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Situated on the dorsal surface of the foot are two short digital extensor muscles, the

extensor hallucis brevis and extensor digitorum brevis. These thin muscle sheets

help the long digital extensors of the anterior compartment extend the digits. Like the

Foot Muscles

anterior compartment muscles, they are innervated by the deep fi bular nerve. The plantar muscles of the foot are much

more substantial than the thin dorsal muscles of the foot. These muscles sit beneath the thick subcutaneous fat pad on the

bottom of the foot. From superfi cial to deep, the plantar muscles form four layers.

Dissection of foot, plantar aponeurosis removed

Plantar view

Dissection of foot, fi rst muscle layer removed

Plantar view

Layer one

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Foot Muscles

 1 Abductor hallucis

 2 Flexor digitorum brevis

 3 Abductor digiti minimi

 4 Quadratus plantae

 5 Lumbricals

 6 Flexor hallucis brevis

 7 Adductor hallucis

 8 Flexor digiti minimi brevis

 9 Plantar interossei

10 Dorsal interossei

Other Muscles and Structures

 11 Fibularis longus (tendon)

12 Flexor digitorum longus (tendon)

13 Flexor hallucis longus (tendon)

14 Long plantar ligament

Dissection of foot, second muscle layer removed

Plantar view

Dissection of foot, third muscle layer removed

Plantar view

Layer three

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Foot Muscles

 1 Extensor hallucis brevis

 2 Extensor digitorum brevis

Other Muscles and Structures

 3 Tibialis anterior (tendon)

 4 Extensor hallucic longus (cut)

 5 Extensor digitorum longus (cut)

 6 Fibularis longus (tendon)

 7 Fibularis brevis (tendon)

 8 Deep fibular nerve

 9 Dorsalis pedis artery

Foot Muscles

Dissection of left foot

Dorsal view

Dorsal foot muscles

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Look around your city

or town and notice the telephone wires that run from telephone

pole to telephone pole along the city streets, eventually reaching

the homes and places of business throughout the city. Th ey might

not always be visible because in some cities they run underground.

Regardless of where they occur, these wires criss-cross throughout the

city distributing electrical current from phone to phone in our homes and

places of school, work, and entertainment. Th ese wires are not complex

structures; they are simply metal wires that can conduct an electric charge

from one phone to another. Th ese telephone wires in our cities and homes are

typically insulated from one another and protectively wrapped to prevent damage. Th eir pathways through the city are not complex; they simply follow logical

routes to diff erent parts of the city. Th e wires are bundled in common groups that

follow shared pathways to similar locations. As these wires course through the

city they relay to telephone centers operated by the telephone

companies. At these centers the wires enter control

rooms where they form complex circuits. Th is

complex circuitry allows the electrical messages

to be processed and directed to the proper

phones.

Like the telephone wires of our cities and

homes, the nerves of the peripheral nervous

system are really rather simple structures.

Th ey consist of long, insulated axons bundled together in protective collagenous

wrappings. Th ese axons pass in bundled

groups that follow logical routes to the different regions of the body where they communicate with receptor (sensory receptors)

or eff ector strucutres (muscles or glands).

Like telephone wires, these neuronal wires

conduct electrical messages to and from the

central processing center (brain and spinal

cord). Th is chapter will depict the basic design of the structures called nerves and

demonstrate the pathways of the nerves

throughout the body.

13 Peripheral Nervous System

Find more information

about the peripheral

nervous system in

REAL ANATOMY

212

Dissection of sciatic nerve

Posterior view

Photomicrograph of nerve cross-section

200x

Photomicrograph of multipolar neuron

400x

Nerves are bundles of axons running between the central nervous

system and the peripheral tissues of the body. While all nerves have

a similar basic structure, they vary in the types and numbers of

Structure of a Nerve

neurons bundled within. The basic design of a nerve consists of neurons wrapped by neurolemmocytes to form the nerve fi ber.

The fi bers are protectively wrapped and nourished by a vascular loose connective tissue, the endoneurium. Many endoneurial

wrapped fi bers are surrounded by a collagenous perineurium to form the fasciculus of the nerve, and all the fasciculi are

wrapped in a collagenous sheath, the epineurium, to form the nerve.

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Dissection of spinal cord, thoracic vertebral bodies removed

Anterior view

Dissection of cervical spinal cord

Superior view

The spinal nerves arise from the Spinal Nerve Structure spinal cord as a series of small

neuronal bundles called rootlets — ventral (motor)rootlets and dorsal (sensory) rootlets. Each series

of ventral rootlets converges to form larger ventral roots. Likewise each series of dorsal rootlets converges to form larger dorsal roots. The dorsal and ventral roots project laterally and converge to form

the spinal nerve trunk. A ganglion, the dorsal root ganglion, is present on the dorsal root just prior to

the spinal nerve trunk. Branching from the trunk are two large branches and a variable series of

smaller branches. Each branch follows a specifi c course to different peripheral regions. The two largest

branches, the ventral ramus and dorsal ramus, are somatic branches that run in the musculoskeletal

wall of the body. Smaller visceral branches, the meningeal nerve, the white and gray communicating

rami, and the parasympathetic splanchnic nerves form the autonomic pathways to smooth muscle

and glandular tissue.

Structure of a Nerve

 1 Sciatic nerve

 2 Epineurium

 3 Perineurium

 4 Endoneurium

 5 Myelin sheath

 6 Axon

 7 Cell body

 8 Dendrite

Spinal Nerve Structures

 9 Ventral rootlets

10 Dorsal rootlets

 11 Dorsal root

12 Dorsal root ganglion

13 Ventral root

14 Spinal nerve trunk

15 Ventral ramus

16 Dorsal ramus

Other Structures

17 Spinal cord

18 Cervical vertebra

19 Vertebral artery

20 Common carotid artery

21 Internal jugular vein

22 Laryngopharynx

23 Larynx

24 Thyroid cartilage

25 Cricoid cartilage

26 Vocalis muscle

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Spinal Nerves

 1 Spinal nerve

 2 Cervical dorsal rootlets

 3 Thoracic dorsal rootlets

 4 Lumbosacral dorsal rootlets

 5 Dorsal rami

 6 Cauda equina

 7 Filum terminale

Dissection revealing spinal cord and brain

Posterior view

Dissection exposing cauda equina

Posterior view

With slight variation, the basic pattern of the spinal nerve repeats itself thirty-one

times along the entire length of the spinal cord. With the exception of the fi rst spinal

nerve, each spinal nerve level emerges from within the vertebral column to pass

Spinal Nerves

Other Structures

 8 Cerebrum

 9 Cerebellum

10 Medulla oblongata

 11 Spinal cord

peripherally between successive vertebrae. Because of the developmental differences in the growth rate of the vertebral

column and associated spinal cord, the lower roots of the spinal nerves are dragged downward by the lengthening vertebral

column. With each succeeding spinal nerve level the roots become longer and more oblique in their course, eventually

extending beyond the end of the spinal cord as the vertically oriented cauda equina.

12 Dura mater

13 Superior sagittal sinus

14 Transverse sinus

15 Opening of straight sinus

16 Confluence of sinuses

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Erector spinae muscle removed to expose dorsal rami

Posterior view

Deep dissection exposing dorsal rami

Posterior view

Dissection of cervical dorsal rami

Posterior view

Dorsal Rami

 1 Greater occipital nerve

 2 Least occipital nerve

 3 Dorsal ramus

 4 Medial branch

 5 Lateral branch

The dorsal rami of the spinal nerves arise at all spinal

levels and pursue a posterior course into the muscles,

connective tissue, and skin of the back. They innervate all

Dorsal Rami

Other Structures

 6 Rectus capitis posterior major muscle

 7 Rectus capitis posterior minor muscle

 8 Obliquus superioris muscle

 9 Obliquus inferioris muscle

10 Posterior digastricus muscle

the epaxial muscles comprising the extensors of the vertebral column. The cutaneous distribution of the dorsal rami spans from the top of the head, down the posterior trunk, to the superior

half of the gluteal region. With the exception of levels C1, S4, S5, and the coccygeal, the dorsal

rami split into lateral and medial branches as they course posteriorly into the back.

 11 Semispinalis cervicis muscle

12 Intertransversarii thoracic muscle

13 Levatores costarum muscles

14 External intercostal muscle

15 External oblique muscle

16 Internal oblique muscle

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Cervical Plexus Nerves

 1 Lesser occipital nerve

 2 Great auricular nerve

 3 Transverse cutaneous nerve

 4 Supraclavicular nerve

 5 Phrenic nerve

 6 Ansa cervicalis

 7 Nerve to geniohyoid muscle

 8 Nerve to thyrohyoid muscle

 9 Nerve to superior omohyoid muscle

10 Nerve to sternohyoid muscle

 11 Nerve to sternothyroid muscle

12 Nerve to inferior omohyoid muscle

Other Nerves and Structures

13 Hypoglossal nerve

14 Vagus nerve

15 Superior trunk of brachial plexus

16 Common carotid artery

17 Carotid sinus

18 Internal carotid artery

19 External carotid artery

20 Parotid gland

21 Sternocleidomastoid muscle

22 Thyrohyoid muscle

23 Omohyoid muscle

24 Sternohyoid muscle

25 Sternothyroid muscle

26 Anterior scalene muscle

27 Middle scalene muscle

28 Levator scapulae muscle

Dissection of cervical plexus

Anterior view

This next series of pages illustrates the ventral rami of

the spinal nerves. The ventral rami innervate the

majority of the skeletal muscles (all hypaxial and limb

Cervical Plexus

muscles). The cervical plexus forms from the ventral rami of the fi rst four cervical spinal nerves. As

these ventral rami pass laterally between the middle and internal layers of the lateral cervical body

wall, they form ascending and descending branches that communicate to form the cervical plexus.

Emerging from this plexus are the nerves that innervate the muscles of the hypaxial cervical wall, as

well as cutaneous branches that serve the overlying skin of the lateral head, neck and upper thorax.

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