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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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First arch muscles

Second arch muscles

Somitic muscles

of head

Third arch muscles

Fourth arch muscles

Head muscles, tongue muscles exposed

Lateral view, right half of mandible removed

 1 Anterior belly of digastricus

 2 Mylohyoid

 3 Tensor veli palatini

 4 Orbicularis oculi

 5 Orbicularis oris

 6 Buccinator

 7 Mentalis

 8 Stylohyoid

 9 Posterior belly of digastricus (cut)

10 Stylopharyngeus

 11 Levator veli palatini

12 Superior pharyngeal constrictor

13 Middle pharyngeal constrictor

14 Inferior pharyngeal constrictor

15 Cricothyroid

16 Genioglossus

17 Hyoglossus

18 Styloglossus

19 Inferior longitudinal muscle

20 Geniohyoid

21 Mucosa of tongue

22 External acoustic meatus

23 Mastoid process

24 Thyroid cartilage

25 Trachea

26 Hyoid bone

27 Mandible (cut)

28 Zygomatic arch

29 Maxilla

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The dissections on this and the opposing page are deep

dissections of the head and neck that expose the palate

and muscular wall of the pharynx and larynx (muscles

Head Muscles

that arise from the third, fourth, and sixth pharyngeal arches). These are the deepest muscles of

the head and neck, and they form the muscular walls of the upper end of the embryonic gut tube.

The dissection below depicts the posterior wall of the pharynx. On the opposing page the pharyngeal wall has been sectioned to reveal the inside of the palate and larynx from behind.

First arch muscles

Second arch muscles

Third arch muscles

Fourth arch muscles

Head muscles, posterior wall of pharynx exposed

Posterior view, cervical vertebrae and occipital bone removed

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First arch muscles

Second arch muscles

Third arch muscles

Fourth arch muscles

Sixth arch muscles

Somitic muscles

of head

Head muscles, posterior wall of pharynx cut and refl ected

Posterior view, cervical vertebrae and occipital bone removed

 1 Masseter

 2 Medial pterygoid

 3 Lateral pterygoid

 4 Stylohyoid

 5 Posterior belly of digastricus

 6 Stylopharyngeus

 7 Levator veli palatini

 8 Palatopharyngeus

 9 Musculus uvulae

10 Superior pharyngeal constrictor

 11 Middle pharyngeal constrictor

12 Inferior pharyngeal constrictor

13 Salpingopharyngeus

14 Posterior crico-arytenoid

15 Oblique arytenoid

16 Transverse arytenoid

17 Styloglossus

18 Pharyngotympanic tube

19 Bony nasal septum

20 Palatine tonsil

21 Tongue

22 Epiglottis

23 Cricoid cartilage

24 Esophagus

25 Trachea

26 Greater cornu of hyoid bone

27 Aryepiglottic fold

28 Pharyngobasilar fascia

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Sectional anatomy broadens perspective and showcases anatomical relationships in ways that are not possible to achieve by dissection alone. The frontal and

parasagittal sections on these pages depict and clarify the relationships of many

of the head muscles and show the relationships these muscles have with other

structures of the head.

Head Muscles

First arch muscles

Second arch muscles

Somitic muscles

of head

Head muscles, frontal section through orbits, nasal cavity, and oral cavity

Posterior view

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Head muscles, parasagittal section through oral cavity and pharynx

Posterior view, section is 1.2 cm lateral to the midline

First arch muscles Second arch muscles Fourth arch muscles Somitic muscles

of head and neck

 1 Temporalis

 2 Masseter

 3 Anterior digastricus

 4 Mylohyoid

 5 Frontal belly of occipitofrontalis

 6 Orbicularis oris

 7 Transversus menti

 8 Risorius

 9 Buccinator

10 Mentalis

 11 Platysma

12 Palatopharyngeus

13 Musculus uvulae

14 Superior pharyngeal constrictor

15 Middle pharyngeal constrictor

16 Inferior pharyngeal constrictor

17 Salpingopharyngeus

18 Superior rectus

19 Inferior rectus

20 Medial rectus

21 Lateral rectus

22 Superior oblique

23 Levator palpebrae superioris

24 Genioglossus

25 Hyoglossus

26 Superior longitudinal muscle

27 Inferior longitudinal muscle

28 Transversus muscle

29 Vertical muscle

30 Geniohyoid

31 Longus capitis

32 Hard palate

33 Mandible

34 Occipital bone

35 Atlas

36 Axis

37 Intervertebral disc

38 Hyoid bone

39 Epiglottis

40 Frontal sinus

41 Frontal lobe of cerebrum

42 Periorbital fat

43 Ethmoidal air cells

44 Superior nasal conchae

45 Middle nasal conchae

46 Inferior nasal conchae

47 Bony nasal septum

48 Maxillary sinus

49 Optic nerve

50 Occipital condyle

51 Torus tubarius of

 pharyngotympanic tube

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The dissection on this page exposes the deepest of the head muscles, those of

the sixth pharyngeal arch. This group, found within the wall of the larynx, is the small

series of muscles that are responsible for sound production. Contractions of these

Head Muscles

muscles vary the tension on the vocal folds and adjust the size of the rima glottidis. A cut anterior portion of the cricothyroid

is also visible; however this muscle is actually the anterior continuation of the inferior pharyngeal constrictor and develops

from the fourth pharyngeal arch.

 1 Posterior crico-arytenoid

 2 Lateral crico-arytenoid

 3 Thyro-arytenoid

 4 Thyro-epiglottic part of thyro-arytenoid

 5 Oblique arytenoid

 6 Ary-epiglottic part of oblique arytenoid

 7 Transverse arytenoid

 8 Cricothyroid (cut)

 9 Hyoid bone

10 Epiglottis

 11 Thyroid cartilage (cut)

12 Cricoid cartilage

13 Trachea

14 Thyrohyoid membrane

Dissection of the larynx, right lamina and horns removed

Posterolateral view

Fourth arch muscle

Sixth arch muscles

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Th e trunk, which

is defi ned by the span of the vertebral column, includes

the neck (span of the cervical vertebrae), the thorax

(span of the thoracic vertebrae), the abdomen (span

of the lumbar vertebrae), and the pelvis (span of

the sacral vertebrae). Th e muscles of the trunk are

the most primitive muscles in the vertebrate

body. Th is series of muscles arises as epithelial

migrations from the myotomes of the embryonic

somites and forms a distinct muscle pattern

throughout the length of the trunk. Th e trunk

muscle pattern has two distinct subdivisions, the

epaxial muscles and the hypaxial muscles, which

are separated by a transverse intermuscular septum. Th e epaxial muscles, situated posterior to the

 vertebral axis, are the extensor muscles of the vertebral

column that develop from the epimere of the myotomes.

Th e dorsal rami of the spinal nerves innervate these muscles.

Th e hypaxial muscles, positioned primarily anterior and lateral to

the vertebral axis, develop from the hypomere of the myotomes

and are supplied by the ventral rami of the spinal nerves.

Th e epaxial muscles form a number of muscle layers that anatomists typically describe as a series of groups. From superfi cial to deep

the groups are the spinotransversales muscles, the erector spinae muscles,

the transversospinales muscles, and the deepest groups (most of which

are intersegmental) consisting of the interspinales, intertransversarii,

and suboccipital muscles.

Th e hypaxial muscles form a distinct pattern throughout the trunk

wall. Th is pattern consists of a subvertebral musculature (positioned on the

anterior and lateral aspect of the vertebral bodies), a four-layered lateral

wall of muscles situated on the lateral aspect of the trunk wall, and a

 ventral strap of musculature on the anterior trunk wall.

Th e photos in this chapter clearly depict the trunk muscles and the

patterns outlined above.

10 Trunk Muscles

Find more information

about the muscles of the

trunk in

157

REAL ANATOMY

158

span the entire length of the vertebral column to the posterior aspect of the occipital bone. They comprise the

intrinsic muscles of the vertebral column, which are often referred to as the “true back muscles.” The vertebral

extensors form four distinct muscle groups. These groups are, from superfi cial to deep, the spinotransversales (splenius muscles), the erector spinae, the transversospinales (three layers — the semispinalis,

multifi dus, and rotatores layers), and the intersegmental muscles. However, each of the four groups does not

extend the entire length of the vertebral column, and in some regions not all four layers are represented.

All epaxial muscles receive a nerve supply from the dorsal (posterior) rami of the spinal nerves.

The epaxial muscles, or vertebral extensors, develop on the

dorsal side of the vertebral column and skull. These muscles

arise from the myotomal epimere of all the trunk somites and

Epaxial Muscles

Dissection of epaxial musculature

Posterior view

Epaxial Muscle Layers

 Spinotransversales — Splenius layer

 Erector spinae layer

 Transversospinalis — Semispinalis layer

 Transversospinalis — Multifidus layer

 Transversospinalis — Rotatores layer

 Deep intersegmental layer

 Vertical muscle subdivisions within muscle layers

 Capitis Muscles

 Splenius capitis

 Erector spinae capitis

 Longissimus capitis

 Spinalis capitis

 Transversospinales capitis

 Semispinalis capitis

 Suboccipitales

 Rectus capitis posterior major

 Rectus capitis posterior minor

 Obliquus capitis superior

 Obliquus capitis inferior

 Cervical Muscles

 Splenius cervicis

 Erector spinae cervicis

 Iliocostalis cervicis

 Longissimus cervicis

 Spinalis cervicis

 Transversospinales cervicis

 Semispinalis cervicis

 Multifidus cervicis

 Rotatores cervicis

 Interspinales cervicis

 Intertransversarii posteriores cervicis medialis

 Thoracic Muscles

 Erector spinae thoracis

 Iliocostalis thoracis

 Longissimus thoracis

 Spinalis thoracis

 Transversospinales thoracis

 Semispinalis thoracis

 Multifidus thoracis

 Rotatores thoracis

 Interspinales thoracis

 Intertransversarii thoracis

 Levatores costarum

 Lumbar Muscles

 Erector spinae lumborum

 Iliocostalis lumborum

 Transversospinales lumborum

 Multifidus lumborum

 Rotatores lumborum

 Interspinales lumborum

 Intertransversarii lumborum medialis

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The spinotransversales muscles are the superfi cial-most

Dissection of splenius and erector spinae muscles

Posterior view

Splenius Musculature

 1 Splenius capitis muscle

 2 Splenius cervicis muscle

Other Muscles and Structures

 3 Iliocostalis muscle

 4 Longissimus muscle

 5 Spinalis muscle

 6 Semispinalis muscle

 7 Multifidus muscle

 8 Levatores costarum muscle

 9 Intertransversarii muscle

10 Posterior scalene muscle

 11 External intercostal muscle

12 Internal intercostal muscle

13 Quadratus lumborum muscle

14 External oblique muscle

15 Transversus abdominis muscle

16 Gluteus maximus muscle

17 Fascia of gluteus medius muscle

18 Supraspinous ligament

19 Nuchal ligament

Spinotransversales Muscles

epaxial muscles and are only present in the superior half of the vertebral column. This group is comprised of

two named muscles — the splenus capitis and splenius cervicis. They span from the midthoracic region to the

base of the occipital bone. As their name suggests, the fi bers attach to the spinous processes of the vertebrae

and course laterally to attach to the vertebral transverse processes. These fl at bands of muscle are primary

extensors of the upper vertebral column and head.

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The erector spinae muscles comprise Erector Spinae Muscles the second layer of epaxial muscles.

Dissection of erector spinae muscles

Posterior view

Dissection of erector spinae muscles

Posterolateral view

Unlike the splenius muscles, the erector spinae muscle group spans the entire length of the vertebral

column. The erector spinae is divided into three parts, which from medial to lateral are the spinalis muscle,

the longissimus muscle, and the iliocostalis muscle. This strong group of epaxial muscles consists of muscle

fi bers that course vertically and somewhat laterally as they span multiple vertebral levels. They function as

primary extensors of the vertebral column.

Erector Spinae and Semispinalis Musculature

 1 Iliocostalis lumborum muscle - lumbar part

 2 Iliocostalis lumborum muscle - thoracic part

 3 Iliocostalis cervicis muscle

 4 Longissimus thoracis muscle

 5 Longissimus cervicis muscle

 6 Longissimus capitis muscle

 7 Spinalis thoracis muscle

 8 Spinalis cervicis muscle

 9 Spinalis capitis muscle

10 Semispinalis thoracis muscle

 11 Semispinalis cervicis muscle

12 Semispinalis capitis muscle

Other Muscles and Structures

13 Multifidus muscle

14 Levatores costarum muscle

15 External intercostal muscle

16 Internal intercostal muscle

17 Middle scalene muscle

18 Nuchal ligament

19 Trapezius muscle

20 Rhomboideus major muscle

21 Latissimus dorsi muscle

22 Infraspinatus muscle

23 Teres major muscle

24 Deltoid muscle

25 Triceps muscle

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Dissection of semispinalis muscles

Posterior view

Dissection of semispinalis muscles

Lateral view

Dissection of semispinalis layer on left and limb muscles on right

Posterolateral view

The transversospinales

muscles form the third

layer of epaxial muscles. This deeper layer of muscles has shorter muscle fi bers, on average, than its two

superfi cial counterparts, and the fi bers angle from lateral (transverse processes) to medial (spinous processes)

as they course from sacrum to cranium. Within this group there are three muscles — the semispinalis, multifi dus, and the rotatores muscles. The more superfi cial semispinalis muscle is depicted on this page.

Transversospinales Muscles

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The multifi dus layer of the

transversospinales musculature is highlighted on

Transversospinales Muscles

Dissection of multifi dus and intersegmental muscles

Posterolateral view

Dissection of multifi dus muscles

Posterior view

this page, and the deeper rotatores are evident on the opposite page along with the deeper intersegmental

muscles. The multifi dus 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 superfi cial counterparts with extension of the vertebral

column and play important roles in the maintenance of posture.

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Dissection of upper deep intersegmental muscles on left

Posterior view

Multifidus, Rotatores, and Intersegemental Muscles

 1 Multifidus lumborum muscle

 2 Multifidus thoracis muscle

 3 Multifidus cervicis muscle

 4 Rectus capitis posterior major muscle

 5 Rectus capitis posterior minor muscle

 6 Obliquus capitis superior muscle

 7 Obliquus capitis inferior muscle

 8 Rotatores cervicis muscle

 9 Rotatores thoracis muscle

10 Levatores costarum muscle

Other Muscles and Structures

 11 Semispinalis cervicic muscle

12 Medial lumbar intertransversarii muscle

13 Nuchal ligament

14 External oblique muscle

15 Transversus abdominis muscle

16 Quadratus lumborum muscle

17 External intercostal muscle

18 Internal intercostal muscle

19 Middle scalene muscle

20 Trapezius muscle

21 Deltoid muscle

22 Latissimus dorsi muscle

23 Infraspinatus muscle

24 Teres major muscle

25 Rhomboideus major muscle

26 Triceps muscle

27 Posterior digastricus msucle

28 Auricularis posterior muscle

29 Transversus nuchae muscle

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 signifi cant 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 modifi ed and enlarged to function

with their specialized vertebral counterparts — the axis, atlas, and occipital bone.

Intersegmental Muscles

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