General Terminology

Students of anatomy and physiology should learn the standard terms for body directions and subdivisions of the body. These will provide a basic introduction to the study of the body and also point out the need for the use of correct terminology.

The body planes are imaginary lines used for reference; they include the median plane, the coronal plane, and the transverse plane. A section is a real or imaginary cut made along a plane. A cut along the median plane is a sagittal section. A cut along the coronal plane is a frontal section, and a cut through the transverse plane is a cross-section. When describing the body, visualize the anatomic position. The body is erect, the feet are slightly apart, the head is held high, and the palms of the hands are facing forward.

Important terms of direction to review include superior (above), inferior (below), anterior (facing forward), posterior (toward the back), medial (toward the midline), and lateral (away from the midline or toward the sides). Proximal and distal are terms of direction usually used in reference to limbs. Proximal means closer to the point of attachment, and distal refers to further away from the point of attachment. Figure 7-1 depicts the directional terms.

Figure 7-1 Planes and directions of the body.

(From Patton KT, Thibodeau GA: Anatomy and physiology, ed 7, St Louis, 2010, Mosby.)

Major body cavities are divided into the dorsal cavity (includes the cranial and spinal cavities) and the ventral cavity (includes the orbits and the nasal, oral, thoracic, and abdominopelvic cavities).

Additional useful terminology is defined later in this chapter.

Histology

Histology is the study of tissues. A tissue is a group of cells that act together to perform specific functions. The four fundamental tissues are epithelial, connective, muscle, and nerve tissues (Figure 7-2). Epithelial cells cover, line, and protect the body and its internal organs. Connective tissue is the framework of the body, providing support and structure for the organs. Nerve tissue is composed of neurons and connective tissue cells that are referred to as neuroglia. Muscle tissues have the ability to contract or shorten. Muscle tissue is classified as voluntary muscle (skeletal muscles) or involuntary muscle (smooth muscle and cardiac muscle tissue).

Figure 7-2 Major tissues of the body. A, Epithelial tissue; B, connective tissue; C, muscle tissue; D, nervous tissue.

(From Patton KT, Thibodeau GA: Anatomy and physiology, ed 7, St Louis, 2010, Mosby.)

The major parts of the cell should be reviewed. The cell is the basic unit of life and the building block of tissues and organs. Within the cell, each organelle has a specific function. The nucleus, which contains deoxyribonucleic acid (DNA), and ribosomes are especially important in the synthesis of proteins. Proteins include the enzymes that regulate all chemical reactions within the body.

Mitosis and Meiosis

Mitosis is necessary for growth and repair. In this process, the DNA is duplicated and distributed evenly to two daughter cells. Meiosis is the special cell division that takes place in the gonads, that is, the ovaries and testes. In the process of meiosis, the chromosome number is reduced from 46 to 23, so when the egg and the sperm unite in fertilization the zygote will have the correct number of chromosomes.

Skin

The skin is the largest organ of the body. The skin consists of two layers: the epidermis (the outermost protective layer of dead keratinized epithelial cells) and the dermis (the underlying layer of connective tissue with blood vessels, nerve endings, and the associated skin structures). The dermis rests on the subcutaneous tissue that connects the skin to the superficial muscles.

The layers of the epidermis, from outer layer to inner layer, are the stratum corneum, the stratum lucidum, the stratum granulosum, and the innermost stratum germinativum (includes stratum basale and stratum spinosum), where mitosis occurs. Epidermal cells contain the protein pigment called melanin, which protects against radiation from the sun.

The inner layer of the skin is the dermis, composed of fibrous connective tissue with blood vessels, sensory nerve endings, hair follicles, and glands. There are two types of sweat glands. The most widely distributed sweat glands regulate body temperature by releasing a watery secretion that evaporates from the surface of the skin. This type of sweat gland is known as eccrine. The other sweat glands, mainly in the armpits and groin area, display apocrine secretion. This secretion contains bits of cytoplasm from the secreting cells. This cell debris attracts bacteria, and the presence of the bacteria on the skin results in body odor. The sebaceous glands release an oily secretion (sebum) through the hair follicles that lubricates the skin and prevents drying. Oil is produced by holocrine secretion, in which whole cells of the gland are part of the secretion. These glands are susceptible to becoming clogged and attracting bacteria, particularly during adolescence.

The appendages of the skin include hair and nails. Both are composed of a strong protein called keratin. Skin structure is illustrated in Figure 7-3. Hair, nails, and skin may show changes in disease that may be used in the diagnosis of clinical conditions. For example, skin cancer is a clinical condition that is associated with the skin.

Figure 7-3 Diagram of skin structure. A, Thick skin, found on surfaces of the palms and soles of the feet. B, Thin skin, found on most surface areas of the body. In each diagram, the epidermis is raised at one corner to reveal the papillae of the dermis.

(From Patton KT, Thibodeau GA: Anatomy and physiology, ed 7, St Louis, 2010, Mosby.)

Skeletal System

The body framework consists of bone, cartilage, ligaments, and joints. Functions of the skeletal system include support, movement, blood cell formation (hemopoiesis), protection of internal organs, detoxification (removal of poisons), provision for muscle attachment, and mineral storage (particularly calcium and phosphorus).

Individual bones are classified by shape. There are long bones, short bones, flat bones, irregular bones, and sesamoid bones. A typical long bone has an irregular epiphysis at each end, composed mainly of spongy (cancellous) bone, and a shaft or diaphysis, composed mainly of compact bone. The cells that form compact bone are called osteoblasts; when they become fixed in the dense bone matrix, they stop dividing but continue to maintain bone tissue as osteocytes.

The axial skeleton (Figure 7-4) consists of the 28 bones of the skull. These are separated into the 14 facial bones and the 14 bones of the cranium. The facial bones are two nasal bones, two maxillary bones, two zygomatic bones, one mandible (the only moveable bone of the skull), two palatine bones, one vomer, two lacrimal bones, and two inferior nasal conchae. The bones of the cranium are the single occipital, frontal, ethmoid, and sphenoid and the paired parietal, temporal, and ossicles of the ear (malleus, incus, and stapes).

Figure 7-4 Anterior view of the skeleton.

(From Patton KT, Thibodeau GA: Anatomy and physiology, ed 7, St Louis, 2010, Mosby.)

The axial skeleton also has 33 bones of the vertebral column, as depicted in Figure 7-5. There are seven cervical vertebrae, 12 thoracic vertebrae, five lumbar vertebrae, five sacral vertebrae (fused to form the sacrum), and the coccygeal vertebrae (known as the tailbone). The final portion of the axial skeleton consists of the bones of the thorax, the sternum, and the 12 pairs of ribs.

Figure 7-5 Anterior view of the vertebral column.

(From Patton KT, Thibodeau GA: Anatomy and physiology, ed 7, St Louis, 2010, Mosby.)

The appendicular skeleton (see Figure 7-4) includes the girdles and the limbs. The upper portion consists of the pectoral or shoulder girdle, the clavicle and scapula, and the upper extremity. The bones of the arm are the humerus, the radius and ulna, the carpals (wrist bones), the metacarpals (bones of the hand), and the phalanges (bones of the fingers). The lower portion of the appendicular skeleton is made up of the pelvic girdle or os coxae. Each of the os coxae consists of a fused ilium, ischium, and pubis. Bones of the lower extremity include the femur (thighbone), the tibia and fibula, the tarsals (ankle bones), the metatarsals (bones of the foot), and the phalanges.

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