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Dissection of lower deep intersegmental muscles on left

Posterolateral view

Intersegmental Muscles The intertransversarii muscles are a mixed group that

are technically misnamed. The epaxial intertransverse

muscles (present at cervical, thoracic, and lumbar

levels) are the “true intertransverse” muscles. They attach to the transverse elements of the vertebral arch. The hypaxial

intertransverse muscles should be named intercostal muscles. They are only present in the cervical and lumbar regions and

attach to the costal processes (ribs) of the cervical and lumbar vertebrae, which are unfortunately named transverse processes even though they are not homologous with the thoracic transverse processes. These cervical and lumbar transverse

processes are homologous with the thoracic ribs. There are no thoracic hypaxial intertransverse muscles because they are

already present as the intercostal muscles and in this region they are properly named.

Rotatores and Intersegemental Muscles

 1 Rotatores thoracis muscle

 2 Rotatores lumborum muscle

 3 Levatores costarum muscle

 4 Interspinales thoracis muscle

 5 Interspinales lumborum muscle

 6 Thoracic intertransversarii muscle

 7 Medial lumbar intertransversarii muscle

Other Muscles and Structures

 8 Intertransversarii laterales

 lumborum muscle - dorsal part

 9 Intertransversarii laterales

 lumborum muscle - ventral part

10 Internal intercostal muscle

 11 Quadratus lumborum muscle

12 Iliocostalis muscle (cut)

13 Multifidus muscle (cut)

14 Trapezius muscle

15 Latissimus dorsi muscle

16 Rib 12

17 Iliac crest

18 Thoracolumbar fascia

19 Supraspinous ligament

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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 clarifi es trunk wall anatomy, but also helps simplify the task of learning the myriad

of hypaxial trunk muscles. These hypaxial trunk muscles are the fl exors 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 Muscles 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

Hypaxial Muscle Pattern

 Ventral musculature

 Four-layered lateral musculature

 Supracostal or outermost muscle layer

 External muscle layer

 Middle muscle layer

 Internal muscle layer

 Subvertebral musculature

Lumbar Hypaxial Muscles

 Ventral musculature

 Rectus abdominis muscle

 Pyramidalis muscle

 Four-layered lateral musculature

 Supracostal layer

 External oblique muscle - superficial lamina

 External layer

 External oblique muscle - deep lamina

 Middle layer

 Internal oblique muscle

 Cremaster muscle

 Intertransversarii laterales lumborum muscle - dorsal part

 Internal layer

 Transversus abdominis muscle

 Quadratus lumborum muscle

 Intertransversarii laterales lumborum muscle - ventral part

 Subvertebral musculature

 Psoas major muscle (annexed by the limb)

 Psoas minor muscle

Pelvis/Perineal Hypaxial Muscles

 Ventral musculature

 Not present as it terminates on the pubic crest

 Four-layered lateral musculature

 Supracostal layer

 Not present

 External layer

 Obturator externus muscle (annexed by the limb)

 Bulbospongiosus muscle

 Ischiocavernosus muscle

 Superficial transverse perinei muscle

 Superficial external anal sphincter

 Middle layer

 Obturator internus muscle (annexed by the limb)

 Deep transverse perinei - male

 Compressor urethrae - female

 Sphincter urethrovaginalis -female

 External urethral sphincter

 Deep external anal sphincter

 Internal layer

 Levator ani muscle

 Ischiococcygeus muscle

 Subvertebral musculature

 Not present as psoas is annexed by the limb

Cervical Hypaxial Muscles

 Ventral musculature

 Geniohyoid muscle

 Thyrohyoid muscle

 Superior omohyoid muscle

 Inferior omohyoid muscle

 Sternothyroid muscle

 Sternohyoid muscle

 Four-layered lateral musculature

 Supracostal layer

 Levator scapulae muscle

 External layer

 Posterior scalene muscle

 Middle layer

 Middle scalene muscle

 Lateral posterior cervical intertransversarii muscle

 Internal layer

 Anterior scalene muscle

 Anterior cervical intertransversarii muscle

 Subvertebral musculature

 Longus capitis muscle

 Longus colli muscle

Thoracic Hypaxial Muscles

 Ventral musculature

 Sternalis muscle (present in about 10% of people)

 Four-layered lateral musculature

 Supracostal layer

 Serratus posterior superior muscle

 Serratus posterior inferior muscle

 Rhomboideus major muscle (annexed by the limb)

 Rhomboideus minor muscle (annexed by the limb)

 Serratus anterior muscle (annexed by the limb)

 External layer

 External intercostal muscle

 Middle layer

 Internal intercostal muscle

 Internal layer

 Innermost intercostal muscle

 Subcostal muscle

 Transversus thoracis muscle

 Diaphragm

 Subvertebral musculature

 Longus capitis muscle

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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

Hypaxial Muscle Pattern

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.

Ventral hypaxial muscles

Anterior view

Lateral supracostal hypaxial muscles

Lateral view

Lateral external hypaxial muscles

Lateral view

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Lateral middle hypaxial muscles

Lateral view

Lateral internal hypaxial muscles

Lateral view

Subvertebral hypaxial muscles

Lateral view

Ventral Musculature

 1 Sternohyoid muscle

 2 Sternothyroid muscle

 3 Thyrohyoid muscle

 4 Omohyoid muscle

 5 Rectus abdominis muscle

Lateral Supracostal Musculature

 6 Levator scapulae muscle

 7 Serratus anterior muscle

 8 Serratus posterior inferior muscle

 9 External oblique muscle (superficial lamina)

Lateral External Musculature

10 Posterior scalene muscle

 11 External intercostal muscle

12 External oblique muscle (deep lamina)

Lateral Middle Musculature

13 Middle scalene muscle

14 Internal intercostal muscle

15 Internal oblique muscle

Lateral Internal Musculature

16 Anterior scalene muscle

17 Innermost intercostal muscle

18 Transversus abdominis muscle

Subvertebral Musculature

19 Longus capitis muscle

20 Longus colli muscle

21 Psoas major muscle

22 Psoas minor muscle

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The muscular wall of the neck arises from the

hypomeres of the cervical somites and develops in accordance with the anterior and lateral

Cervical Hypaxial Muscles

body wall muscle pattern. A close scrutiny of the cervical hypaxial muscles reveals a ventral muscle, which has split into

numerous subdivisions, a four-layered lateral muscle wall where the muscles have lost their sheet-like structure, and a

subvertebral muscle on the anterior surface of the neck vertebrae. The cervical trunk muscles have a variety of functions.

Some of the muscles function to stabilize and move the cervical vertebral column. Some of the muscles assist in raising the

upper ribs. Some are annexed by the upper limb to support the pectoral girdle. The strap-like ventral muscles, which run

from sternum to larynx to hyoid bone to mandible, are active during mastication, swallowing, respiration, and sound production. These seemingly varied muscles are all innervated by the anterior rami of the cervical spinal nerves.

Dissection of neck muscles

Anterior view

Cervical Hypaxial Muscles

 1 Sternohyoid muscle

 2 Sternothyroid muscle

 3 Thyrohyiod muscle

 4 Omohyoid muscle

 5 Geniohyoid muscle

 6 Anterior scalene muscle

 7 Middle scalene muscle

 8 Posterior scalene muscle

 9 Levator scapulae muscle

10 Longus colli muscle

Other Muscles and Structures

 11 Anterior digastricus muscle

12 Mylohyoid muscle

13 Sternocleidomastoid muscle

14 Trapezius muscle

15 Deltoid muscle

16 Pectoralis major muscle

17 Serratus anterior muscle

18 Cricothyroid muscle

19 Stylohyoid muscle

20 Posterior digastricus muscle

21 Subclavian artery

22 Root of brachial plexus

23 Common carotid artery

24 Vagus nerve

25 Thyroid cartilage

26 Thyroid gland

27 Trachea

28 External intercostal muscle

29 Internal intercostal muscle

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Dissection of cervical hypaxial muscles

Anterior view

Dissection of cervical hypaxial muscles

Anterolateral view

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