Transversospinales Muscles The transversospinales

The multifidus layer of the transversospinales musculature is highlighted on

this page, and the deeper rotatores are evident on the opposite page along with the deeper intersegmental muscles. The multifidus muscles span three to fi ve vertebral levels in their span from the sacrum to the second cervical vertebra, while the deepest member, the rotatores, typically span only one to two vertebrae. The transversospinales muscles assist their more superficial counterparts with extension of the vertebral column and play important roles in the maintenance of posture.

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Dissection of multifidus muscles

Posterior view

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Dissection of multifidus and intersegmental muscles

The small intersegmental muscles — the interspinales muscles, intertransversarii muscles, levatores costarum,

and subocciptal muscles — in general span a single intervertebral joint. The interspinales and intertransversarii muscles contribute little to any significant vertebral movements. They contain large numbers of sensory neurons within their muscultendinous fasciculi. These spindle-like sensory receptors in the muscles monitor muscle tension. These small muscles, with their poor mechanical advantage, probably function as receptors that monitor the regional movements of the vertebral column and supply feedback that infl uences the action of the larger surrounding muscles. Associated deep in the junction of the cranium and vertebral column are the four suboccipital muscles. The suboccipital muscles are homologous to the other deep muscles at more inferior vertebral levels, but are developmentally modified and enlarged to function with their specialized vertebral counterparts — the axis, atlas, and occipital bone.

11Semispinalis cervicic muscle

12Medial lumbar intertransversarii muscle

17External intercostal muscle

18Internal intercostal muscle

19Middle scalene muscle

Dissection of upper deep intersegmental muscles on left

The hypaxial muscles develop from the hypomere of each somite’s myotome and form the lateral and ventral muscle wall of the trunk. As the hypomeres migrate to form the ventrolateral muscle wall of the

trunk, a repeating segmental pattern emerges. This common muscle pattern is present in the anterior and lateral muscles of the neck, the thorax, the abdomen, and in a modifi ed form in the wall and fl oor of the pelvis. Each hypomere contributes six basic muscles, per side, to the trunk wall. The six muscles are a ventral muscle, a series of four superfi cial to deep lateral muscles, and a subvertebral muscle. This simple, eloquent design runs the entire length of the trunk. Understanding and recognizing this pattern of design not only clarifies trunk wall anatomy, but also helps simplify the task of learning the myriad of hypaxial trunk muscles. These hypaxial trunk muscles are the flexors and rotators of the vertebral column. They also support the internal viscera of the abdomen and thorax and play important roles in respiration, vocalization, urination, and defecation. The ventral (anterior) ramus of each spinal nerve supplies all of the hypaxial muscles. The hypaxial muscle pattern and the muscles that form the pattern are summarized below. On the next two pages the pattern is clearly demonstrated.

Hypaxial Muscle Pattern

Ventral musculature

Four-layered lateral musculature

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The dissection photos on this and the facing page clearly depict the pattern of design that arises from the hypomere migration in the trunk wall. Note that

both the ventral and subvertebral muscles are reduced in the thorax because the sturdy thoracic cage leads to a lack of mobility in the thoracic vertebral column. Also, note that the lateral supracostal muscles of the neck and thorax are annexed by the pectoral girdle to support the unattached upper limb. The clear relationship of the serratus anterior and its abdominal homologue – the superfi cial lamina of the external oblique muscle – is also evident, as well as the continuity of the deep lamina of the external oblique and its homologue, the external intercostal muscle. Finally, note how the subvertebral psoas major is annexed away from the sacrum and onto the lower limb.

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