awinml/medqa-textbooks-chunked · Datasets at Hugging Face

id stringlengths 14 28	file_name stringclasses 18 values	content stringlengths 1 722k
Anatomy_Gray_400	Anatomy_Gray.txt	Lymph nodes may become diffusely enlarged in certain systemic illnesses (e.g., viral infection), or local groups may become enlarged with primary lymph node malignancies, such as lymphoma (Fig. 1.31).
Anatomy_Gray_401	Anatomy_Gray.txt	In the clinic
Anatomy_Gray_402	Anatomy_Gray.txt	A knowledge of dermatomes and myotomes is absolutely fundamental to carrying out a neurological examination. A typical dermatome map is shown in Fig. 1.38.
Anatomy_Gray_403	Anatomy_Gray.txt	Clinically, a dermatome is that area of skin supplied by a single spinal nerve or spinal cord level. A myotome is that region of skeletal muscle innervated by a single spinal nerve or spinal cord level. Most individual muscles of the body are innervated by more than one spinal cord level, so the evaluation of myotomes is usually accomplished by testing movements of joints or muscle groups.
Anatomy_Gray_404	Anatomy_Gray.txt	In the clinic
Anatomy_Gray_405	Anatomy_Gray.txt	Referred pain occurs when sensory information comes to the spinal cord from one location but is interpreted by the CNS as coming from another location innervated by the same spinal cord level. Usually, this happens when the pain information comes from a region, such as the gut, which has a low amount of sensory output. These afferents converge on neurons at the same spinal cord level that receive information from the skin, which is an area with a high amount of sensory output. As a result, pain from the normally low output region is interpreted as coming from the normally high output region.
Anatomy_Gray_406	Anatomy_Gray.txt	Pain is most often referred from a region innervated by the visceral part of the nervous system to a region innervated, at the same spinal cord level, by the somatic side of the nervous system.
Anatomy_Gray_407	Anatomy_Gray.txt	to another. For example, irritation of the peritoneum on the inferior surface of the diaphragm, which is innervated by the phrenic nerve, can be referred to the skin on the top of the shoulder, which is innervated by other somatic nerves arising at the same spinal cord level.
Anatomy_Gray_408	Anatomy_Gray.txt	A young man sought medical care because of central abdominal pain that was diffuse and colicky. After some hours, the pain began to localize in the right iliac fossa and became constant. He was referred to an abdominal surgeon, who removed a grossly inflamed appendix. The patient made an uneventful recovery.
Anatomy_Gray_409	Anatomy_Gray.txt	When the appendix becomes inflamed, the visceral sensory fibers are stimulated. These fibers enter the spinal cord with the sympathetic fibers at spinal cord level T10. The pain is referred to the dermatome of T10, which is in the umbilical region (Fig. 1.50). The pain is diffuse, not focal; every time a peristaltic wave passes through the ileocecal region, the pain recurs. This intermittent type of pain is referred to as colic.
Anatomy_Gray_410	Anatomy_Gray.txt	In the later stages of the disease, the appendix contacts and irritates the parietal peritoneum in the right iliac fossa, which is innervated by somatic sensory nerves. This produces a constant focal pain, which predominates over the colicky pain that the patient felt some hours previously. The patient no longer interprets the referred pain from the T10 dermatome.
Anatomy_Gray_411	Anatomy_Gray.txt	Although this is a typical history for appendicitis, it should always be borne in mind that the patient’s symptoms and signs may vary. The appendix is situated in a retrocecal position in approximately 70% of patients; therefore it may never contact the parietal peritoneum anteriorly in the right iliac fossa. It is also possible that the appendix is long and may directly contact other structures. As a consequence, the patient may have other symptoms (e.g., the appendix may contact the ureter, and the patient may then develop urological symptoms).
Anatomy_Gray_412	Anatomy_Gray.txt	Although appendicitis is common, other disorders, for example of the bowel and pelvis, may produce similar symptoms.
Anatomy_Gray_413	Anatomy_Gray.txt	The Body
Anatomy_Gray_414	Anatomy_Gray.txt	In the clinic—cont’d
Anatomy_Gray_415	Anatomy_Gray.txt	The back consists of the posterior aspect of the body and provides the musculoskeletal axis of support for the trunk. Bony elements consist mainly of the vertebrae, although proximal elements of the ribs, superior aspects of the pelvic bones, and posterior basal regions of the skull contribute to the back’s skeletal framework (Fig. 2.1).
Anatomy_Gray_416	Anatomy_Gray.txt	Associated muscles interconnect the vertebrae and ribs with each other and with the pelvis and skull. The back contains the spinal cord and proximal parts of the spinal nerves, which send and receive information to and from most of the body.
Anatomy_Gray_417	Anatomy_Gray.txt	The skeletal and muscular elements of the back support the body’s weight, transmit forces through the pelvis to the lower limbs, carry and position the head, and brace and help maneuver the upper limbs. The vertebral column is positioned posteriorly in the body at the midline. When viewed laterally, it has a number of curvatures (Fig. 2.2):
Anatomy_Gray_418	Anatomy_Gray.txt	The primary curvature of the vertebral column is concave anteriorly, reflecting the original shape of the embryo, and is retained in the thoracic and sacral regions in adults.
Anatomy_Gray_419	Anatomy_Gray.txt	Secondary curvatures, which are concave posteriorly, form in the cervical and lumbar regions and bring the center of gravity into a vertical line, which allows the body’s weight to be balanced on the vertebral column in a way that expends the least amount of muscular energy to maintain an upright bipedal stance.
Anatomy_Gray_420	Anatomy_Gray.txt	As stresses on the back increase from the cervical to lumbar regions, lower back problems are common.
Anatomy_Gray_421	Anatomy_Gray.txt	Muscles of the back consist of extrinsic and intrinsic groups:
Anatomy_Gray_422	Anatomy_Gray.txt	The extrinsic muscles of the back move the upper limbs and the ribs.
Anatomy_Gray_423	Anatomy_Gray.txt	The intrinsic muscles of the back maintain posture and move the vertebral column; these movements include flexion (anterior bending), extension, lateral flexion, and rotation (Fig. 2.3).
Anatomy_Gray_424	Anatomy_Gray.txt	Although the amount of movement between any two vertebrae is limited, the effects between vertebrae are additive along the length of the vertebral column. Also, freedom of movement and extension are limited in the thoracic region relative to the lumbar part of the vertebral column. Muscles in more anterior regions flex the vertebral column.
Anatomy_Gray_425	Anatomy_Gray.txt	In the cervical region, the first two vertebrae and associated muscles are specifically modified to support and position the head. The head flexes and extends, in the nodding motion, on vertebra CI, and rotation of the head occurs as vertebra CI moves on vertebra CII (Fig. 2.3).
Anatomy_Gray_426	Anatomy_Gray.txt	Protection of the nervous system
Anatomy_Gray_427	Anatomy_Gray.txt	The vertebral column and associated soft tissues of the back contain the spinal cord and proximal parts of the spinal nerves (Fig. 2.4). The more distal parts of the spinal nerves pass into all other regions of the body, including certain regions of the head.
Anatomy_Gray_428	Anatomy_Gray.txt	The major bones of the back are the 33 vertebrae (Fig. 2.5). The number and specific characteristics of the vertebrae vary depending on the body region with which they are associated. There are seven cervical, twelve thoracic, five lumbar, five sacral, and three to four coccygeal vertebrae. The sacral vertebrae fuse into a single bony element, the sacrum. The coccygeal vertebrae are rudimentary in structure, vary in number from three to four, and often fuse into a single coccyx.
Anatomy_Gray_429	Anatomy_Gray.txt	A typical vertebra consists of a vertebral body and a vertebral arch (Fig. 2.6).
Anatomy_Gray_430	Anatomy_Gray.txt	The vertebral body is anterior and is the major weightbearing component of the bone. It increases in size from vertebra CII to vertebra LV. Fibrocartilaginous intervertebral discs separate the vertebral bodies of adjacent vertebrae.
Anatomy_Gray_431	Anatomy_Gray.txt	The vertebral arch is firmly anchored to the posterior surface of the vertebral body by two pedicles, which form the lateral pillars of the vertebral arch. The roof of the vertebral arch is formed by right and left laminae, which fuse at the midline.
Anatomy_Gray_432	Anatomy_Gray.txt	The vertebral arches of the vertebrae are aligned to form the lateral and posterior walls of the vertebral canal, which extends from the first cervical vertebra (CI) to the last sacral vertebra (vertebra SV). This bony canal contains the spinal cord and its protective membranes, together with blood vessels, connective tissue, fat, and proximal parts of spinal nerves.
Anatomy_Gray_433	Anatomy_Gray.txt	The vertebral arch of a typical vertebra has a number of characteristic projections, which serve as: attachments for muscles and ligaments, levers for the action of muscles, and sites of articulation with adjacent vertebrae.
Anatomy_Gray_434	Anatomy_Gray.txt	A spinous process projects posteriorly and generally inferiorly from the roof of the vertebral arch.
Anatomy_Gray_435	Anatomy_Gray.txt	On each side of the vertebral arch, a transverse process extends laterally from the region where a lamina meets a pedicle. From the same region, a superior articular process and an inferior articular process articulate with similar processes on adjacent vertebrae.
Anatomy_Gray_436	Anatomy_Gray.txt	Each vertebra also contains rib elements. In the thorax, these costal elements are large and form ribs, which articulate with the vertebral bodies and transverse processes.
Anatomy_Gray_437	Anatomy_Gray.txt	In all other regions, these rib elements are small and are incorporated into the transverse processes. Occasionally, they develop into ribs in regions other than the thorax, usually in the lower cervical and upper lumbar regions.
Anatomy_Gray_438	Anatomy_Gray.txt	Muscles in the back can be classified as extrinsic or intrinsic based on their embryological origin and type of innervation (Fig. 2.7).
Anatomy_Gray_439	Anatomy_Gray.txt	The extrinsic muscles are involved with movements of the upper limbs and thoracic wall and, in general, are innervated by anterior rami of spinal nerves. The superficial group of these muscles is related to the upper limbs, while the intermediate layer of muscles is associated with the thoracic wall.
Anatomy_Gray_440	Anatomy_Gray.txt	All of the intrinsic muscles of the back are deep in position and are innervated by the posterior rami of spinal nerves. They support and move the vertebral column and participate in moving the head. One group of intrinsic muscles also moves the ribs relative to the vertebral column.
Anatomy_Gray_441	Anatomy_Gray.txt	The spinal cord lies within a bony canal formed by adjacent vertebrae and soft tissue elements (the vertebral canal) (Fig. 2.8):
Anatomy_Gray_442	Anatomy_Gray.txt	The anterior wall is formed by the vertebral bodies of the vertebrae, intervertebral discs, and associated ligaments.
Anatomy_Gray_443	Anatomy_Gray.txt	The lateral walls and roof are formed by the vertebral arches and ligaments.
Anatomy_Gray_444	Anatomy_Gray.txt	Within the vertebral canal, the spinal cord is surrounded by a series of three connective tissue membranes (the meninges):
Anatomy_Gray_445	Anatomy_Gray.txt	The pia mater is the innermost membrane and is intimately associated with the surface of the spinal cord.
Anatomy_Gray_446	Anatomy_Gray.txt	The second membrane, the arachnoid mater, is separated from the pia by the subarachnoid space, which contains cerebrospinal fluid.
Anatomy_Gray_447	Anatomy_Gray.txt	The thickest and most external of the membranes, the dura mater, lies directly against, but is not attached to, the arachnoid mater.
Anatomy_Gray_448	Anatomy_Gray.txt	In the vertebral canal, the dura mater is separated from surrounding bone by an extradural (epidural) space containing loose connective tissue, fat, and a venous plexus.
Anatomy_Gray_449	Anatomy_Gray.txt	The 31 pairs of spinal nerves are segmental in distribution and emerge from the vertebral canal between the pedicles of adjacent vertebrae. There are eight pairs of cervical nerves (C1 to C8), twelve thoracic (T1 to T12), five lumbar (L1 to L5), five sacral (S1 to S5), and one coccygeal (Co). Each nerve is attached to the spinal cord by a posterior root and an anterior root (Fig. 2.9).
Anatomy_Gray_450	Anatomy_Gray.txt	After exiting the vertebral canal, each spinal nerve branches into: a posterior ramus—collectively, the small posterior rami innervate the back; and an anterior ramus—the much larger anterior rami innervate most other regions of the body except the head, which is innervated predominantly, but not exclusively, by cranial nerves.
Anatomy_Gray_451	Anatomy_Gray.txt	The anterior rami form the major somatic plexuses (cervical, brachial, lumbar, and sacral) of the body. Major visceral components of the PNS (sympathetic trunk and prevertebral plexus) of the body are also associated mainly with the anterior rami of spinal nerves.
Anatomy_Gray_452	Anatomy_Gray.txt	Cervical regions of the back constitute the skeletal and much of the muscular framework of the neck, which in turn supports and moves the head (Fig. 2.10).
Anatomy_Gray_453	Anatomy_Gray.txt	The brain and cranial meninges are continuous with the spinal cord meninges at the foramen magnum of the skull. The paired vertebral arteries ascend, one on each side, through foramina in the transverse processes of cervical vertebrae and pass through the foramen magnum to participate, with the internal carotid arteries, in supplying blood to the brain.
Anatomy_Gray_454	Anatomy_Gray.txt	Thorax, abdomen, and pelvis
Anatomy_Gray_455	Anatomy_Gray.txt	The different regions of the vertebral column contribute to the skeletal framework of the thorax, abdomen, and pelvis (Fig. 2.10). In addition to providing support for each of these parts of the body, the vertebrae provide attachments for muscles and fascia, and articulation sites for other bones. The anterior rami of spinal nerves associated with the thorax, abdomen, and pelvis pass into these parts of the body from the back.
Anatomy_Gray_456	Anatomy_Gray.txt	The bones of the back provide extensive attachments for muscles associated with anchoring and moving the upper limbs on the trunk. This is less true of the lower limbs, which are firmly anchored to the vertebral column through articulation of the pelvic bones with the sacrum. The upper and lower limbs are innervated by anterior rami of spinal nerves that emerge from cervical and lumbosacral levels, respectively, of the vertebral column.
Anatomy_Gray_457	Anatomy_Gray.txt	During development, the vertebral column grows much faster than the spinal cord. As a result, the spinal cord does not extend the entire length of the vertebral canal (Fig. 2.11).
Anatomy_Gray_458	Anatomy_Gray.txt	In the adult, the spinal cord typically ends between vertebrae LI and LII, although it can end as high as vertebra TXII and as low as the disc between vertebrae LII and LIII.
Anatomy_Gray_459	Anatomy_Gray.txt	Spinal nerves originate from the spinal cord at increasingly oblique angles from vertebrae CI to Co, and the nerve roots pass in the vertebral canal for increasingly longer distances. Their spinal cord level of origin therefore becomes increasingly dissociated from their vertebral column level of exit. This is particularly evident for lumbar and sacral spinal nerves.
Anatomy_Gray_460	Anatomy_Gray.txt	Each spinal nerve exits the vertebral canal laterally through an intervertebral foramen (Fig. 2.12). The foramen is formed between adjacent vertebral arches and is closely related to intervertebral joints:
Anatomy_Gray_461	Anatomy_Gray.txt	The superior and inferior margins are formed by notches in adjacent pedicles.
Anatomy_Gray_462	Anatomy_Gray.txt	The posterior margin is formed by the articular processes of the vertebral arches and the associated joint.
Anatomy_Gray_463	Anatomy_Gray.txt	The anterior border is formed by the intervertebral disc between the vertebral bodies of the adjacent vertebrae.
Anatomy_Gray_464	Anatomy_Gray.txt	Any pathology that occludes or reduces the size of an intervertebral foramen, such as bone loss, herniation of the intervertebral disc, or dislocation of the zygapophysial joint (the joint between the articular processes), can affect the function of the associated spinal nerve.
Anatomy_Gray_465	Anatomy_Gray.txt	Innervation of the back
Anatomy_Gray_466	Anatomy_Gray.txt	Posterior branches of spinal nerves innervate the intrinsic muscles of the back and adjacent skin. The cutaneous distribution of these posterior rami extends into the gluteal region of the lower limb and the posterior aspect of the head. Parts of dermatomes innervated by the posterior rami of spinal nerves are shown in Fig. 2.13.
Anatomy_Gray_467	Anatomy_Gray.txt	Skeletal components of the back consist mainly of the vertebrae and associated intervertebral discs. The skull, scapulae, pelvic bones, and ribs also contribute to the bony framework of the back and provide sites for muscle attachment.
Anatomy_Gray_468	Anatomy_Gray.txt	There are approximately 33 vertebrae, which are subdivided into five groups based on morphology and location (Fig. 2.14):
Anatomy_Gray_469	Anatomy_Gray.txt	The seven cervical vertebrae between the thorax and skull are characterized mainly by their small size and the presence of a foramen in each transverse process (Figs. 2.14 and 2.15).
Anatomy_Gray_470	Anatomy_Gray.txt	The 12 thoracic vertebrae are characterized by their articulated ribs (Figs. 2.14 and 2.16); although all vertebrae have rib elements, these elements are small and are incorporated into the transverse processes in regions other than the thorax; but in the thorax, the ribs are separate bones and articulate via synovial joints with the vertebral bodies and transverse processes of the associated vertebrae.
Anatomy_Gray_471	Anatomy_Gray.txt	Inferior to the thoracic vertebrae are five lumbar vertebrae, which form the skeletal support for the posterior abdominal wall and are characterized by their large size (Figs. 2.14 and 2.17).
Anatomy_Gray_472	Anatomy_Gray.txt	Next are five sacral vertebrae fused into one single bone called the sacrum, which articulates on each side with a pelvic bone and is a component of the pelvic wall.
Anatomy_Gray_473	Anatomy_Gray.txt	Inferior to the sacrum is a variable number, usually four, of coccygeal vertebrae, which fuse into a single small triangular bone called the coccyx.
Anatomy_Gray_474	Anatomy_Gray.txt	In the embryo, the vertebrae are formed intersegmentally from cells called sclerotomes, which originate from adjacent somites (Fig. 2.18). Each vertebra is derived from the cranial parts of the two somites below, one on each side, and the caudal parts of the two somites above. The spinal nerves develop segmentally and pass between the forming vertebrae.
Anatomy_Gray_475	Anatomy_Gray.txt	A typical vertebra consists of a vertebral body and a posterior vertebral arch (Fig. 2.19). Extending from the vertebral arch are a number of processes for muscle attachment and articulation with adjacent bone.
Anatomy_Gray_476	Anatomy_Gray.txt	The vertebral body is the weight-bearing part of the vertebra and is linked to adjacent vertebral bodies by intervertebral discs and ligaments. The size of vertebral bodies increases inferiorly as the amount of weight supported increases.
Anatomy_Gray_477	Anatomy_Gray.txt	The vertebral arch forms the lateral and posterior parts of the vertebral foramen.
Anatomy_Gray_478	Anatomy_Gray.txt	The vertebral foramina of all the vertebrae together form the vertebral canal, which contains and protects the spinal cord. Superiorly, the vertebral canal is continuous, through the foramen magnum of the skull, with the cranial cavity of the head.
Anatomy_Gray_479	Anatomy_Gray.txt	The vertebral arch of each vertebra consists of pedicles and laminae (Fig. 2.19):
Anatomy_Gray_480	Anatomy_Gray.txt	The two pedicles are bony pillars that attach the vertebral arch to the vertebral body.
Anatomy_Gray_481	Anatomy_Gray.txt	The two laminae are flat sheets of bone that extend from each pedicle to meet in the midline and form the roof of the vertebral arch.
Anatomy_Gray_482	Anatomy_Gray.txt	A spinous process projects posteriorly and inferiorly from the junction of the two laminae and is a site for muscle and ligament attachment.
Anatomy_Gray_483	Anatomy_Gray.txt	A transverse process extends posterolaterally from the junction of the pedicle and lamina on each side and is a site for muscle and ligament attachment, and for articulation with ribs in the thoracic region.
Anatomy_Gray_484	Anatomy_Gray.txt	Also projecting from the region where the pedicles join the laminae are superior and inferior articular processes (Fig. 2.19), which articulate with the inferior and superior articular processes, respectively, of adjacent vertebrae.
Anatomy_Gray_485	Anatomy_Gray.txt	Between the vertebral body and the origin of the articular processes, each pedicle is notched on its superior and inferior surfaces. These superior and inferior vertebral notches participate in forming intervertebral foramina.
Anatomy_Gray_486	Anatomy_Gray.txt	The seven cervical vertebrae are characterized by their small size and by the presence of a foramen in each transverse process. A typical cervical vertebra has the following features (Fig. 2.20A):
Anatomy_Gray_487	Anatomy_Gray.txt	The vertebral body is short in height and square shaped when viewed from above and has a concave superior surface and a convex inferior surface.
Anatomy_Gray_488	Anatomy_Gray.txt	Each transverse process is trough shaped and perforated by a round foramen transversarium.
Anatomy_Gray_489	Anatomy_Gray.txt	The spinous process is short and bifid.
Anatomy_Gray_490	Anatomy_Gray.txt	The vertebral foramen is triangular.
Anatomy_Gray_491	Anatomy_Gray.txt	The first and second cervical vertebrae—the atlas and axis—are specialized to accommodate movement of the head.
Anatomy_Gray_492	Anatomy_Gray.txt	Vertebra CI (the atlas) articulates with the head (Fig. 2.21). Its major distinguishing feature is that it lacks a vertebral body (Fig. 2.20B). In fact, the vertebral body of CI fuses onto the body of CII during development to become the dens of CII. As a result, there is no intervertebral disc between CI and CII. When viewed from above, the atlas is ring shaped and composed of two lateral masses interconnected by an anterior arch and a posterior arch.
Anatomy_Gray_493	Anatomy_Gray.txt	Each lateral mass articulates above with an occipital condyle of the skull and below with the superior articular process of vertebra CII (the axis). The superior articular surfaces are bean shaped and concave, whereas the inferior articular surfaces are almost circular and flat.
Anatomy_Gray_494	Anatomy_Gray.txt	The atlanto-occipital joint allows the head to nod up and down on the vertebral column.
Anatomy_Gray_495	Anatomy_Gray.txt	The posterior surface of the anterior arch has an articular facet for the dens, which projects superiorly from the vertebral body of the axis. The dens is held in position by a strong transverse ligament of atlas posterior to it and spanning the distance between the oval attachment facets on the medial surfaces of the lateral masses of the atlas.
Anatomy_Gray_496	Anatomy_Gray.txt	The dens acts as a pivot that allows the atlas and attached head to rotate on the axis, side to side.
Anatomy_Gray_497	Anatomy_Gray.txt	The transverse processes of the atlas are large and protrude further laterally than those of the other cervical vertebrae and act as levers for muscle action, particularly for muscles that move the head at the atlanto-axial joints.
Anatomy_Gray_498	Anatomy_Gray.txt	The axis is characterized by the large tooth-like dens, which extends superiorly from the vertebral body (Figs. 2.20B and 2.21). The anterior surface of the dens has an oval facet for articulation with the anterior arch of the atlas.
Anatomy_Gray_499	Anatomy_Gray.txt	The two superolateral surfaces of the dens possess circular impressions that serve as attachment sites for strong alar ligaments, one on each side, which connect the dens to the medial surfaces of the occipital condyles. These alar ligaments check excessive rotation of the head and atlas relative to the axis.

Anatomy_Gray_400

Anatomy_Gray.txt

Lymph nodes may become diffusely enlarged in certain systemic illnesses (e.g., viral infection), or local groups may become enlarged with primary lymph node malignancies, such as lymphoma (Fig. 1.31).

Anatomy_Gray_401

Anatomy_Gray.txt

In the clinic

Anatomy_Gray_402

Anatomy_Gray.txt

A knowledge of dermatomes and myotomes is absolutely fundamental to carrying out a neurological examination. A typical dermatome map is shown in Fig. 1.38.

Anatomy_Gray_403

Anatomy_Gray.txt

Clinically, a dermatome is that area of skin supplied by a single spinal nerve or spinal cord level. A myotome is that region of skeletal muscle innervated by a single spinal nerve or spinal cord level. Most individual muscles of the body are innervated by more than one spinal cord level, so the evaluation of myotomes is usually accomplished by testing movements of joints or muscle groups.

Anatomy_Gray_404

Anatomy_Gray.txt

In the clinic

Anatomy_Gray_405

Anatomy_Gray.txt

Referred pain occurs when sensory information comes to the spinal cord from one location but is interpreted by the CNS as coming from another location innervated by the same spinal cord level. Usually, this happens when the pain information comes from a region, such as the gut, which has a low amount of sensory output. These afferents converge on neurons at the same spinal cord level that receive information from the skin, which is an area with a high amount of sensory output. As a result, pain from the normally low output region is interpreted as coming from the normally high output region.

Anatomy_Gray_406

Anatomy_Gray.txt

Pain is most often referred from a region innervated by the visceral part of the nervous system to a region innervated, at the same spinal cord level, by the somatic side of the nervous system.

Anatomy_Gray_407

Anatomy_Gray.txt

to another. For example, irritation of the peritoneum on the inferior surface of the diaphragm, which is innervated by the phrenic nerve, can be referred to the skin on the top of the shoulder, which is innervated by other somatic nerves arising at the same spinal cord level.

Anatomy_Gray_408

Anatomy_Gray.txt

A young man sought medical care because of central abdominal pain that was diffuse and colicky. After some hours, the pain began to localize in the right iliac fossa and became constant. He was referred to an abdominal surgeon, who removed a grossly inflamed appendix. The patient made an uneventful recovery.

Anatomy_Gray_409

Anatomy_Gray.txt

When the appendix becomes inflamed, the visceral sensory fibers are stimulated. These fibers enter the spinal cord with the sympathetic fibers at spinal cord level T10. The pain is referred to the dermatome of T10, which is in the umbilical region (Fig. 1.50). The pain is diffuse, not focal; every time a peristaltic wave passes through the ileocecal region, the pain recurs. This intermittent type of pain is referred to as colic.

Anatomy_Gray_410

Anatomy_Gray.txt

In the later stages of the disease, the appendix contacts and irritates the parietal peritoneum in the right iliac fossa, which is innervated by somatic sensory nerves. This produces a constant focal pain, which predominates over the colicky pain that the patient felt some hours previously. The patient no longer interprets the referred pain from the T10 dermatome.

Anatomy_Gray_411

Anatomy_Gray.txt

Although this is a typical history for appendicitis, it should always be borne in mind that the patient’s symptoms and signs may vary. The appendix is situated in a retrocecal position in approximately 70% of patients; therefore it may never contact the parietal peritoneum anteriorly in the right iliac fossa. It is also possible that the appendix is long and may directly contact other structures. As a consequence, the patient may have other symptoms (e.g., the appendix may contact the ureter, and the patient may then develop urological symptoms).

Anatomy_Gray_412

Anatomy_Gray.txt

Although appendicitis is common, other disorders, for example of the bowel and pelvis, may produce similar symptoms.

Anatomy_Gray_413

Anatomy_Gray.txt

The Body

Anatomy_Gray_414

Anatomy_Gray.txt

In the clinic—cont’d

Anatomy_Gray_415

Anatomy_Gray.txt

The back consists of the posterior aspect of the body and provides the musculoskeletal axis of support for the trunk. Bony elements consist mainly of the vertebrae, although proximal elements of the ribs, superior aspects of the pelvic bones, and posterior basal regions of the skull contribute to the back’s skeletal framework (Fig. 2.1).

Anatomy_Gray_416

Anatomy_Gray.txt

Associated muscles interconnect the vertebrae and ribs with each other and with the pelvis and skull. The back contains the spinal cord and proximal parts of the spinal nerves, which send and receive information to and from most of the body.

Anatomy_Gray_417

Anatomy_Gray.txt

The skeletal and muscular elements of the back support the body’s weight, transmit forces through the pelvis to the lower limbs, carry and position the head, and brace and help maneuver the upper limbs. The vertebral column is positioned posteriorly in the body at the midline. When viewed laterally, it has a number of curvatures (Fig. 2.2):

Anatomy_Gray_418

Anatomy_Gray.txt

The primary curvature of the vertebral column is concave anteriorly, reflecting the original shape of the embryo, and is retained in the thoracic and sacral regions in adults.

Anatomy_Gray_419

Anatomy_Gray.txt

Secondary curvatures, which are concave posteriorly, form in the cervical and lumbar regions and bring the center of gravity into a vertical line, which allows the body’s weight to be balanced on the vertebral column in a way that expends the least amount of muscular energy to maintain an upright bipedal stance.

Anatomy_Gray_420

Anatomy_Gray.txt

As stresses on the back increase from the cervical to lumbar regions, lower back problems are common.

Anatomy_Gray_421

Anatomy_Gray.txt

Muscles of the back consist of extrinsic and intrinsic groups:

Anatomy_Gray_422

Anatomy_Gray.txt

The extrinsic muscles of the back move the upper limbs and the ribs.

Anatomy_Gray_423

Anatomy_Gray.txt

The intrinsic muscles of the back maintain posture and move the vertebral column; these movements include flexion (anterior bending), extension, lateral flexion, and rotation (Fig. 2.3).

Anatomy_Gray_424

Anatomy_Gray.txt

Although the amount of movement between any two vertebrae is limited, the effects between vertebrae are additive along the length of the vertebral column. Also, freedom of movement and extension are limited in the thoracic region relative to the lumbar part of the vertebral column. Muscles in more anterior regions flex the vertebral column.

Anatomy_Gray_425

Anatomy_Gray.txt

In the cervical region, the first two vertebrae and associated muscles are specifically modified to support and position the head. The head flexes and extends, in the nodding motion, on vertebra CI, and rotation of the head occurs as vertebra CI moves on vertebra CII (Fig. 2.3).

Anatomy_Gray_426

Anatomy_Gray.txt

Protection of the nervous system

Anatomy_Gray_427

Anatomy_Gray.txt

The vertebral column and associated soft tissues of the back contain the spinal cord and proximal parts of the spinal nerves (Fig. 2.4). The more distal parts of the spinal nerves pass into all other regions of the body, including certain regions of the head.

Anatomy_Gray_428

Anatomy_Gray.txt

The major bones of the back are the 33 vertebrae (Fig. 2.5). The number and specific characteristics of the vertebrae vary depending on the body region with which they are associated. There are seven cervical, twelve thoracic, five lumbar, five sacral, and three to four coccygeal vertebrae. The sacral vertebrae fuse into a single bony element, the sacrum. The coccygeal vertebrae are rudimentary in structure, vary in number from three to four, and often fuse into a single coccyx.

Anatomy_Gray_429

Anatomy_Gray.txt

A typical vertebra consists of a vertebral body and a vertebral arch (Fig. 2.6).

Anatomy_Gray_430

Anatomy_Gray.txt

The vertebral body is anterior and is the major weightbearing component of the bone. It increases in size from vertebra CII to vertebra LV. Fibrocartilaginous intervertebral discs separate the vertebral bodies of adjacent vertebrae.

Anatomy_Gray_431

Anatomy_Gray.txt

The vertebral arch is firmly anchored to the posterior surface of the vertebral body by two pedicles, which form the lateral pillars of the vertebral arch. The roof of the vertebral arch is formed by right and left laminae, which fuse at the midline.

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The vertebral arches of the vertebrae are aligned to form the lateral and posterior walls of the vertebral canal, which extends from the first cervical vertebra (CI) to the last sacral vertebra (vertebra SV). This bony canal contains the spinal cord and its protective membranes, together with blood vessels, connective tissue, fat, and proximal parts of spinal nerves.

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The vertebral arch of a typical vertebra has a number of characteristic projections, which serve as: attachments for muscles and ligaments, levers for the action of muscles, and sites of articulation with adjacent vertebrae.

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A spinous process projects posteriorly and generally inferiorly from the roof of the vertebral arch.

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On each side of the vertebral arch, a transverse process extends laterally from the region where a lamina meets a pedicle. From the same region, a superior articular process and an inferior articular process articulate with similar processes on adjacent vertebrae.

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Each vertebra also contains rib elements. In the thorax, these costal elements are large and form ribs, which articulate with the vertebral bodies and transverse processes.

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In all other regions, these rib elements are small and are incorporated into the transverse processes. Occasionally, they develop into ribs in regions other than the thorax, usually in the lower cervical and upper lumbar regions.

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Muscles in the back can be classified as extrinsic or intrinsic based on their embryological origin and type of innervation (Fig. 2.7).

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The extrinsic muscles are involved with movements of the upper limbs and thoracic wall and, in general, are innervated by anterior rami of spinal nerves. The superficial group of these muscles is related to the upper limbs, while the intermediate layer of muscles is associated with the thoracic wall.

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All of the intrinsic muscles of the back are deep in position and are innervated by the posterior rami of spinal nerves. They support and move the vertebral column and participate in moving the head. One group of intrinsic muscles also moves the ribs relative to the vertebral column.

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The spinal cord lies within a bony canal formed by adjacent vertebrae and soft tissue elements (the vertebral canal) (Fig. 2.8):

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The anterior wall is formed by the vertebral bodies of the vertebrae, intervertebral discs, and associated ligaments.

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The lateral walls and roof are formed by the vertebral arches and ligaments.

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Within the vertebral canal, the spinal cord is surrounded by a series of three connective tissue membranes (the meninges):

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The pia mater is the innermost membrane and is intimately associated with the surface of the spinal cord.

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The second membrane, the arachnoid mater, is separated from the pia by the subarachnoid space, which contains cerebrospinal fluid.

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The thickest and most external of the membranes, the dura mater, lies directly against, but is not attached to, the arachnoid mater.

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In the vertebral canal, the dura mater is separated from surrounding bone by an extradural (epidural) space containing loose connective tissue, fat, and a venous plexus.

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The 31 pairs of spinal nerves are segmental in distribution and emerge from the vertebral canal between the pedicles of adjacent vertebrae. There are eight pairs of cervical nerves (C1 to C8), twelve thoracic (T1 to T12), five lumbar (L1 to L5), five sacral (S1 to S5), and one coccygeal (Co). Each nerve is attached to the spinal cord by a posterior root and an anterior root (Fig. 2.9).

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After exiting the vertebral canal, each spinal nerve branches into: a posterior ramus—collectively, the small posterior rami innervate the back; and an anterior ramus—the much larger anterior rami innervate most other regions of the body except the head, which is innervated predominantly, but not exclusively, by cranial nerves.

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The anterior rami form the major somatic plexuses (cervical, brachial, lumbar, and sacral) of the body. Major visceral components of the PNS (sympathetic trunk and prevertebral plexus) of the body are also associated mainly with the anterior rami of spinal nerves.

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Cervical regions of the back constitute the skeletal and much of the muscular framework of the neck, which in turn supports and moves the head (Fig. 2.10).

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The brain and cranial meninges are continuous with the spinal cord meninges at the foramen magnum of the skull. The paired vertebral arteries ascend, one on each side, through foramina in the transverse processes of cervical vertebrae and pass through the foramen magnum to participate, with the internal carotid arteries, in supplying blood to the brain.

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Thorax, abdomen, and pelvis

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The different regions of the vertebral column contribute to the skeletal framework of the thorax, abdomen, and pelvis (Fig. 2.10). In addition to providing support for each of these parts of the body, the vertebrae provide attachments for muscles and fascia, and articulation sites for other bones. The anterior rami of spinal nerves associated with the thorax, abdomen, and pelvis pass into these parts of the body from the back.

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The bones of the back provide extensive attachments for muscles associated with anchoring and moving the upper limbs on the trunk. This is less true of the lower limbs, which are firmly anchored to the vertebral column through articulation of the pelvic bones with the sacrum. The upper and lower limbs are innervated by anterior rami of spinal nerves that emerge from cervical and lumbosacral levels, respectively, of the vertebral column.

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During development, the vertebral column grows much faster than the spinal cord. As a result, the spinal cord does not extend the entire length of the vertebral canal (Fig. 2.11).

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In the adult, the spinal cord typically ends between vertebrae LI and LII, although it can end as high as vertebra TXII and as low as the disc between vertebrae LII and LIII.

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Spinal nerves originate from the spinal cord at increasingly oblique angles from vertebrae CI to Co, and the nerve roots pass in the vertebral canal for increasingly longer distances. Their spinal cord level of origin therefore becomes increasingly dissociated from their vertebral column level of exit. This is particularly evident for lumbar and sacral spinal nerves.

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Each spinal nerve exits the vertebral canal laterally through an intervertebral foramen (Fig. 2.12). The foramen is formed between adjacent vertebral arches and is closely related to intervertebral joints:

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The superior and inferior margins are formed by notches in adjacent pedicles.

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The posterior margin is formed by the articular processes of the vertebral arches and the associated joint.

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The anterior border is formed by the intervertebral disc between the vertebral bodies of the adjacent vertebrae.

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Any pathology that occludes or reduces the size of an intervertebral foramen, such as bone loss, herniation of the intervertebral disc, or dislocation of the zygapophysial joint (the joint between the articular processes), can affect the function of the associated spinal nerve.

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Innervation of the back

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Posterior branches of spinal nerves innervate the intrinsic muscles of the back and adjacent skin. The cutaneous distribution of these posterior rami extends into the gluteal region of the lower limb and the posterior aspect of the head. Parts of dermatomes innervated by the posterior rami of spinal nerves are shown in Fig. 2.13.

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Skeletal components of the back consist mainly of the vertebrae and associated intervertebral discs. The skull, scapulae, pelvic bones, and ribs also contribute to the bony framework of the back and provide sites for muscle attachment.

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There are approximately 33 vertebrae, which are subdivided into five groups based on morphology and location (Fig. 2.14):

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The seven cervical vertebrae between the thorax and skull are characterized mainly by their small size and the presence of a foramen in each transverse process (Figs. 2.14 and 2.15).

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The 12 thoracic vertebrae are characterized by their articulated ribs (Figs. 2.14 and 2.16); although all vertebrae have rib elements, these elements are small and are incorporated into the transverse processes in regions other than the thorax; but in the thorax, the ribs are separate bones and articulate via synovial joints with the vertebral bodies and transverse processes of the associated vertebrae.

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Inferior to the thoracic vertebrae are five lumbar vertebrae, which form the skeletal support for the posterior abdominal wall and are characterized by their large size (Figs. 2.14 and 2.17).

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Next are five sacral vertebrae fused into one single bone called the sacrum, which articulates on each side with a pelvic bone and is a component of the pelvic wall.

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Inferior to the sacrum is a variable number, usually four, of coccygeal vertebrae, which fuse into a single small triangular bone called the coccyx.

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In the embryo, the vertebrae are formed intersegmentally from cells called sclerotomes, which originate from adjacent somites (Fig. 2.18). Each vertebra is derived from the cranial parts of the two somites below, one on each side, and the caudal parts of the two somites above. The spinal nerves develop segmentally and pass between the forming vertebrae.

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A typical vertebra consists of a vertebral body and a posterior vertebral arch (Fig. 2.19). Extending from the vertebral arch are a number of processes for muscle attachment and articulation with adjacent bone.

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The vertebral body is the weight-bearing part of the vertebra and is linked to adjacent vertebral bodies by intervertebral discs and ligaments. The size of vertebral bodies increases inferiorly as the amount of weight supported increases.

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The vertebral arch forms the lateral and posterior parts of the vertebral foramen.

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The vertebral foramina of all the vertebrae together form the vertebral canal, which contains and protects the spinal cord. Superiorly, the vertebral canal is continuous, through the foramen magnum of the skull, with the cranial cavity of the head.

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The vertebral arch of each vertebra consists of pedicles and laminae (Fig. 2.19):

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The two pedicles are bony pillars that attach the vertebral arch to the vertebral body.

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The two laminae are flat sheets of bone that extend from each pedicle to meet in the midline and form the roof of the vertebral arch.

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A spinous process projects posteriorly and inferiorly from the junction of the two laminae and is a site for muscle and ligament attachment.

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A transverse process extends posterolaterally from the junction of the pedicle and lamina on each side and is a site for muscle and ligament attachment, and for articulation with ribs in the thoracic region.

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Also projecting from the region where the pedicles join the laminae are superior and inferior articular processes (Fig. 2.19), which articulate with the inferior and superior articular processes, respectively, of adjacent vertebrae.

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Between the vertebral body and the origin of the articular processes, each pedicle is notched on its superior and inferior surfaces. These superior and inferior vertebral notches participate in forming intervertebral foramina.

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The seven cervical vertebrae are characterized by their small size and by the presence of a foramen in each transverse process. A typical cervical vertebra has the following features (Fig. 2.20A):

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The vertebral body is short in height and square shaped when viewed from above and has a concave superior surface and a convex inferior surface.

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Each transverse process is trough shaped and perforated by a round foramen transversarium.

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The spinous process is short and bifid.

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The vertebral foramen is triangular.

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The first and second cervical vertebrae—the atlas and axis—are specialized to accommodate movement of the head.

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Vertebra CI (the atlas) articulates with the head (Fig. 2.21). Its major distinguishing feature is that it lacks a vertebral body (Fig. 2.20B). In fact, the vertebral body of CI fuses onto the body of CII during development to become the dens of CII. As a result, there is no intervertebral disc between CI and CII. When viewed from above, the atlas is ring shaped and composed of two lateral masses interconnected by an anterior arch and a posterior arch.

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Each lateral mass articulates above with an occipital condyle of the skull and below with the superior articular process of vertebra CII (the axis). The superior articular surfaces are bean shaped and concave, whereas the inferior articular surfaces are almost circular and flat.

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The atlanto-occipital joint allows the head to nod up and down on the vertebral column.

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The posterior surface of the anterior arch has an articular facet for the dens, which projects superiorly from the vertebral body of the axis. The dens is held in position by a strong transverse ligament of atlas posterior to it and spanning the distance between the oval attachment facets on the medial surfaces of the lateral masses of the atlas.

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The dens acts as a pivot that allows the atlas and attached head to rotate on the axis, side to side.

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The transverse processes of the atlas are large and protrude further laterally than those of the other cervical vertebrae and act as levers for muscle action, particularly for muscles that move the head at the atlanto-axial joints.

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The axis is characterized by the large tooth-like dens, which extends superiorly from the vertebral body (Figs. 2.20B and 2.21). The anterior surface of the dens has an oval facet for articulation with the anterior arch of the atlas.

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The two superolateral surfaces of the dens possess circular impressions that serve as attachment sites for strong alar ligaments, one on each side, which connect the dens to the medial surfaces of the occipital condyles. These alar ligaments check excessive rotation of the head and atlas relative to the axis.