Overview of the Locomotor System

The Trunk

The trunk consists of three segments: thorax, abdomen, and pelvis, each of which is bounded by body wall and contains a cavity. The thoracic cavity lies cranial to the diaphragm, whereas the abdominal cavity lies caudal. The pelvic cavity is defined by the borders of the bony pelvis and communicates with the abdominal cavity.

Dorsally, the roof of all three cavities is formed by the spinal column and associated muscles. Vertebrae develop segmentally from somitic sclerotomes, whereas muscles develop from somitic myotomes. Within each myotome is found a single nerve leaving the central neural tube. Motor innervation in the adult is therefore segmental. Sensory innervation in the adult is also segmental, although the patterning of neuroectoderm to form skin is not clearly defined, and dermatomes often overly one another.

The ventral and lateral body walls are initally formed by somatopleure (ectoderm and lateral plate mesoderm), but is later invaded by somitic cells migrating ventrally. These differentiate to form ribs and sternum and associated muscles.