Spinal cord pdf - The back

Basic Anatomy

first lumbar vertebra (Fig. 12.7). In the young child, it is

inferiorly in the adult at the level of the lower border of the

ous with the medulla oblongata of the brain. It terminates

begins above at the foramen magnum, where it is continu

The spinal cord is a cylindrical, grayish white structure that

For dermatomes of the back, see Figure 1.25.

posterior ramus divides into a medial and a lateral branch.

causes diminished, but not total, loss of sensation. Each

skin areas supplied occurs so that section of a single nerve

foramen from which they emerge. Considerable overlap of

posterior median sulcus.

face a shallow furrow, the

and on the posterior sur

anterior median fissure,

sure, the

possesses in the midline anteriorly a deep longitudinal fis

back of the coccyx (Figs. 12.7, 12.11, and 12.15). The cord

descends to be attached to the

filum terminale,

mater, the

from the apex of which a prolongation of the pia

ullaris,

conus med

Inferiorly, the spinal cord tapers off into the

lumbar enlargements.

cervical

called

gin to the lumbosacral plexus, has fusiform enlargements

the lower thoracic and lumbar regions, where it gives ori

region, where it gives origin to the brachial plexus, and in

the third lumbar vertebra. The spinal cord in the cervical

relatively longer and ends inferiorly at the upper border of

697

and

-
-

Spinal Cord

anterior gray column

posterior gray column

dura mater

arachnoid mater

pia mater

gray matter

white matter

posterior rootlets of spinal nerve

posterior root

ganglion

spinal nerve

anterior rootlets of spinal nerve

arachnoid mater

dura mater

spinal nerve

cauda
equina

posterior root ganglion

filum terminale

conus medullaris

pia mater

arachnoid mater

dura mater

filum terminale

ligamentum denticulatum

posterior gray column or horn

dura mater

arachnoid mater

pia mater

posterior root

posterior root ganglion

spinal nerve

anterior root

subarachnoid space filled with cerebrospinal fluid

central canal

anterior gray column or horn

lateral gray column or horn

ligamentum denticulatum

FIGURE 12.11

showing the meninges and the position of the cerebrospinal fluid.

Transverse section through the spinal cord

showing the anterior and posterior roots of the spinal nerves and meninges.

Lower end of the spinal cord and the cauda equina. B. Section through the thoracic part of the spinal cord

698

CHAPTER 12

Here,

spinal nerve.

foramina, where they unite to form a

cord segment to the level of their respective intervertebral

The spinal nerve roots pass laterally from each spinal

rise to peripheral and central nerve fibers.

possesses a posterior root ganglion, the cells of which give

responding segment of the cord. Each posterior nerve root

series of rootlets, which extend the whole length of the cor

12.14, and 12.15). Each root is attached to the cord by a

(Figs. 12.11, 12.13,

sensory, roots

posterior,

motor, roots

anterior,

31 pairs of spinal nerves by the

Along the whole length of the spinal cord are attached

(Figs. 12.11 and 12.15).

cauda equina

terminale. The inferior nerve roots together are called the

the adult) form a vertical leash of nerves around the filum

of the cord (lower border of the first lumbar vertebra in

and sacral nerves inferior to the level of the termination

and run almost horizontally, but the roots of the lumbar

the upper cervical region, the spinal nerve roots are short

sively from above downward (Figs. 12.12 and 12.15). In

the spinal cord, the length of the roots increases progres

vertebral column during development compared to that of

ers. Because of the disproportionate growth in length of the

nerve is made up of a mixture of motor and sensory fib

the motor and sensory fibers become mixed so that a spinal

The Back

Roots of the Spinal Nerves

and the

atlas

axis

seventh cervical vertebra

first thoracic vertebra

first lumbar vertebra

twelfth thoracic vertebra

sacrum

coccyx

C1 spinal nerve

cervical
segments
of spinal
cord

coccygeal spinal nerve one

lower end of spinal cord

lumbar, sacral, and
coccygeal segments
of spinal cord

thoracic
segments
of spinal
cord

fifth lumbar vertebra

T12

FIGURE 12.12

Posterior view of the spinal cord showing the origins of the roots of the spinal nerves and their relationship

nerve roots.

to the different vertebrae. On the right, the laminae have been removed to expose the right half of the spinal cord and the

Basic Anatomy

anterior spinal arteries,

The

close to the attachments of the posterior spinal nerve roots.

the vertebral arteries, run down the side of the spinal cord,

which arise either directly or indirectly from

nal arteries,

posterior spi

The

anterior spinal artery.

arteries and one

longitudinally running arteries: the two posterior spinal

The spinal cord receives its arterial supply from three small,

motor and sensory fibers.

which contain both

posterior ramus,

and a smaller

ramus

anterior

spinal nerve immediately divides into a large

After emergence from the intervertebral foramen, each

699

Blood Supply of the Spinal Cord

which arise from the

ebral

vert

cavity of lateral
ventricle

septum
pellucidum

midbrain

III c. nerve

pons

V c. nerve

occipital bone

spinal root of
accessory nerve

vertebral artery

X c. nerve

rootlets of
posterior root
of third cervical
spinal n.

carotid artery

posterior root
ganglion of
fourth cervical
spinal n.

fourth cervical
spinal n.

corpus

callosum

POSTERIOR

ANTERIOR

fornix

thalamus

tentorium

cerebelli

occipital lobe

of cerebrum

cerebellum

fourth

ventricle

medulla

oblongata

occipital

bone

foramen

magnum

spinal

cord

post vertebral

muscles

FIGURE 12.13

tal section

Dissection of the skull and the upper part of the cervical vertebral column showing the brain in sagit

of the spinal cord drain into the internal

veins

The

through the intervertebral foramina.

which enter the vertebral canal

radicular arteries,

The posterior and anterior spinal arteries are reinforced

within the anterior median fissure.

arteries, unite to form a single artery, which runs down

intervertebral foramina.

Note also the roots of the cervical spinal nerves and the trunks of the spinal nerves as they emerge through the dissected

and the intact spinal cord in situ. Note the continuity of the medulla oblongata and the spinal cord at the foramen magnum.

vertebral venous plexus.

mater (see Figs. 12.11, 12.14, and 12.15).

meninges: the dura mater, the arachnoid mater, and the pia

The spinal cord, like the brain, is surrounded by three

Meninges of the Spinal Cord

700

CHAPTER 12

The Back

periosteal
layer of dura

cut edge of periosteal
layer of dura just below
transverse sinus

right cerebellar
hemisphere

foramen of
magendie

medulla
oblongata

spinal cord

posterior roots of
cervical spinal nerves
surrounded by
meningeal sheaths

cut edge of

skull bone

site of superior

sagittal sinus

reflected cut

flap of dura and

arachnoid

posterior root

of cervical spinal

nerve

cut edge

of skin

dural sheath

for spinal

cord

FIGURE 12.14

he greater part of the occipital bone has been removed expos

Dissection of the back of the head and neck. T

intervertebral foramen. The inner surface of the dura mater

(epineurium)

tissue surrounding each spinal nerve

each nerve root and becomes continuous with connective

vertebral venous plexus. The dura mater extends along

space). This contains loose areolar tissue and the internal

(epidural

extradural space

the walls of the canal by the

lies loosely in the vertebral canal and is separated from

of the second sacral vertebra (Fig. 12.7). The dural sheath

ends on the filum terminale at the level of the lower border

the meningeal layer of dura covering the brain. Inferiorly, it

continuous superiorly through the foramen magnum with

and cauda equina (Figs. 12.10, 12.11, 12.14, and 12.15). It is

dense, strong, fibrous sheet that encloses the spinal cord

The dura mater is the most external membrane and is a

Dura Mater

nerves leaving the vertebral canal enveloped in a meningeal sheath.

been incised in the midline to expose the spinal cord and rootlets of the cervical spinal nerves. Note the cervical spinal

to expose the cerebellum and the medulla oblongata in the posterior cranial fossa. In the neck the dura and arachnoid have

ing the periosteal layer of dura. On the right side a window has been made in the dura below the transverse venous sinus

at the

is separated from the arachnoid mater by the potential

border of the second sacral vertebra (Figs. 12.7 and 12.15).

orly, it ends on the filum terminale at the level of the lower

magnum with the arachnoid covering the brain. Inferi

The arachnoid is continuous above through the foramen

(Fig. 12.11).

cerebrospinal fluid

which is filled with

space,

subarachnoid

arated from the pia mater by a wide space, the

that contains a thin film of tissue fluid. The arachnoid is sep

12.11). It is separated from the dura by the subdural space

internally and the dura mater externally (Figs. 12.10 and

covering the spinal cord and lying between the pia mater

The arachnoid mater is a delicate impermeable membrane

Arachnoid Mater

subdural space.

Basic Anatomy

701

spinal cord

conus

medullaris

lower end of

spinal cord

filum

terminale

lower end of

subarachnoid

space

reflected cut

edge of dura

and arachnoid

anterior and

posterior nerve

roots of

cauda equina

cut lamina

of sacrum

filum

terminale

FIGURE 12.15

Dissection of the lower part of the back including a complete laminectomy of the lumbar and sacral regions of

roots of the lumbar and sacral spinal nerves forming the cauda equina.

lower end of the spinal cord, and the cauda equina. Note the filum terminale surrounded by the anterior and posterior nerve

the vertebral column. The meningeal sheath has been incised and reflected laterally exposing the subarachnoid space, the

Nerve Root Pain

is, the cervicothoracic junction and the lumbosacral junction. In

The discs most commonly affected are those in areas where

the disc is undergoing degenerative changes that result in her

sudden shocks, particularly if the vertebral column is flexed and

on their shoulders. Nevertheless, the discs are vulnerable to

umn is the growth of osteophytes, which commonly encroach on

cesses and joints (Fig. 12.5). In the lumbar region, the largest

disc and the vertebral body, and posteriorly by the articular pro

Spinal nerve roots exit from the vertebral canal through the

intervertebral foramina. Each foramen is bounded superiorly

and inferiorly by the pedicles, anteriorly by the intervertebral

foramen is between the first and second lumbar vertebrae and

the smallest is between the fifth lumbar and first sacral vertebra.

One of the complications of osteoarthritis of the vertebral col-

the intervertebral foramina, causing pain along the distribution

of the segmental nerve. The fifth lumbar spinal nerve is the larg-

est of the lumbar spinal nerves, and it exits from the vertebral

column through the smallest intervertebral foramen. For this rea-

son, it is the most vulnerable.

Osteoarthritis as a cause of root pain is suggested by the

patient’s age, its insidious onset, and a history of back pain of

long duration. A prolapsed disc usually occurs in a younger age

group and often has an acute onset.

Herniated Intervertebral Discs

The structure and function of the intervertebral disc are

described on pages 689 and 690. The resistance of these discs

to compression forces is substantial, as seen, for example, in cir-

cus acrobats who can support four or more of their colleagues

niation of the nucleus pulposus.

a mobile part of the column joins a relatively immobile part—that

these areas, the posterior part of the anulus fibrosus ruptures,

and the nucleus pulposus is forced posteriorly like toothpaste out

C L I N I C A L N O T E S

(continued)

Basic Anatomy

703

Summary of Important Features Found in Cervical and Lumbosacral Root Syndromes

T A B L E 1 2 . 1

Toe extension, ankle

Tibialis anterior, quadriceps

Triceps

Triceps and flexor carpi

Wrist extensors

Movement Weakness

Root Injury Dermatome Pain

Muscle Supplied

Reflex Involved

Lower lateral aspect of upper arm

Deltoid and biceps

Shoulder abduction, elbow

flexion

Biceps

Lateral aspect of forearm

Extensor carpi radialis longus

and brevis

Brachioradialis

Middle finger

radialis

Extension of elbow and flexion

of wrist

Medial aspect of forearm

Flexor digitorum superficialis

and profundus

Finger flexion

None

Groin

Iliopsoas

Hip flexion

Cremaster

Anterior aspect of thigh

Iliopsoas, sartorius, hip

adductors

Hip flexion, hip adduction

Cremaster

Medial aspect of knee

Iliopsoas, sartorius,

quadriceps, hip adductors

Hip flexion, knee extension,

hip adduction

Patellar

Medial aspect of calf

Foot inversion, knee extension

Patellar

Lateral part of lower leg and

dorsum of foot

Extensor hallucis longus,

extensor digitorum longus

dorsiflexion

None

Lateral edge of foot

Gastrocnemius, soleus

Ankle plantar flexion

Ankle jerk

Posterior part of thigh

Flexor digitorum longus, flexor

hallucis longus

Ankle plantar flexion, toe

flexion

None

disc

occipital bone

atlas

nucleus
pulposus

anulus
fibrosus

foramen magnum

L5—S1

FIGURE 12.16

A, B.

the S1 motor nerve root produces weakness of plantar flexion of the ankle joint.

Pressure on the L5 motor nerve root produces weakness of dorsiflexion of the ankle; pressure on

pulposus posteriorly.

An intervertebral disc that has herniated its nucleus

and the first sacral vertebra showing pressure on the S1 nerve root.

Posterolateral herniation of the nucleus pulposus of the intervertebral disc between the fifth lumbar vertebra

vertebrae.

the pedicle of the fourth lumbar vertebra and are not related to the intervertebral disc between the fourth and fifth lumbar

but only seven cervical vertebrae. In the lumbar region, for example, the emerging L4 nerve roots pass out laterally close to

exist between the herniated nucleus pulposus and the spinal nerve roots. Note that there are eight cervical spinal nerves

Posterior views of vertebral bodies in the cervical and lumbar regions showing the relationship that might

704

CHAPTER 12

(Fig. 12.11).

the connective tissue surrounding each spinal nerve

along each nerve root and becomes continuous with

in the middle of the dural sheath. The pia mater extends

dura. It is by this means that the spinal cord is suspended

which passes laterally to be attached to the

ticulatum,

ligamentum den

between the nerve roots to form the

terminale. The pia mater is thickened on either side

covering the brain; inferiorly, it fuses with the filum

superiorly through the foramen magnum with the pia

The Back

Spinal Cord Ischemia

gia in flexion and extension are beyond the scope of this book. For

of function of afferent and efferent nerve tracts below the level of

of function at the level of the lesion and partial or complete loss

Injury to the spinal cord can produce partial or complete loss

bar vertebra. Second, the large size of the vertebral foramen

only down as far as the level of the lower border of the first lum

facts aid the patient. First, the spinal cord in the adult extends

ing severe injury. However, when considerable displacement

of the vertebral canal often results in the spinal cord escap

running anterior and posterior spinal arteries are of small and

The blood supply to the spinal cord is surprisingly meager, con-

sidering the importance of this nervous tissue. The longitudinally

variable diameter, and the reinforcing segmental arteries vary in

number and in size. Ischemia of the spinal cord can easily follow

minor damage to the arterial supply as a result of regional anes-

thesia, pain block procedures, or aortic surgery.

Spinal Cord Injuries

The degree of spinal cord injury at different vertebral levels is

largely governed by anatomic factors. In the cervical region,

dislocation or fracture dislocation is common, but the large size

occurs, the cord is sectioned and death occurs immediately.

Respiration ceases if the lesion occurs above the segmental ori-

gin of the phrenic nerves (C3, 4, and 5).

In fracture dislocations of the thoracic region, displacement

is often considerable, and the small size of the vertebral canal

results in severe injury to the spinal cord.

In fracture dislocations of the lumbar region, two anatomic

in this region gives the roots of the cauda equina ample room.

Nerve injury may therefore be minimal in this region.

the lesion. The symptoms and signs of spinal shock and paraple-

further information, a textbook of neurology should be consulted.

Relationships of Spinal Cord Segments to Vertebral
Numbers
Because the spinal cord is shorter than the vertebral column,

the spinal cord segments do not correspond numerically with

the vertebrae that lie at the same level (Fig. 12.12). The following

list helps determine which spinal segment is contiguous with a

given vertebral body.

Vertebrae

Spinal Segment

Cervical

Add 1

Upper thoracic

Add 2

Lower thoracic (T7 to 9)

Add 3

Tenth thoracic

L1 and 2 cord segments

Eleventh thoracic

L3 and 4 cord segments

Twelfth thoracic

L5 cord segment

First lumbar

Sacral and coccygeal cord segments

C L I N I C A L N O T E S

Between the levels of the conus medullaris and the lower end

The pia mater is a vascular membrane that closely covers

Pia Mater

space.

roots, forming small lateral extensions of the subarachnoid

The arachnoid mater is continued along the spinal nerve

equina bathed in cerebrospinal fluid (Figs. 12.11 and 12.15).

of the subarachnoid space lie the nerve roots of the cauda

the spinal cord (Figs. 12.10 and 12.11). It is

uous

contin

Lumbar Puncture (Spinal Tap)

with a stylet, is passed into the vertebral canal above or below

region is opened to a maximum (Fig. 12.17). An imaginary line

this region usually pushes the nerve roots to one side without

bra. The lower lumbar part of the vertebral canal is thus occu

terminates inferiorly at the level of the lower border of the first

Lumbar puncture may be performed to withdraw a sample of

cerebrospinal fluid for examination. Fortunately, the spinal cord

lumbar vertebra in the adult. (In the infant, it may reach as low

as the third lumbar vertebra.) The subarachnoid space extends

inferiorly as far as the lower border of the second sacral verte-

pied by the subarachnoid space, which contains the cauda

equina—that is, the lumbar and sacral nerve roots and the filum

terminale. A needle introduced into the subarachnoid space in

causing damage.

With the patient lying on the side with the vertebral column

well flexed, the space between adjoining laminae in the lumbar

joining the highest points on the iliac crests passes over the

fourth lumbar spine (Fig. 12.35). With a careful aseptic technique

and under local anesthesia, the lumbar puncture needle, fitted

C L I N I C A L N O T E S

(continued)

706

CHAPTER 12

The Back

cauda equina (anterior and posterior nerve roots)

posterior longitudinal ligament

anterior longitudinal ligament

intervertebral disc

fourth lumbar spinal nerve

articular process

transverse process

cauda equina

arachnoid mater

dura mater

supraspinous ligament

ligamentum flavum

interspinous ligament

lumbar puncture needle

skin

internal vertebral veins

spine

superficial fascia

FIGURE 12.17

Sagittal section through the lumbar part of the vertebral column in flexion. Note that the spines and laminae

are well separated in this position, enabling one to introduce a lumbar puncture needle into the subarachnoid space.

fourth sacral

spinous process

sacral hiatus

coccyx

sacral cornu

lamina of fourth
sacral vertebra

lateral mass

of sacrum

sacrococcygeal

ligament

sacrococcygeal

membrane

fourth posterior
sacral foramen

dura and
arachnoid

coccyx

sacrococcygeal

membrane

coccyx

sacral
hiatus

sacral hiatus

filum terminale

lower limit of

subarachnoid space

posterior rami
of spinal nerves

filum terminale

extradural space

subarachnoid space filum

terminale

fourth posterior

sacral foramen

third sacral spinous process

third posterior sacral foramen

FIGURE 12.18

Longitudinal section through the sacrum showing the anatomy of caudal anesthesia.

removed.

sheath (thecal sac) around the lower end of the spinal cord and spinal nerves in the sacral canal; the laminae have been

The dural

lower end of the sacrum and the coccyx showing the sacrococcygeal membrane covering the sacral hiatus.

Posterior surface of the

indicate the position of important bony landmarks.

Black dots

The sacral hiatus.

Basic Anatomy

effectively protects the spinal cord from trauma.

with the bony and ligamentous walls of the vertebral canal,

medium that surrounds the spinal cord. This fluid, together

neuronal activity, the cerebrospinal fluid provides a fluid

In addition to removing waste products associated with

venous sinus.

superior sagittal

the dural venous sinuses, in particular the

into

arachnoid villi

bloodstream by passing through the

filum terminale (Fig. 12.7). Eventually, the fluid enters the

second sacral vertebra, where the arachnoid fuses with the

noid space extends down as far as the lower border of the

around the spinal cord. The spinal part of the subarach

over the surface of the cerebral hemispheres and downward

fourth ventricle (see page 548). It circulates both upward

noid space through the three foramina in the roof of the

through the ventricular system and enters the subarach

and fourth ventricles of the brain. The fluid circulates

within the lateral, third,

choroid plexuses,

mainly by the

The cerebrospinal fluid is a clear, colorless fluid formed

707

Cerebrospinal Fluid

cases with prolapse of an intervertebral disc (Fig. 12.16) (see

Relationship of the Vertebral Body to the Spinal

Nerve

Since the fully developed vertebral body is intersegmental in

position, each spinal nerve leaves the vertebral canal through

the intervertebral foramen and is closely related to the inter-

vertebral disc. This fact is of great clinical significance in

page 701).

C L I N I C A L N O T E S

Development of the Vertebral Column

only the neural arch for the atlas, which grows anteriorly and

neural arch (Fig. 12.19). The two centers for the centrum usually

At about the ninth week of development, primary ossification

pass between the myotomes to form the mesenchymal

nerves to fuse with their fellows of the opposite side and form

sclerotomic mesenchyme situated between adjacent vertebral

of the intervertebral disc is derived from

anulus fibrosus,

(Fig. 12.20). The surrounding fibrocartilage,

of the

pletely in the region of the vertebral body, but in the interver

(Figs. 12.19 and 12.20). Each vertebral body is thus

(Fig. 12.19). The caudal half of each

migrate medially during the fourth week of development and

The mesenchymal cells of the sclerotome rapidly divide and

Early in development, the embryonic mesoderm becomes differ-

entiated into three distinct regions: paraxial mesoderm, interme-
diate mesoderm, and lateral mesoderm. The paraxial mesoderm

is a column of tissue situated on either side of the midline of the

embryo, and at about the fourth week, it becomes divided into

blocks of tissue called somites. Each somite becomes differenti-

ated into a ventromedial part (the sclerotome) and a dorsolateral

part (the dermatomyotome). The dermatomyotome now further

differentiates into the myotome and the dermatome (Fig. 12.19).

surround the notochord

sclerotome now fuses with the cephalic half of the immedi-

ately succeeding sclerotome to form the mesenchymal verte-
bral body

an intersegmental structure. The notochord degenerates com-

tebral region, it enlarges to form the nucleus pulposus
intervertebral discs

the

bodies (Fig. 12.20).

Meanwhile, the mesenchymal vertebral body gives rise

to dorsal and lateral outgrowths on each side. The dorsal out-

growths grow around the neural tube between the segmental

the mesenchymal neural arch (Fig. 12.19). The lateral outgrowths

costal

processes, or primordia of the ribs.

Two centers of chondrification appear in the middle of each

mesenchymal vertebral body. These quickly fuse to form a car-

tilaginous centrum (Fig. 12.19). A chondrification center forms in

each half of the mesenchymal neural arch and spreads dorsally

to fuse behind the neural tube with its fellow of the opposite side.

These centers also extend anteriorly to fuse with the cartilagi-

nous centrum and laterally into the costal processes. The con-

densed mesenchymal or membranous vertebra has thus been

converted into a cartilaginous vertebra.

In the thoracic region, each costal process forms a carti-

laginous rib. The costal processes in the cervical region remain

short and form the lateral and anterior boundaries of the foramen
transversarium of each vertebra. In the lumbar region, the cos-

tal process forms part of the transverse process; in the sacral

region, the costal processes fuse together to form the lateral
mass of the sacrum.

centers appear: two for each centum and one for each half of the

unite quickly, but the complete union of all the primary centers

does not occur until several years after birth.

During adolescence, secondary centers appear in the car-

tilage covering the superior and inferior ends of the vertebral

body, and the epiphyseal plates are formed. A secondary center

also appears at the tip of each transverse process and at the tip

of the spinous process. By the 25th year, all the secondary cen-

ters have fused with the rest of the vertebra.

The atlas and axis develop somewhat differently. The cen-

trum of the atlas fuses with that of the axis and becomes the part

of the axis vertebra known as the odontoid process. This leaves

finally fuses in the midline to form the characteristic ring shape

of the atlas vertebra.

In the sacral region, the bodies of the individual vertebrae

are separated from each other in early life by intervertebral

discs. At about the 18th year, the bodies start to become united

E M B R Y O L O G I C N O T E S

(continued)

708

CHAPTER 12

The Back

neural tube

dermatome

myotome

sclerotome

notochord

mesenchymal neural arch

myotome

centers of chondrification

mesenchymal

costal process

mesenchymal

vertebral body

cartilaginous neural arch

primary centers

of ossification

cartilaginous

costal process

developing rib

fusion between

cartilage centers

notochord remains

cartilaginous

vertebral body

(centrum)

secondary centers

of ossification

site of neurocentral fusion

FIGURE 12.19

The stages in the formation of a thoracic vertebra.

by bone; this process starts caudally. Usually by the 13th

in the affected region. The types of spina bifida are shown in

tube and the surface ectoderm, to form the vertebral arches

ure of the mesenchyme, which grows in between the neural

this defect, the meninges and spinal cord may or may not be

in the lower thoracic, lumbar, and sacral regions. Beneath

vertebrae fail to develop. The condition occurs most frequently

In spina bifida, the spines and arches of one or more adjacent

The prevertebral and postvertebral muscles develop from the

develops. Toward the end of the first year, when the child stands

his or her head, the cervical curve, which is convex anteriorly,

uous anterior (ventral) concavity. When the child begins to raise

(ventral) concavity. Later, the sacrovertebral angle develops. At

The embryonic vertebral column shows one continuous anterior

year, all the sacral vertebrae are united. In the coccygeal region,

segmental fusion also takes place, and in later life the coccyx

often fuses with the sacrum.

Development of the Curves of the Vertebral Column

birth, the cervical, thoracic, and lumbar regions show one contin-

up, the lumbar curve, which is convex anteriorly, develops.

Development of the Muscles of the Vertebral Column

segmental myotomes.

Scoliosis
Scoliosis results from a congenital hemivertebra. A hemiverte-

bra is caused by a failure in development of one of the two ossi-

fication centers that appear in the centrum of the body of each

vertebra (Fig. 12.21).

Spina Bifida

involved in varying degrees. This condition is a result of fail-

Figures 12.22 and 12.23.

Basic Anatomy

709

sclerotomes

myotomes

notochord

intervertebral disc

sclerotomes

costal

process

myotomes

spinal nerves

anulus fibrosus

nucleus pulposus

FIGURE 12.20

The formation of each mesenchymal vertebral body by the fusion of the caudal half of each sclerotome with

spinal nerve and each intervertebral disc.

costal processes grow out between adjacent myotomes. Also shown is the close relationship that exists between each

the cephalic half of the immediately succeeding sclerotome. Each vertebral body is thus an intersegmental structure. The

FIGURE 12.21

Posterior view of a woman with scoliosis

resulting from a congenital hemivertebra in the lower

thoracic region.

spina bifida occulta

meningocele

meningomyelocele

myelocele

syringomyelocele

FIGURE 12.22

Different types of spina bifida.