its irritability and the conduction of electrical impulses. The functional cellular unit of the nervous system is the neuron. Neurons come in varying shapes and sizes; the giant motor neurons where cell bodies are found in the ventral horn of the spinal cord are among the largest cells  in the body.

Neurons and their supporting cells and tissues comprise the nervous system, which can be organized in either of two ways: according to structure, or according to function.

The neuron: this cellular unit of the nervous system consists of a cell body, which is usually found in the gray matter of the central nervous system or in ganglia of the peripheral nervous system, and two kinds of processes. DENDRITES are processes which conduct impulses toward the cell body (AFFERENT, SENSORY). Axons are processes which conduct impulses away from the cell body (EFFERENT, MOTOR). Neurons communicate sith other neurons, with muscle cells, or with specialized receptors at SYNAPSES, highly organized points of intercellular contact. Chemical substances called NEUROTRANSMITTERS are released at these points of contact.

Axons may be encased in a fatty myelin sheath, which increases the speed at which an impulse is conducted. In general, the large nerves of the peripheral nervous system which are the subject of gross dissection are collections of myelinated nerve fibers, or cell processes.

Note: although it is proper to distinguish between afferent and efferent fibers as axons and dentrites on the cellular level, it is common usage in gross anatomy to refer to all dissectible bundles of nerve fibers as collections of “axons” – regardless of the direction of impulse propagation.

Organization of the Nervous System

There are two ways to organize one’s thinking about the nervous system: structurally and functionally.

Structural organization: the nervous system can be divided into two parts: the CENTRAL NERVOUS SYSTEM (CNS) consists of the brain and spinal cord. The PERIPHERAL NERVOUS SYSTEM (PNS) consists of 31 pairs of spinal nerves, 12 pairs of cranial nerves and associated ganglia.

Functional organization: the nervous system can (again) be divided into two parts. The SOMATIC NERVOUS SYSTEM has a voluntary motor (general somatic efferent: GFE) component which supplies skeletal muscles. It also includes a general sensory (general somatic afferent: GSA) component which conveys sensation from the skin and mucous membranes. Among the modalities of sensation conveyed are touch, pressure, pain and temperature, which are transmitted by all spinal nerves and some cranial nerves. In addition to the general sensations, there are so-called “special senses” which are conveyed only by cranial nerves, and relate specifically to the head: sight, hearing, taste. Smell and balance.

The AUTONOMIC NERVOUS SYSTEM (ANS) has two divisions which complement and balance each other. Unlike the somatic nervous system, which is voluntary andinvolves conscious control and perception, the ANS is an involuntary, or “automatic” system. It is a motor system, described as general visceral efferent (GVE), but a visceral sensory component (general visceral afferent GVA) also exists. Examples of GVA include angina, uterine contractions during labor and nausea.

The divisions of the ANS are the sympathetic and parasympathetic nervous systems. The sympathetic division is activated in times of stress or emergency (“fight or flight”), and is augmented by the release of catecholamine hormones (epinephrine and norepinephrine) from the adrenal medulla. Its actions are short-lived and intense, and include increased heart rate, bronchodilation and increased blood flow to the large muscles of the body.

The parasympathetic division of the ANS is restorative in nature, and is operational during the routines of daily existence in the life of the individual. Parasympathetic stimulation decreases the heart rate and promotes digestion by increasing the blood flow to the GI tract and enhancing contraction of smooth muscle in the walls of the abdominal viscera.

Autonomic cell bodies and fibers are found in specific spinal nerves and cranial nerves. Preganglionic sympathetic cell bodies are located in the intermediolateral cell column of spinal nerves T1-L2 (L3). Preganglionic parasympathetic cell bodies are found in parasympathetic cranial nerve nucluei in the brainstem which are associated with cranial nerves III, VII, IX and X, or in the internediolateral cell column of spinal cord segments S2, S3 and S4. Because of this selective localization, the sympathetic division of the ANS may be described as thoracolumbar and the parasympathetic division as craniosacral.