Fig. 1-1. Curvatures of vertebral column.
The vertebral column of the fetus is kyphotic (convex backward); think of the crouching posture of a fetus. After birth, the baby holds its head up to make the cervical vertebrae lordotic (convex forward). As the baby matures, he/she stands up to make the lordosis of his/her lumbar vertebrae. The cervical and lumbar curvatures are thus called the secondary curvatures.
The lordosis of lumbar vertebrae is used as an excuse for a lazy anatomist’s potbelly.
< Cutaneous nerves >
After skinning a cadaver, students can find cutaneous veins and nerves in the subcutaneous tissue. In the trunk, it will suffice for students to search only for the cutaneous nerves (Fig. 3-3), since there are no notable cutaneous veins. Most cutaneous nerves in the trunk are from the intercostal nerves (TN1–TN11) (Fig. 5-5).
Fig. 1-2. Cutaneous nerves of trunk.
The posterior and anterior roots from the spinal cord form the trunk of the spinal nerve (Fig. 1-19), which divides into posterior and anterior rami. The anterior ramus further divides into lateral and anterior cutaneous branches. Branch (English) derives from ramus (Latin), as English derived from Latin.
During dissection, students should try to find six cutaneous nerves (1 to 6 in the figure above). They characteristically emerge from muscles (Fig. 3-3).
The main component of cutaneous nerves is somatic sensory nerve (Figs. 2-10,12) that accounts for the dermatome (Fig. 5-5). Moreover, cutaneous nerves contain visceral motor nerves that innervate smooth muscles of the blood vessel, sweat gland, and so on. (Fig. 3-18).
< Muscles >
If a priest takes off his hood, the hood resembles the shape of trapezius.
Fig. 1-3. Trapezius, latissimus dorsi.
The trapezius is the most superficial back muscle. The main action of the trapezius is the adduction of the scapula. When the superior portion of trapezius contracts, the scapula is elevated (shrugged shoulder); when the inferior portion contracts, the scapula is depressed. A muscle’s broad origin may result in various muscle actions; another instance is the deltoid muscle (Fig. 2-1).
Fig. 1-4. The actions of trapezius (solid arrow), divided into two (dotted arrows).
This figure differs from the usual horizontal plane: The subject is in prone position, rather than supine position for taking computed tomography (Fig. 2-12). Such horizontal plane can conveniently be compared to the posterior view (Fig. 1-3); therefore, this type of illustration will temporarily be employed in chapter 1 (back) and chapter 2 (only scapular region).
In addition to adduction, the trapezius performs retraction of the scapula. This is because the scapula is obliquely placed on curved ribs and the muscle’s oblique action (a vector) can be divided into two. Reverse actions of adduction and retraction are abduction and protraction.
When bilateral scapulae are fixed by other muscles (Fig. 2-5), the contraction of bilateral trapezius leads to the hyperextension of the head (Fig. 1-3). If the insertion is fixed, the origin moves.
Reversely, bilateral sternocleidomastoid muscles are used for flexing the head forward (Fig. 3-4). Both the trapezius and sternocleidomastoid muscle are innervated by XI (Figs. 3-20,24).
Fig. 1-5. Ligaments around cervical vertebrae (midsagittal plane).
The nuchal ligament, which is expansion of the supraspinous ligament, prevents the excessive flexion of the head. The nuchal ligament protects from the injury caused by violent head flexion in cases such as a sudden car stop.
In the midsagittal plane, we can also see the interspinous ligament, ligamentum flavum, and posterior longitudinal ligament which also restrict extreme flexion of the vertebral column. Anterior longitudinal ligament limits excessive hyperextension.
With the exception of the nuchal ligament, other ligaments exist throughout the entire vertebral column. The intervertebral discs are between all vertebral bodies.
Fig. 1-6. Two types of cartilaginous joint.
Joints are classified into fibrous joints (Figs. 4-14,29), cartilaginous joints, and synovial joints (Fig. 2-59). The cartilaginous joints are more mobile than the fibrous joints, but less mobile than the synovial joints. The cartilaginous joints are further categorized into synchondrosis (with one hyaline cartilage) and symphysis (with two hyaline cartilages and one fibrous cartilage).
Two hyaline cartilages and one fibrous cartilage (intervertebral disc) exist between vertebral bodies (Fig. 1-5). The intervertebral disc with the two hyaline cartilages is the representative of symphysis.
Fig. 1-7. Intervertebral disc.
An intervertebral “disc” (There is no “disk” in anatomy.) is composed of “anulus” fibrosus (There is neither “annulus” nor “annular” in anatomy.) and nucleus pulposus. The disc resembles a ring-shaped doughnut that contains fruit jam in its center.
Just as jam leaks when one bites into the doughnut, nucleus pulposus can herniate through a torn anulus fibrosus. Herniation usually occurs in the cervical and lumbar vertebrae that are very mobile. Thoracic vertebrae which are attached to the ribs are less mobile and thus less prone to herniation (Fig. 2-5). Sacral vertebrae which are fused to form the sacrum have no intervertebral discs between them (Fig. 7-18).
The number of spinal nerves are somewhat similar to the number of vertebrae because the spinal nerves pass the intervertebral foramina (Fig. 1-19). CN1 is superior to CV1 as an exception; while TN1, LN1, SN1 are inferior to TV1, LV1, SV1, respectively.
LN4 passes the intervertebral foramen between LV4 and LV5. However, if the nucleus pulposus is herniated between LV4 and LV5, it presses the LN5. This is because the herniation occurs posteriorly rather than laterally.
The next muscle, partly covered by the trapezius, is the latissimus dorsi. Latissimus means the muscle’s origin is the largest (lower six thoracic vertebrae, thoracolumbar fascia, iliac crest, ribs, and scapula) (Fig. 1-3). It is innervated by the “thoracodorsal” nerve (Figs. 2-14,15). The name of this nerve originates from the location of the latissimus dorsi which partly covers the “thorax’ dorsum” (posterior thoracic wall).
Latissimus dorsi performs adduction, medial rotation, and extension of the humerus (Fig. 2-3). Latissimus dorsi is used when one climbs a rope or rows a boat.
These two superficial back muscles build the auscultation triangle and the lumbar triangle (Fig. 1-3). Anatomy teachers’ secret: Triangles such as these are frequently asked on the anatomy exam not because they are especially meaningful, but because they are easy to grade.
The auscultation triangle can be enlarged by protraction and elevation of the scapula (Figs. 1-3,4), which can be achieved by gathering the upper limbs in front and flexing the trunk. A clinician may ask a patient to pose like this to listen to the lung sound by putting a stethoscope on the back.
Fig. 1-8. Thoracolumbar fascia.
The horizontal plane passing the lumbar triangle is illustrated above. The triangle’s medial border (latissimus dorsi), lateral border (external oblique muscle), and base (internal oblique muscle) (Fig. 6-1) are represented (Fig. 1-3).
The figure also shows that the thoracolumbar fascia, enclosing the deep back muscles, functions as an aponeurosis (a broad and flat tendon) of the latissimus dorsi (Fig. 1-3), the internal oblique muscle, and the transversus abdominis (Fig. 6-1). Generally, named fasciae are thick and play the aponeurosis role of other muscles (Fig. 4-15). The thoracolumbar fascia is a good example of this case.
Unlike Fig. 1-3, the thoracolumbar fascia extends to the “thoracic” vertebrae level. This is the etymology of the “thoraco”lumbar fascia.
Fig. 1-9. Levator scapulae, rhomboid minor and major.
Among the three deep muscles between the vertebrae and the scapula, the “levator scapulae” “elevates” the “scapula.” Such is done because the levator scapulae lies obliquely. As the rhomboid minor and major contract, the scapula is adducted.
Levator scapulae and the two rhomboid muscles are innervated by the dorsal scapular nerve (Fig. 2-14). This nerve runs “dorsal to the scapula” (considering the round trunk) (Fig. 1-4), and thus is named “dorsal scapular” nerve.
Fig. 1-10. Serratus posterior superior and inferior.
The subsequent two thin muscles are the serratus posterior superior (elevating the upper ribs) and the serratus posterior inferior (depressing the lower ribs). They are innervated by the intercostal nerves that run adjacent to them (Fig. 5-4).
The superficial back muscles discussed so far (trapezius, latissimus dorsi, levator scapulae, rhomboid minor and major, serratus posterior superior and inferior) are controlled by XI or by the anterior rami of spinal nerves (thoracodorsal, dorsal scapular, and intercostal nerves). On the other hand, the deep back muscles to be discussed (Figs. 1-12,13,15) are controlled by the posterior rami of spinal nerves (Fig. 1-2).
Fig. 1-11. Development of superficial back muscles.
It may seem odd that the anterior rami are distributed to the superficial back muscles which are located posteriorly, while the posterior rami are distributed to the deep back muscles which are positioned anteriorly. The secret lies in embryology; at an early stage of development, the superficial back muscles were situated at the anterior side of the embryo so the muscles were innervated by the anterior rami. Throughout development, those muscles have moved around the embryo trunk to become known as the superficial back muscles. During such embryological rearrangement, anterior rami have followed the superficial back muscles till the end.
When a developing organ moves to its final position, the nerve that innervates the organ always follows the organ, unlike artery and vein. Another example is the phrenic nerve in the following chapter (Fig. 3-16).
Fig. 1-12. Splenius capitis and cervicis.
Deep back muscles are classified into erector spinae and transversospinalis (Fig. 1-13). Among the erector spinae muscles, superficial muscles are the splenius capitis and cervicis. “Capitis” and “cervicis” indicate the insertions of muscles. The two short muscles exist only at the superior one third of the vertebral column.
Splenius capitis is superficial to the levator scapulae (Fig. 1-9). Splenius capitis appears in the posterior cervical triangle (Fig. 3-24).
Erector spinae consists of iliocostalis, longissimus, and spinalis (from lateral to medial) (Fig. 1-13). The representative origin of the three muscles are the iliac crest (Fig. 1-3), the transverse processes (Figs. 1-13,14), and the spinous processes, in that order. These muscles are almost as long as the entire vertebral column.
Fig. 1-13. Deep back muscles consisting of erector spinae (circled X), transversospinalis (arrows).
These three muscles consist of several parts. The Latin adjectives (lumborum, thoracis, cervicis, and capitis) mark the muscle’s insertion level. Exceptively, the insertion of iliocostalis lumborum is the lower ribs (thoracic level).
The deep portion of the deep back muscles (the deepest of the deep) is the transversospinalis. The transversospinalis is composed of the semispinalis, multifidus, and rotatores.
In erector spinae and spinal cord, the word “spine” means the “vertebral column.” The “semispinalis” (thoracis, cervicis, capitis) occupy the upper “half of vertebral column.” The main function of the semispinalis is to extend the vertebral column and the head like the erector spinae.
Fig. 1-14. Transversospinalis excluding semispinalis.
The “multifidus” has “multiple origins.” The multifidus crosses at least two intervertebral discs (Fig. 1-5), and therefore is situated obliquely and has the function of extending and rotating the vertebrae.
Due to the fact that the deep “rotatores” is short and crosses only one disc, it is closer to the horizontal plane and its main function is to “rotate” the vertebrae.
Conventionally, the muscles are named in one of three ways. First, like in the transversospinalis, the muscle’s origin (transverse processes) and insertion (spinous processes) are used for naming. Second, like in the semispinalis and multifidus, muscle’s location and shape are used. Third, like in the rotatores, muscle’s action is used (Figs. 1-13,14).
Fig. 1-15. Suboccipital triangle.
Situated deep to the semispinalis capitis (Fig. 1-13) are two straight muscles (rectus capitis posterior minor and major) and two oblique muscles (obliquus capitis superior and inferior). Except for the rectus capitis posterior minor, the three muscles form borders of the suboccipital triangle below the occipital bone. (Inferior nuchal line is a structure of the occipital bone.) All four muscles have similar functions as the transversospinalis: to extend and rotate the head (Fig. 1-14).
From the suboccipital triangle, comes out the suboccipital nerve (posterior ramus of CN1) (Fig. 1-2) to innervate the four muscles. Keep realizing that the posterior ramus is for the deep back muscles (Fig. 1-11).
The vertebral artery of subclavian artery (Fig. 3-26) appears in the suboccipital triangle, just before entering the foramen magnum.
< Spinal cord >
Fig. 1-16. Spinal meninges, resultant spaces.
Prior to discussing the spinal cord as a whole, we must discuss the spinal meninges. Spinal meninges are composed of the pia, arachnoid, and dura maters. The pia mater is hardly detectable with bare eyes, the arachnoid mater is entangled like a spider’s web, and the dura mater is thick.
Here is a tip on the origin of the term “mater.” The “maTers” covering the brain and spinal cord have only one T in its name, while the gray and white “maTTers” consisting the brain and spinal cord have two Ts. What is the maTer? In etymology, the word “maTer” originates from the “moTher” who covers the baby. The “dura maTer” reminds us of “durable moTher.”
Since the pia mater cannot be detached from the spinal cord, there are only three identifiable spaces: subarachnoid, subdural, and epidural spaces (Fig. 1-16). Among them, subarachnoid space containing the cerebrospinal fluid and epidural space containing the fat are the two actual spaces.
On the other hand, subdural space is a potential space; its volume is close to zero, only to be increased by bleeding, and so on (Fig. 4-7). Therefore, Fig. 1-16 which shows considerable volume of subdural space is not an accurate portrayal of reality.
Fig. 1-17. Spinal cord enlargements, cauda equina.
The caudal (inferior) part of the spinal cord is called the conus medullaris. The term “medulla” refers to the spinal cord. Medulla oblongata, which is the caudal part of the brain, is regarded as the elongated spinal cord. “Medulla” may also mean the bone marrow (e.g., the medullary cavity of bone (Fig. 2-24)), or the internal part of an organ (e.g., renal medulla, adrenal medulla (Fig. 6-51)).
The cervical enlargement of the spinal cord exists for the brachial plexus innervating the upper limb (CN5–TN1) (Fig. 2-13). Likewise, the lumbosacral enlargement exists for the lumbosacral plexus innervating the lower limb (LN2–SN3) (Fig. 7-15). So, in case of snakes which have no limbs, there are no enlargements in the spinal cord.
At very early stage of development, the vertebral column used to be as long as the spinal cord. However, during development, the vertical growth of the vertebral column is faster than that of the spinal cord. Because of this discrepancy in length, LN2–SN3 and lower spinal nerves are elongated from the spinal cord to reach the intervertebral foramina. These spinal nerves are collectively named the cauda equina (horsetail).
Even after birth, the vertical growth of the vertebral column is faster than that of the spinal cord. Therefore, the conus medullaris ends at LV2–LV3 level in newborn and at LV1–LV2 level in adult.
The conus medullaris is connected to the coccyx via the terminal filum. The terminal filum can be thought as the trace of the conus medullaris tip that has relatively gone upward from the coccyx.
In case of sacral nerves, the trunk of spinal nerve bifurcates (Fig. 1-2) in the sacral canal. The anterior and posterior rami go through the anterior and posterior sacral foramina, separately (Fig. 7-18).
During the development, the five sacral vertebrae (SV1–SV5) are fused into a single sacrum (Fig. 7-18). Nevertheless, the sacral vertebrae are conveniently used for describing the level.
The dura mater ends at the SV2. The subarachnoid space ends at the SV2 as well because the subdural space is a potential space (Fig. 1-16).
To obtain the cerebrospinal fluid form examination, doctors insert needle to the subarachnoid space. Theoretically, the needle must be injected above SV2. Moreover, the puncture must be done below LV2 in all ages to avoid injuring the spinal cord. However, the actual practice is performed just above or just below LV4; therefore, it is called the “lumbar” puncture, not “sacral” puncture.
During a lumbar puncture, the needle will have to pass through the skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, dura mater, arachnoid mater, then to the subarachnoid space (Figs. 1-5,16).
One might be worried that the needle would injure the spinal nerves of the cauda equina. Hopefully, these spinal nerves are mobile in the cerebrospinal fluid, so the needle hardly ever injures the nerves.
Fig. 1-18. Structures between spinal cord and dura mater (posterior view).
The spinal cord is stabilized by the (string-like) terminal filum, extending inferiorly to the coccyx (Fig. 1-17), as well as by the (sheet-like) denticulate ligaments extending laterally to the dura mater (Fig. 1-19). Denticulate ligaments do not match the spinal nerves in number.
Students who begin anatomy learn that ligaments connect bones to other bones to restrict the range of the joint movement (Fig. 2-58). However, like the denticulate ligament, there are many exceptional ligaments that do not connect bones to bones (Fig. 6-21).
Fig. 1-19. Structures between spinal cord and dura mater (horizontal plane).
Examine Figs. 1-18,19 together. The several rootlets arising from the spinal cord unite to become a posterior root or an anterior root. Posterior and anterior roots exit the dura mater; the posterior root then forms a spinal ganglion, which is found in the intervertebral foramen. The two roots immediately converge to form the trunk of spinal nerve (Fig. 1-2).
At the level of CN1–CN5, the spinal root of XI is located between posterior roots and denticulate ligaments. The spinal root emerges, ascends, and passes the foramen magnum to enter the cranial cavity (Fig. 3-20).
Around the spinal cord of cadaver, there are one anterior spinal artery and two posterior spinal arteries which are reinforced by the radicular arteries.
The posterior root of the spinal nerve is often called the dorsal root. The dorsal direction is decided in early developmental stage. Despite the dramatic change of the embryo, which shifts locations of the body, the term “dorsal” applies constantly. For example, the dorsa of hand and foot, determined at the initial development of the limbs, are consistent even after one is born and grown up (Fig. 8-24).
Fig. 1-20. Direction terms during head fold.
During the head fold (Fig. 6-16), the brain is flexed in different extents according to the brain levels. The term “dorsal” is constant regardless of the angle of the flexion. Thus, the term “dorsal” is preferred over terms such as superior, superoposterior, or posterior. The same reasoning is applied to the terms “ventral, cranial, and caudal.”
The same does not apply to the spinal cord, since the spinal cord is not flexed during the development. So, we can simply use the terms “posterior and anterior” when describing the spinal cord and spinal nerves.