1: General Concepts

Chapter 1imageGeneral Concepts

1 The Study of Anatomy

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.

2 Terminology

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.

TERMS OF RELATIONSHIP

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.

Term Definition
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

3 Skeleton

CARTILAGE

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.

During early development, most of the fetal skeleton is present as cartilage, and most of this cartilage is subsequently replaced by bone during fetal and postnatal development.

BONE

Bone, like cartilage, is a living tissue consisting of cells or osteoblasts, which give rise to osteocytes within an organic framework or matrix.

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.

Classification

By Shape

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.

Surface Features

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.

Following is a list of bony features that will be encountered in the study of bones as they are presented throughout the book.

Raised Markings or Elevations

Development

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.

Endochondral Ossification

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.

4 Joints

A joint is an articulation or union between two or more bones. Joints may be classified according to the degree of possible movement and by the tissues that bind the bones together.

BY JOINT TISSUES

Joints between bones may be composed of fibrous connective tissue, cartilage, a combination of connective tissue and cartilage, or cartilage and a joint cavity.

Fibrous Joints

There are three types of fibrous joints: (1) suture, (2) syndesmosis, and (3) gomphosis.

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 (Synchondroses)

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.

Secondary Cartilaginous Joints (Symphyses)

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.

SYNOVIAL JOINTS

A synovial joint is freely movable (diarthrodial) and is typical of nearly all the joints of the upper and lower limbs. Synovial joints have a number of characteristic features (Figure 1-9, A).

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.

Classification

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.

5 Muscular System

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

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.

Parts

Skeletal muscle consists of a fleshy portion and a fibrous or tendinous portion.

Jan 5, 2015 | Posted by in General Dentistry | Comments Off on 1: General Concepts
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