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.

Types

Hyaline (from the Greek word hyalos, meaning “glass”) cartilage is a bluish-white, translucent structure. Nearly all of the fetal skeleton is hyaline cartilage. In the adult, its remnants are:

1. Articular cartilage, which is smooth and slippery and persists at the ends of cartilaginous bones to line articular surfaces of movable joints

2. Costal cartilages, which persist at the sternal ends of the ribs

3. Respiratory cartilages, consisting of the movable external nose and septum, larynx, trachea, and bronchial tree

4. Auditory cartilages, which include the external auditory meatus and the cartilaginous portion of the auditory (pharyngotympanic) tube

Elastic cartilage is pliable and yellowish in color because of the presence of elastin fibers. It is found in the external ear and in the epiglottis.

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).

Growth

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

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.

Functions

Bone has the following functions:

1. Support. Bones provide a rigid framework for the body.

2. Movement. Bones act as levers for muscles. Muscle usually attaches to approximating bones, and these attachments are able to move one bone in relation to another.

3. Protection. The brain and the thoracic viscera are protected by bone.

4. Hemopoiesis. The principal blood cells are formed in the marrow space of bone.

5. Storage. Calcium and phosphorus are stored in bone as body reserves.

Classification

By Region

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.

Figure 1-3 Skeleton. A, Anterior. B, Posterior.

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.

Figure 1-4 Features of a long bone. A, Adult tibia. B, Longitudinal cut to show internal features. C, Tibia of a child.

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.

Figure 1-5 Three stages in development of a long bone.

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

Depressions and Openings

Fossa

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)

Groove, or Sulcus

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)

Foramen

A hole that allows structures (usually nerves and vessels) to pass through the bone (e.g., the foramen ovale [round hole] and the foramen magnum [large hole] in the skull)

Canal

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)

Notch

A “bite” from the edge of the bone that transmits vessels and nerves, like an incomplete foramen (e.g., the suprascapular notch of the scapula and the mandibular notch of the mandible)

Fissure

An elongated space between two bones of the skull (e.g., the superior and inferior orbital fissures)

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.

Intramembranous Ossification

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.

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.

Postnatal Growth

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.

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 DEGREE OF MOVEMENT

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.

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).

Figure 1-6 Sutures. A, Squamous. B, Serrated. C, Denticulate.

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.

Figure 1-7 Other examples of fibrous joints. A, Syndesmosis. B, Gomphosis.

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.

Figure 1-8 Cartilaginous joints. A, Primary cartilaginous joint (synchondrosis). B, Secondary cartilaginous joint (symphysis).

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).

Figure 1-9 Synovial joint. A, Typical. B, With articular disc.

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.

Blood and Nerve Supply

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.

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.