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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
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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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