The trunk is the central part of the body from which the neck and limbs extend. Its dorsal aspect consists of a group of structures that comprise the back. The structures of the back include skin and subcutaneous tissue, bony and muscular elements, spinal cord and meninges, vessels and nerves. The bony and muscular elements of the dorsal trunk support the body’s weight, transmit forces to the lower limbs through the pelvis, carry and position the head, and maneuver the upper limb and vertebral column.
The skeletal framework of the dorsal trunk mainly consists of the vertebral column, formed by approximately 33 vertebrae and their associated intervertebral discs and ligaments. These vertebrae vary in number and characteristics, depending on their region. There are seven cervical, twelve thoracic, five lumbar, five sacral, and three or four coccygeal vertebrae. The dorsal trunk is also formed by the proximal posterior portion of the ribs.
Muscles of the back are divided into two major groups, based on embryological origin and innervation. The extrinsic back muscles include both superficial and intermediate muscles. The superficial group of muscles consists of muscles related to and involved in upper limb movement, this includes the trapezius, latissimus dorsi, levator scapulae, and the rhomboids. The intermediate group of muscles consists of muscles attached to the rib, and they are the serratus posterior inferior and serratus posterior superior. These two thin sheets of muscles are involved in thoracic (respiratory) movement. Both groups are generally innervated by the anterior rami of spinal nerves, except for the trapezius muscle (innervated by accessory nerve CN XI).
The intrinsic back muscles consist of deep muscles developed in the back. They extend from the pelvis to the skull, and include the spinotransversales muscles, the erector spinae and transversospinalis muscles, and the segmental back muscles. These muscles are responsible for maintaining posture and controlling the movement of the vertebral column and head. Another group of deep muscles responsible for head movement are the suboccipital muscles. They are located in the upper cervical region, at the base of the occipital bone. All extrinsic muscles of the back are innervated by the posterior rami of spinal nerves.
The spinal cord is one of the most important structures present in the vertebral canal of the dorsal trunk. It serves as a major reflex center and conduction pathway between the body and brain. The spinal cord is a continuation of the medulla oblongata. It is protected by the vertebrae and their associated ligaments and muscles, the spinal meninges, and the cerebrospinal fluid. The spinal meninges are membranes that encompass the spinal cord and its nerve roots, and contain cerebrospinal fluid in which these structures are suspended in. They are the dura, arachnoid, and pia maters.
The vasculature of the dorsal trunk includes the occipital, vertebral, transverse cervical, posterior intercostal, subcostal and lumbar arteries, and all their associated veins. The main nerves that supply the dorsal trunk region are the greater occipital, lesser occipital, suboccipital, supraclavicular, intercostal, and the lateral cutaneous branches of the intercostal nerves.