Chapter 1General Concepts
The word anatomy was coined from two Greek root words that mean “to cut up.” This is precisely the way in which the early anatomists studied the structure of once living things—by dissecting animal or human remains, observing structures, and then speculating as to what function these structures might perform. The scope of anatomy has broadened considerably. Human anatomy is now the study of the structure of the human body through a variety of approaches, and these approaches have given rise to specialized subfields of human anatomy.
Gross anatomy is concerned with the study of human form and structure as seen with the naked eye. When applicable, applied or clinical anatomy is introduced to illustrate the connection with everyday clinical problems in the health sciences. There are two classic approaches to the study of gross anatomy, both of which are used in this book.
A systemic approach is one in which the various systems of the body are studied as separate entities. This system of study is favored by college anatomy courses that do not include laboratory dissections.
A regional approach divides the body into a number of regions, which are then studied in turn. All the structures belonging to various systems are considered within the region being studied. This system of study follows the sequence of events encountered in a dissection and is favored in anatomy courses that include a dissecting laboratory component or a program that uses prosected specimens (specimens that were previously dissected).
This textbook largely follows a regional approach, because most dental school anatomy programs include a laboratory component. Bones, joints, muscles, blood vessels, nerves, fascia, and skin are found in every region of the body. For this reason a brief overview of the systems that give rise to these elements is presented in this chapter.
Histology is the study of smaller details of structure as seen through a microscope. It is the study of human tissues and ranges from basic tissue and cell architecture, with use of light and confocal microscopes, to ultrastructural elements of tissues and cells, with use of the electron microscope. Biochemical techniques combined with histological techniques have given rise to applied disciplines of histochemistry and immunocytochemistry.
Neuroanatomy is the study of the central nervous system (CNS), meaning the brain and spinal cord, as viewed in gross dissection and histological preparations, as well as the study of pathways through immunocytochemical tracers.
Developmental anatomy is the study of age-related changes in size, complexity, shape, and ability to function. Prenatal development follows the development of the individual from the time of conception to birth. Embryology is particularly concerned with the first 2 months of life in utero, during which the organ systems are formed. Postnatal development traces the various changes in form and function after birth, and through infancy, childhood, adolescence, and adulthood.
Surface anatomy (living anatomy) deals with the surface or topography of the living person. Superficial structures can be readily located and deeper structures can be located and envisioned based on surface landmarks.
Imaging anatomy is the noninvasive study of living or dead subjects as revealed by conventional radiography, magnetic resonance imaging (MRI), and ultrasonography. The use of serialized radiographs taken at ever-increasing depths through the body (computed tomography [CT]) and MRI has rekindled interest in sectional anatomy (i.e., the study of structures as they appear on the surface of cross-sectional or longitudinal sections through a cadaver).
The basis for all communication in human gross anatomy and related basic and clinical sciences is standardized and universally accepted. A precise terminology enables us to name structures to distinguish them from all other structures and to relate the position of these named structures to the rest of the body so they can be located with consistency and precision.
In the dissecting laboratory, we assume, by convention, that our subject is standing in the anatomical position (Figure 1-1): that is, standing erect with (1) the toes pointed forward, (2) the eyes directed to the horizon, (3) the arms by the sides, and (4) the palms of the hands facing forward.
There are two basic ways to visualize deep structures of the human body. One is to dissect down to the area of interest; the other is to cut through the cadaver in defined planes (see Figure 1-1). Sections also can be obtained from a living patient with CT or MRI.
The following terms are presented in pairs because each term has an opposite (Figure 1-2). Again, the assumption is that our subject is in the anatomical position.
|Anterior (ventral)||Toward the front of the body|
|Posterior (dorsal)||Toward the back of the body|
|Superior (cranial)||Toward the top of the head|
|Inferior (caudal)||Toward the soles of the feet|
|Medial||Toward the median plane|
|Lateral||Away from the median plane|
|Proximal (central)||Toward the trunk|
|Distal (peripheral)||Away from the trunk|
|Superficial||Toward the skin or body surface|
|Deep||Toward the interior of the body|
|Ipsilateral (homolateral)||On the same side|
|Contralateral||On the opposite side|
|Palmar surface of hand||Anterior surface of hand|
|Dorsal surface of hand||Posterior surface of hand|
|Plantar surface of foot||Inferior surface of foot|
|Dorsal surface of foot||Superior surface of foot|
Cartilage is a specialized supporting connective tissue. It consists of cells (chondroblasts, which give rise to chondrocytes) contained within a ground substance in the form of a rigid gel. There are no neurovascular elements within cartilage; instead, nutrients diffuse through the ground substance to the enclosed chondrocytes. No calcium salts are present; therefore cartilage does not appear on radiographs.
Fibrocartilage contains proportionately more collagen fibers, which are arranged in a parallel fashion for high tensile strength. It is found in tendon insertions and intervertebral discs (not including the pulpal nucleus).
Although cartilage is a rigid tissue, its unique structure allows it to grow as most soft tissues do. Cartilage can increase in size in two ways: (1) by internal growth, in which young chondrocytes proliferate within the cartilage, and (2) by appositional growth, in which a surface perichondrium consisting of a fibrous outer layer and a chondroblastic inner layer lay down surface cartilage.
Bone is unlike cartilage in that the intercellular matrix becomes calcified for greater rigidity and strength. Calcification, however, prevents diffusion of nutrients, and each cell within the matrix must therefore have a direct vascular supply.
Because of its rigid structure, interstitial growth is not possible. Appositional growth takes place only below the covering periosteal layer of bone. Periosteum consists of a fibrous outer layer and a cellular inner layer of osteoblasts, which form the bony matrix.
The adult skeleton is divided into an axial and an appendicular skeleton. The axial skeleton comprises the skull, the vertebral or spinal column, the ribs, and the sternum. The appendicular skeleton includes the bones of the upper and lower limbs. The individual bones and their numbers are illustrated in Figure 1-3.
Long bones are hollow tubes, shafts, or diaphyses that are capped at both ends by knoblike epiphyses. A section through a long bone (Figure 1-4) reveals (1) an outer compact layer for rigidity, (2) an inner cancellous or spongy layer consisting of trabeculated bone for inner support, and (3) a marrow space containing blood cell–forming tissues in active red marrow or just plain fat in inactive yellow marrow.
The blood supply to long bones (Figure 1-5) is from the following three different sources: (1) nutrient arteries pierce the shaft and supply all layers to the marrow cavity within, (2) periosteal arteries supply periosteum and some adjacent compact bone, and (3) epiphyseal arteries supply the epiphyses and the adjacent joint structures.
Short bones are similar to long bones, except they are cuboidal rather than tubular in shape and lack the shaft of long bones. They are usually six-sided, with cartilage covering the articular surfaces. Short bones consist of the same layered structures as long bones but have no epiphyses. The carpal bones of the wrist and the tarsal bones of the ankle are short bones.
Flat bones are thin and flat and are found in the vault of the skull and the scapula. They consist of a sandwich: two layers of compact bone encasing a cancellous layer called the diploë. The diploic layer contains red bone marrow.
Irregular bones are bones that fit none of the previous descriptions. Some irregular bones are mainly cancellous bone covered with only thin layers of compact bone. Others, such as the lacrimal bone (a small delicate bone of the orbit), consist only of a single compact layer. Still others, such as the maxilla (upper jaw), are invaded and hollowed by nasal mucosa during development, resulting in pneumatic bones. Pneumatic bones consist of thin compact bone surrounding an air-filled cavity or sinus.
Sesamoid bones (from the Greek word sesamon, meaning “like a seed”) are not actually part of the skeleton. They occur rather in some tendons of the hands, feet, and knee where the tendon rubs against bone. The patella (knee cap) is a smooth, rounded, sesamoid bone found within the tendon of the quadriceps femoris muscle. Articular cartilage covers the areas in contact with bone.
The surface of individual bones is marked by several features that reflect (1) attachments of muscles and ligaments, producing raised areas; (2) passage of nerves and vessels through or over the bone, producing openings and depressions; and (3) articulations with other bones, producing joint surfaces that are raised or depressed. Some terms are self-descriptive, but most are not intuitive without a background in Latin and Greek.
(From the Greek, meaning “knuckle”) The rounded or widened end of a bone with a smooth articular surface covered by cartilage (e.g., medial and lateral condyles of the femur and the condyles of the mandible)
Large bony traction processes found only on the superior end of the femur (e.g., the greater and lesser trochanter of the femur, which provide attachment for large, powerful muscles of the lower limb)
Two bony prominences found only on bones of the leg that serve to bind the lower leg to the ankle below (e.g., the medial malleolus on the inferior end of the tibia and the lateral malleolus on the inferior end of the fibula)
A gently rounded depression that in some cases provides space for the muscles (e.g., supraspinous and infraspinous fossae of the scapula) and in other cases denotes the smooth concave area for joint surfaces (e.g., the glenoid fossa of the scapula and the mandibular fossa of the skull)
Linear bony depressions that accommodate cylindrical or tubular structures (e.g., the bicipital or intertubercular groove of the humerus accommodates a tendon of the biceps muscle, and the superior sagittal sulcus accommodates the superior sagittal venous sinus within the skull)
An opening that has length through bone; when a canal emerges onto the surface of the bone, that surface opening is sometimes referred to as a foramen (e.g., the infraorbital canal exits onto the face as the infraorbital foramen)
Bone develops from embryonic mesenchyme by one of two mechanisms—intramembranous ossification or endochrondral ossification. Once bone is formed, however, there is no difference in appearance or properties between intramembranous and endochondral bone. The former replaces membrane; the latter replaces cartilage.
During embryonic skeletal development, mesenchymal cells condense as a membrane in the area of the future bone. Osteoblasts differentiate from the mesenchymal cells and lay down a bone matrix at multiple sites that gradually coalesce to form a single bone. Bones of the skull vault and face develop in this fashion and are separated by fibrous sutures that are remnants of the bone precursor membrane. The clavicle develops in this fashion.
The remainder of the skeleton undergoes a slightly more complicated process of endochondral ossification (see Figure 1-5). Each of these bones is preformed in cartilage during early embryonic development. During the sixth to eighth weeks of embryonic development, cartilage within the center of the future bone shaft dies and is replaced by invading osteoblasts that form the primary center of ossification. The perichondrium surrounding the shaft becomes periosteum and it lays down an intramembranous collar of bone around the primary center. All the primary centers develop before birth. Invading vascular tissue hollows the shaft to form the medullary cavity that contains red bone marrow.
At birth, secondary centers of ossification develop in the epiphyses, or ends, of the long bones and increase in size until they ultimately fuse with the primary centers to form a complete bone. Up until maturation after puberty, a plate of remaining cartilage, the epiphyseal plate, separates the epiphyses from the shaft. Short bones do not have a shaft and develop in the same manner as secondary centers of ossification.
The cartilage of the epiphyseal plate continues to proliferate at these sites and contributes to the increase in length of the entire bone. The shape of the bone is maintained by selective apposition and resorption (bone remodeling). During adolescence, two competing phenomena occur. The growth rate of long bones accelerates, and at the same time, hormonal changes cause gradual ossification of the epiphyseal plates (synostosis). Thus complete ossification of long bones results in cessation of growth in the adult. Cartilage remains at both ends, covering the epiphyses as articular cartilage.
Mineralized bone appears on radiographs, but cartilage does not. Secondary centers and short bones begin to ossify and mineralize after birth in a more or less predictable sequence as the child grows. Knowing when these various centers ossify enables us to determine bone or skeletal ages of children.
Synarthrodial joints allow no movement between the bones they unite. A good example is the flat bones of the skull, which are bound together as a rigid entity. Amphiarthrodial joints are partially movable, and diarthrodial joints are freely movable.
Sutures are found only between the bones of the skull (Figure 1-6). In the fetal skull the sutures are wide, and the bones present smooth opposing surfaces. This spacing between the flat bones of the skull allows a slight degree of movement between the skull bones during passage of the head through the birth canal (birth molding).
After birth, the sutures become quite rigid (synarthrodial) during infancy and early childhood, allowing no movement between skull bones. The developing sutures differentiate into one of three types (see Figure 1-6): (1) a squamous suture in which the bones simply overlap obliquely but are rendered immobile by intervening fibrous tissue; (2) a serrated suture, which develops sawtoothed interdigitating projections from the opposing bones; and (3) a denticulate suture, which features interlocking dove-tailed surfaces.
A syndesmosis, unlike other fibrous joints, is partially movable (amphiarthrodial) and is a joint in which the two bony components are farther apart, united by a fibrous interosseous membrane (Figure 1-7, A). Examples are the joint between the two bones of the forearm (radius and ulna) and the joint between the bones of the leg (fibula and tibia). Syndesmoses also are found between the laminae of the vertebrae.
A gomphosis is a unique joint in the form of a peg-and-socket articulation between the roots of the teeth and the maxillary and mandibular aleveolar processes (Figure 1-7, B). Fibrous tissue organized as the periodontal ligament anchors the tooth securely in the socket. Mobility of this joint indicates a pathological state affecting the supporting structures of the tooth.
Primary cartilaginous joints develop between two bones of endochondral origin. They are characterized by a solid plate of hyaline cartilage between apposing surfaces (Figure 1-8, A). The cartilage plate functions in exactly the same manner as the epiphyseal plate between primary and secondary centers of long bones and provides an area of growth between bones. An example is the sphenooccipital synchondrosis in the young skull between the sphenoid bone and the occipital bone of the skull. This joint fuses after adolescence.
A secondary cartilaginous joint, or symphysis, is a partially movable (amphiarthrodial) joint in which the apposing bony surfaces are covered with cartilage but separated by intervening fibrous tissue or fibrocartilage (Figure 1-8, B). Symphyses are found in the midline of the body and include the joints between vertebral bodies (intervertebral discs), between right and left pubic bones (symphysis pubis), and in the newborn skull between the right and left halves of the mandible (symphysis menti). The symphysis menti starts to fuse during the first year of life to form a single bone, the mandible.
Articular cartilage coats the surfaces of the apposing bones. The cartilage may be hyaline (where bones of endochondral origin articulate) or may be fibrocartilage (where bones of intramembranous origin articulate). Typical of cartilage, this layer contains no blood vessels or nerves but must instead be nourished from the epiphyseal vessels of the bone and derives nourishment from the synovial lubricating fluid within the joint. A joint cavity exists between the articular surfaces of the apposing bones. The joint cavity is not large but contains enough space to allow a thin intervening film of synovial fluid. A capsular ligament surrounds the joint like a fibrous sleeve and attaches to the circumference of both bones to completely enclose the joint cavity. A synovial membrane consisting of loose areolar tissue contains a rich supply of capillaries. This membrane lines the inner aspect of the capsular ligament but does not line the articular surfaces of the cartilage. The synovial membrane secretes a lubricating synovial fluid, or synovium, into the joint cavity. Some joints contain discs interposed between the articular surfaces (Figure 1-9, B). A disc or meniscus is a fibrocartilaginous or sometimes condensed fibrous structure found within some joint cavities. These padlike structures divide the joint cavity into two compartments allowing for two types of movements, one for each subdivided joint compartment. The temporomandibular, or jaw, joint is an example of a synovial joint containing a disc.
Epiphyseal arteries that supply the epiphyses of long bones also supply the synovial joints between the long bones. Hilton’s law states that the nerves that supply the muscles that move a synovial joint send sensory branches to supply the joint. There are two types of sensory nerve endings that convey two kinds of messages to the CNS. Proprioceptive endings, or Ruffini corpuscles, in the capsular ligament convey a sense of position and degree of movement of a joint. Pain receptors in the synovial membrane indicate whether the allowable degree of movement is being overtaxed.
Freely movable synovial joints may be classified in different ways (Figure 1-10). One classification is based on the number of axes in which a joint can be moved (i.e., uniaxial, biaxial, or multiaxial). The shape or form of the opposing bony surfaces determines the degree of movement.
One apposing bony surface is cylindrical, the other is reciprocally concave, and the joint allows movement in one plane. An example is the humeroulnar (elbow) joint that allows only flexion and extension.
Muscle is a specialized tissue that has the ability to contract and produce movement. The three kinds of muscle tissue within the body differ from each other in their histo-logical appearances and in their ability to be controlled voluntarily. These tissues are (1) skeletal muscle, (2) smooth muscle, and (3) cardiac muscle.
Skeletal muscle is so named because of its attachment to bones. Because muscles span joints, they have the ability to move one bone in relation to another; for example, the brachialis muscle flexes the elbow or the masseter muscle elevates the mandible. Contraction of all skeletal muscle is under voluntary control. Though operation of some skeletal muscles is “automatic,” such as that of the muscles of respiration, which continue to work during sleep, we can still voluntarily override them, such as in holding one’s breath. Skeletal muscle is also known as striated muscle because it appears striped in histological sections.
The names of muscles are generally descriptive and give us either an indication of (1) shape (e.g., trapezius muscle), (2) number of origins (e.g., triceps, biceps), (3) location (e.g., temporalis), (4) number of bellies (e.g., digastric), (5) function (e.g., levator veli palatini), or (6) origin and insertion (e.g., thyrohyoid).
Muscle fibers are the basic functional and anatomical units of muscle. Fibers are actually elongated muscle cells, ranging in length from several millimeters to several centimeters and contain several nuclei, specialized protoplasm or sarcoplasm, and myofibrils within the sarcoplasm. The cell membrane, or sarcolemma, encases the cell.
The cells derive their individual blood and nerve supply via sheets of fibrous connective tissue membranes through which the vessels and nerves run. Surrounding the individual muscle cells is a fibrous sheet of endomysium. Surrounding a bundle of several fibers (fasciculus) is a fibrous sheet of perimysium. Finally, surrounding several bundles, there is an overall fibrous coating of epimysium covering the entire muscle. Each of the three levels of surrounding membranes is interconnected, allowing vessels and nerves entering the outer layer eventually to reach individual fibers (Figure 1-11).
Groups of muscles in the limbs can be bound into compartments by intermuscular septa, which separate various groups of muscles. An example is the anterior compartment of the arm containing flexor muscles and the posterior compartment of the arm containing extensor muscles (see Chapter 9).
Most sites of muscular attachment are to bone. The muscle fibers do not attach directly but rather through specialized extensions of the fibrous tissue coverings in the form of tendons. The tendons may take various forms.