Columna Vertebral y Caja torácica

Organization of the Human Skeleton

The human skeletal system contains a total of $206$ bones, primarily categorized into two main divisions: the axial skeleton, which includes the head and trunk, and the appendicular skeleton, which includes the limbs. The axial skeleton consists of exactly $80$ bones. Within the axial skeleton, the skull and its associated bones account for $29$ elements: the cranium ($8$ bones), the face ($14$ bones), the auditory ossicles ($6$ bones), and the hyoid bone ($1$ bone). The thoracic cage (jaula torácica) comprises $25$ bones, consisting of the sternum ($1$ bone) and the ribs ($24$ bones). The vertebral column (columna vertebral) is composed of $26$ distinct bone structures: $24$ individual vertebrae, the sacrum ($1$ bone), and the coccyx ($1$ bone). The appendicular skeleton contains the remaining $126$ bones of the body.

Divisions and Curvatures of the Vertebral Column

The vertebral column is organized into five primary regions, each containing a specific number of vertebrae. The cervical region (región cervical) contains $7$ vertebrae, denoted as $C_1$ through $C_7$. The thoracic region (región torácica) consists of $12$ vertebrae, denoted as $T_1$ through $T_{12}$. The lumbar region (región lumbar) contains $5$ vertebrae, denoted as $L_1$ through $L_5$. Below these are the sacral region (sacra) and the coccygeal region (coccígea). In a healthy adult, the column displays four major curvatures: the cervical curvature, thoracic curvature, lumbar curvature, and sacral curvature.

These curvatures change significantly throughout human development. A fetus at $2$ months and $6$ months, as well as a newborn, possesses a single primary curvature. As development progresses, secondary curvatures emerge. By age $4$, the cervical and lumbar curves begin to stabilize to support head movement and an upright posture. This process continues through age $13$ and into adulthood. The cervical and lumbar curves are secondary curvatures that appear after birth, while the thoracic and sacral curves are primary curvatures present since the fetal stage.

Anatomy and Structure of a Typical Vertebra

A typical vertebra consists of several specialized structures designed for weight-bearing and protection. The cuerpo vertebral (vertebral body) is the thick, anterior portion that supports the weight of the body. The arco vertebral (vertebral arch) forms the posterior portion of the vertebra, enclosing the agujero vertebral (vertebral foramen). This arch is formed by two pedículos (pedicles) and two láminas (laminae). When vertebrae are stacked, the sequence of vertebral foramina creates the conducto vertebral (vertebral canal), through which the spinal cord passes. The gaps between the pedicles of adjacent vertebrae form the agujeros intervertebrales (intervertebral foramina), providing paths for spinal nerves.

The vertebral structure includes seven processes. The apófisis espinosa (spinous process) projects posteriorly and inferiorly, while the two apófisis transversas (transverse processes) project laterally. Four apófisis articulares (articular processes)—two superior and two inferior—extend from the junction of the pedicles and laminae. Each of these processes has a carilla articular (articular facet) that forms a joint with the vertebra above or below it. Between the vertebral bodies, the disco intervertebral (intervertebral disc) provides cushioning and flexibility.

Regional Characteristics of Vertebrae

Regional differences in the structure of vertebrae reflect the specific functional demands of each area of the body. Cervical vertebrae ($7$) are characteristically small with an oval body and curved faces. They have a large vertebral foramen and a long, often bifid (parted) spinous process directed downward. Their most distinct feature is the agujero transverso (transverse foramen) found in each transverse process. Their primary functions are to sustain the cranium, stabilize the brain and spinal cord, and facilitate controlled head movement.

Thoracic vertebrae ($12$) possess a medium-sized, heart-shaped body with flat faces and specialized fositas costales (costal facets) for rib articulation. The vertebral foramen is smaller than in the cervical region. The spinous process is long, thin, and points sharply downward. Except for $T_{11}$ and $T_{12}$, the transverse processes have facets for rib articulation. These vertebrae support the weight of the head, neck, upper extremities, and thoracic organs, while allowing changes in thoracic cage volume during respiration.

Lumbar vertebrae ($5$) are the most massive, with an oval body and flat faces. They have the smallest vertebral foramen of the three regions. The spinous process is thick, flat, and points directly posteriorly. They lack transverse foramina and costal facets. Their primary role is to support the total weight of the head, neck, upper limbs, and the organs of the thoracic and abdominal cavities.

Specialized Vertebrae: Atlas and Axis

The first two cervical vertebrae are uniquely structured to allow specialized movement of the head. The Atlas ($C_1$) is distinct because it lacks a vertebral body and a spinous process. It consists of an anterior arch and a posterior arch, featuring a large vertebral foramen and a fosita del diente for articulation with the second vertebra. The Axis ($C_2$) is characterized by the Diente (dens or odontoid process), which extends upward into the vertebral foramen of the Atlas. The ligamento transverso (transverse ligament) holds the dens in place against the Atlas, creating a pivot joint that allows the head to rotate. The $C_7$ vertebra is known as the vertebra prominens due to its particularly long spinous process that can be felt at the base of the neck.

Clinical Abnormalities of the Spinal Column

Abnormalities in the spinal column can significantly affect functionality and balance. Cifosis (Kyphosis) is an exaggerated posterior curvature of the thoracic region, often resulting in a "hunchback" appearance. Potential causes include osteoporosis leading to vertebral compression, chronic muscle contractions, or abnormal bone growth. Lordosis, or "swayback," is an exaggerated anterior curvature of the lumbar region that causes the abdomen and buttocks to protrude abnormally. It is often caused by abdominal obesity or weakness in the abdominal musculature.

Escoliosis (Scoliosis) is the most common spinal deformity, characterized by an abnormal lateral (side-to-side) curvature of one or more mobile vertebrae. While it can result from developmental issues like incomplete vertebral formation or muscle paralysis (such as in poliomyelitis), $80\%$ of cases are idiopathic, meanings the cause is unknown. Idiopathic scoliosis typically appears in girls during rapid adolescent growth spurts. Treatments involve exercise and corsets (braces), though severe cases may require surgical intervention with the implantation of metallic rods or cables to straighten the spine.

The Sacrum and Coccyx

The sacrum and coccyx are composed of vertebrae that fuse during adulthood. The sacrum (sacro) consists of a base, a prominent ridge called the promontorio del sacro (sacral promontory), and lateral wings known as alas. Its posterior features include the cresta sacra media and lateral, sacral tuberosities, and the carilla auricular for hip articulation. The conducto sacro ends at the hiato sacro. There are four pairs of agujeros sacros (sacral foramina) for nerve passage. The pelvic surface (cara pélvica) shows transversal lines indicating where fusion occurred. The coccyx (cóccix) is a small, terminal bone composed of fused vertebrae with structures called astas that connect to the sacrum.

The Thoracic Cage and Sternum

The jaula torácica (thoracic cage) provides protection for vital organs and a structural framework for breathing. The sternum (esternón) is the central anterior bone, consisting of the manubrio (manubrium) at the top, the cuerpo (body) in the center, and the apófisis xifoides (xiphoid process) at the bottom. The manubrio contains the escotadura yugular (jugular notch) and articular facets for the clavicles.

There are $12$ pairs of ribs. Costillas verdaderas (true ribs, $1$-$7$) attach directly to the sternum via individual cartílagos costales. Costillas falsas (false ribs, $8$-$12$) do not attach directly to the sternum. Within the false ribs, pairs $8$-$10$ are vertebrocondrales because their cartilages fuse together before joining the cartilage of the $7$th rib. Pairs $11$ and $12$ are costillas flotantes (floating ribs) because they lack any anterior attachment. A typical rib features a cabeza (head) for vertebral articulation, a cuello (neck), a tubérculo (tubercle) for transverse process articulation, and a surco costal (costal groove) on the internal surface for nerves and vessels.

Conclusion and Competencies

Integrated knowledge of the structure and function of the human body allows for the identification of homeostatic mechanisms at molecular, cellular, tissue, organic, and system levels throughout the human life cycle. Analyzing structural and functional abnormalities is essential for recognizing the changes manifested during disease processes. This comprehensive study is based on the work of Martini, F. H. from Fundamentals of Anatomy and Physiology ($2008$).