Cervical vertebrae

In vertebrates, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull.

Thoracic vertebrae in all mammalian species are those vertebrae that also carry a pair of ribs, and lie caudal to the cervical vertebrae. Further caudally follow the lumbar vertebrae, which also belong to the trunk, but do not carry ribs. In reptiles, all trunk vertebrae carry ribs and are called dorsal vertebrae.

In many species, though not in mammals, the cervical vertebrae bear ribs. In many other groups, such as lizards and saurischian dinosaurs, the cervical ribs are large; in birds, they are small and completely fused to the vertebrae. The vertebral transverse processes of mammals are homologous to the cervical ribs of other amniotes. Most mammals have 7 cervical vertebrae.[1]

In humans, cervical vertebrae are the smallest of the true vertebrae, and can be readily distinguished from those of the thoracic or lumbar regions by the presence of a foramen (hole) in each transverse process, through which the vertebral artery, vertebral veins and inferior cervical ganglion passes.

The remainder of this article focuses upon human anatomy.

Cervical vertebrae
Cervical vertebrae lateral2
Position of human cervical vertebrae (shown in red). It consists of 7 bones, from top to bottom, C1, C2, C3, C4, C5, C6, and C7.
Gray84
A human cervical vertebra
Details
Identifiers
LatinVertebrae cervicales
MeSHD002574
TAA02.2.02.001
FMA9915 72063, 9915
Anatomical terms of bone

Structure

Gray85
Side view of a typical cervical vertebra
Okapi Giraffe Neck
Despite greatly differing neck lengths, okapi (left) and giraffe (right) both have seven cervical vertebrae. The giraffe's neck is elongated by heterochrony, extension of the time for the embryonic development of these bones.[2]

By convention, the cervical vertebrae are numbered, with the first one (C1) closest to the skull and higher numbered vertebrae (C2–C7) proceeding away from the skull and down the spine. The general characteristics of the third through sixth cervical vertebrae are described here. The first, second, and seventh vertebrae are extraordinary, and are detailed later.

  • The bodies of these four vertebrae are small, and broader from side to side than from front to back.
    • The anterior and posterior surfaces are flattened and of equal depth; the former is placed on a lower level than the latter, and its inferior border is prolonged downward, so as to overlap the upper and forepart of the vertebra below.
    • The upper surface is concave transversely, and presents a projecting lip on either side.
    • The lower surface is concave from front to back, convex from side to side, and presents laterally shallow concavities that receive the corresponding projecting lips of the underlying vertebra.
  • The pedicles are directed laterally and backward, and attach to the body midway between its upper and lower borders, so that the superior vertebral notch is as deep as the inferior, but it is, at the same time, narrower.
  • The laminae are narrow, and thinner above than below; the vertebral foramen is large, and of a triangular form.
  • The spinous process is short and bifid, the two divisions being often of unequal size. Because the spinous processes are so short, certain superficial muscles (the trapezius and splenius capitis) attach to the nuchal ligament rather than directly to the vertebrae; the nuchal ligament itself attaching to the spinous processes of C2–C7 and to the posterior tubercle of the atlas.
  • The superior and inferior articular processes of cervical vertebrae have fused on either or both sides to form articular pillars, columns of bone that project laterally from the junction of the pedicle and lamina.
  • The articular facets are flat and of an oval form:
    • the superior face backward, upward, and slightly medially.
    • the inferior face forward, downward, and slightly laterally.
  • The transverse processes are each pierced by the foramen transversarium, which, in the upper six vertebrae, gives passage to the vertebral artery and vein, as well as a plexus of sympathetic nerves. Each process consists of an anterior and a posterior part. These two parts are joined, outside the foramen, by a bar of bone that exhibits a deep sulcus on its upper surface for the passage of the corresponding spinal nerve.
    • The anterior portion is the homologue of the rib in the thoracic region, and is therefore named the costal process or costal element. It arises from the side of the body, is directed laterally in front of the foramen, and ends in a tubercle, the anterior tubercle.
    • The posterior part, the true transverse process, springs from the vertebral arch behind the foramen, and is directed forward and laterally; it ends in a flattened vertical tubercle, the posterior tubercle.

The anterior tubercle of the sixth cervical vertebra is known as the carotid tubercle or Chassaignac tubercle. This separates the carotid artery from the vertebral artery and the carotid artery can be massaged against this tubercle to relieve the symptoms of supraventricular tachycardia. The carotid tubercle is also used as a landmark for anaesthesia of the brachial plexus and cervical plexus.

The cervical spinal nerves emerge from above the cervical vertebrae. For example, the cervical spinal nerve 3 (C3) passes above C3.

Atlas and axis

The atlas (C1) and axis (C2) are the two topmost vertebrae.

The atlas, C1, is the topmost vertebra, and along with the axis; forms the joint connecting the skull and spine. Its chief peculiarity is that it has no body because the body of the atlas has been fused with that of the axis.

The axis, C2, forms the pivot on which the atlas rotates. The most distinctive characteristic of this bone is the strong odontoid process (dens) that rises perpendicularly from the upper surface of the body. The body is deeper in front than behind, and prolonged downward anteriorly so as to overlap the upper and front part of the third vertebra.

Vertebra prominens

C7 animation small
Position of C7 shown in red.

The vertebra prominens, or C7, has a distinctive long and prominent spinous process, which is palpable from the skin surface. Sometimes, the seventh cervical vertebra is associated with an abnormal extra rib, known as a cervical rib, which develops from the anterior root of the transverse process. These ribs are usually small, but may occasionally compress blood vessels (such as the subclavian artery or subclavian vein) or nerves in the brachial plexus, causing pain, numbness, tingling, and weakness in the upper limb, a condition known as thoracic outlet syndrome. Very rarely, this rib occurs in a pair.

The long spinous process of C7 is thick and nearly horizontal in direction. It is not bifurcated, and ends in a tubercle that the ligamentum nuchae attaches to. This process is not always the most prominent of the spinous processes, being found only about 70% of the time, C6 or T1 can sometimes be the most prominent.

The transverse processes are of considerable size, their posterior roots are large and prominent, while the anterior are small and faintly marked; the upper surface of each has usually a shallow sulcus for the eighth spinal nerve, and its extremity seldom presents more than a trace of bifurcation.

The transverse foramen may be as large as that in the other cervical vertebrae, but is generally smaller on one or both sides; occasionally it is double, sometimes it is absent.

On the left side it occasionally gives passage to the vertebral artery; more frequently the vertebral vein traverses it on both sides; but the usual arrangement is for both artery and vein to pass in front of the transverse process, and not through the foramen.

Function

The movement of nodding the head takes place predominantly through flexion and extension at the atlanto-occipital joint between the atlas and the occipital bone. However, the cervical spine is comparatively mobile, and some component of this movement is due to flexion and extension of the vertebral column itself. This movement between the atlas and occipital bone is often referred to as the "yes joint", owing to its nature of being able to move the head in an up-and-down fashion.

The movement of shaking or rotating the head left and right happens almost entirely at the joint between the atlas and the axis, the atlanto-axial joint. A small amount of rotation of the vertebral column itself contributes to the movement. This movement between the atlas and axis is often referred to as the "no joint", owing to its nature of being able to rotate the head in a side-to-side fashion.

Clinical significance

Cervical degenerative changes arise from conditions such as spondylosis, stenosis of intervertebral discs, and the formation of osteophytes. The changes are seen on radiographs which are used in a grading system from 0-4 ranging from no changes (0), to early with minimal development of osteophytes (1), mild with definite osteophytes (2), moderate with additional disc space stenosis or narrowing (3), to the stage of many large osteophytes, severe narrowing of the disc space, and more severe vertebral end plate sclerosis (4).[3][4][5]

Injuries to the cervical spine are common at the level of the second cervical vertebrae, but neurological injury is uncommon. C4 and C5 are the areas that see the highest amount of cervical spine trauma.[6]

If it does occur, however, it may cause death or profound disability, including paralysis of the arms, legs, and diaphragm, which leads to respiratory failure.

Common patterns of injury include the odontoid fracture and the hangman's fracture, both of which are often treated with immobilization in a cervical collar or Halo brace.

A common practice is to immobilize a patient's cervical spine to prevent further damage during transport to hospital. This practice has come under review recently as incidence rates of unstable spinal trauma can be as low as 2% in immobilized patients. In clearing the cervical spine, Canadian studies have developed the Canadian C-Spine Rule (CCR) for physicians to decide who should receive radiological imaging.[7]

Landmarks

The vertebral column is often used as a marker of human anatomy. This includes:

Additional images

Cervical vertebrae animation small

Position of cervical vertebrae (shown in red). Animation.

Blausen 0222 CervicalSpine

Illustration of cervical vertebrae.

Cervical vertebrae - close-up - animation2

Shape of cervical vertebrae (shown in blue and yellow). Animation.

Human cervical vertebra.stl

3D image

Cervical vertebrae lateral3

Cervical vertebrae, lateral view (shown in blue and yellow).

Illu vertebral column

Vertebral column

Vertebral column.

HWS seitlich Annotation

X-Ray of cervical vertebrae.

Cervical XRayFlexionExtension

X-ray of cervical spine in flexion and extension.

Gray86

First cervical vertebra, or Atlas

Gray87

Second cervical vertebra, or epistropheus, from above.

Gray88

Second cervical vertebra, epistropheus, or axis, from the side.

Gray89

Seventh cervical vertebra.

Gray305

Posterior atlanto-occipital membrane and atlantoaxial ligament.

Gray308

Median sagittal section through the occipital bone and first three cervical vertebræ.

Gray384

Section of the neck at about the level of the sixth cervical vertebra.

Cervical Spine Anterior View

Anterior View of Cervical Spine showing the vertebral arteries along with the spinal nerves. See this in 3d here.

See also

References

This article incorporates text in the public domain from page 97 of the 20th edition of Gray's Anatomy (1918)

  1. ^ Varela-Lasheras, Irma; Bakker, Alexander J; Van Der Mije, Steven D; Metz, Johan AJ; Van Alphen, Joris; Galis, Frietson (2011). "Breaking evolutionary and pleiotropic constraints in mammals: On sloths, manatees and homeotic mutations". Evo Devo. 2: 11. doi:10.1186/2041-9139-2-11. Lay summaryScienceDaily (May 6, 2011).
  2. ^ Hillis, David M. (May 2011). Principles of Life. Palgrave Macmillan. pp. 280–. ISBN 978-1-4641-6298-5. Archived from the original on 2018-05-06.
  3. ^ Ofiram, Elisha; Garvey, Timothy A; Schwender, James D; Denis, Francis; Perra, Joseph H; Transfeldt, Ensor E; Winter, Robert B; Wroblewski, Jill M (2009). "Cervical degenerative index: a new quantitative radiographic scoring system for cervical spondylosis with interobserver and intraobserver reliability testing". Journal of Orthopaedics and Traumatology. 10 (1): 21–26. doi:10.1007/s10195-008-0041-3. PMC 2657349. PMID 19384631.
  4. ^ Garfin, Steven R; Bono, Christopher M. "Degenerative Cervical Spine Disorders". spineuniverse. Archived from the original on 28 October 2016. Retrieved 25 October 2016.
  5. ^ Christie, A; Läubli, R; Guzman, R; Berlemann, U; Moore, R J; Schroth, G; Vock, P; Lövblad, K O (2005). "Degeneration of the cervical disc: histology compared with radiography and magnetic resonance imaging". Neuroradiology. 47 (10): 721–729. doi:10.1007/s00234-005-1412-6.
  6. ^ 2012 Annual Report Archived 2014-02-22 at the Wayback Machine., Table 64, page 66
  7. ^ "Canadian C-Spine Rule - Emergency Medicine Research - Ottawa Hospital Research Institute". www.ohri.ca. Archived from the original on 14 May 2017. Retrieved 6 May 2018.
  8. ^ a b MedicalMnemonics.com: 3548

External links

Asiatosaurus

Asiatosaurus (meaning "Asian lizard") was a genus of herbivorous sauropod dinosaur which lived during the early Cretaceous. Its fossils have been found in China and Mongolia. Its type species is known only from teeth, making it difficult to rely on information until more specimens are found to expand our knowledge. The type species, Asiatosaurus mongoliensis, was described by Osborn, in 1924. It was the first sauropod genus named from East-Asia.

Asiatosaurus kwangshiensis was described by Hou, Yeh and Zhao, in 1975 based on a tooth, three cervical vertebrae and multiple ribs from the Xinlong Formation of Guangxi, China. The genus was classified within Brachiosauridae by Hou et al.. Both are now classified as nomina dubia.

Axis (anatomy)

In anatomy, the second cervical vertebra (C2) of the spine is named the axis (from Latin axis, "axle") or epistropheus.

By the atlanto-axial joint, it forms the pivot upon which the first cervical vertebra (the atlas), which carries the head, rotates.

The most distinctive characteristic of this bone is the strong odontoid process known as the dens which rises perpendicularly from the upper surface of the body. That peculiar feature gives to the vertebra a rarely used third name: vertebra dentata. In some judicial hangings the odontoid process may break and hit the medulla oblongata, causing death.

Camarasauridae

Camarasauridae (meaning "chambered lizards") is a family of neosauropod dinosaurs within the clade Macronaria, the sister group to Titanosauriformes. Among sauropods, camarasaurids are small to medium-sized, with relatively short necks. They are visually identifiable by a short skull with large nares, and broad, spatulate teeth filling a thick jaw. Based on cervical vertebrae and cervical rib biomechanics, camarasaurids most likely moved their necks in a vertical, rather than horizontal, sweeping motion, in contrast to most diplodocids. Cladistically, they are defined to be all sauropods more closely related to Camarasaurus supremus than to Saltasaurus loricatus.

Deep cervical vein

The deep cervical vein (posterior vertebral or posterior deep cervical vein) accompanies its artery between the Semispinales capitis and colli.

It begins in the suboccipital region by communicating branches from the occipital vein and by small veins from the deep muscles at the back of the neck.

It receives tributaries from the plexuses around the spinous processes of the cervical vertebræ, and terminates in the lower part of the vertebral vein.

Erketu

Erketu is the name given to a genus of dinosaur from the Late Cretaceous (Cenomanian - Santonian). It was a sauropod with the longest neck relative to its body size. Its fossils were found in Mongolia.

The type species, Erketu ellisoni, was described in March 2006 by Daniel Ksepka and Mark Norell of the American Museum of Natural History. Its neck was estimated to be twice as long as its body, which may be a record for neck to body ratio. The exact ratio is unknown, because no dorsal vertebrae of E. ellisoni have been reported, although some hindlimb material suggests the approximate size of the body. The long neck of Erketu is the result of the individual vertebrae being greatly elongated; it is unknown if the number of cervical vertebrae was increased. Erketu is also diagnosed by bifurcate anterior cervical neural spines, another unusual trait for a titanosauriform. A phylogenetic analysis of sauropods indicates that Erketu is a basal somphospondylian (the clade of all macronarians closer to titanosaurs than to brachiosaurs), and is most closely related to Titanosauria.

This particular sauropod species is named after the American Museum of Natural History's resident paleo-artist, and close friend of Mark Norell, Mick Ellison.

Greater occipital nerve

The greater occipital nerve is a spinal nerve, specifically the medial branch of the dorsal primary ramus of cervical spinal nerve 2. This nerve arises from between the first and second cervical vertebrae, along with the lesser occipital nerve. It ascends after emerging from below the suboccipital triangle beneath the obliquus capitis inferior muscle. It then passes through the semispinalis muscle before ascending to innervate the skin along the posterior part of the scalp to the vertex. It innervates the scalp at the top of the head, over the ear and over the parotid glands.

Longissimus

The longissimus (Latin for 'the longest one') is the muscle lateral to the semispinalis muscles. It is the longest subdivision of the erector spinae muscles that extends forward into the transverse processes of the posterior cervical vertebrae.

Longus colli muscle

The Longus colli muscle (Latin for long muscle of the neck) is a muscle of the human body.

The Longus colli is situated on the anterior surface of the vertebral column, between the atlas and the third thoracic vertebra.

It is broad in the middle, narrow and pointed at either end, and consists of three portions, a superior oblique, an inferior oblique, and a vertical.

The superior oblique portion arises from the anterior tubercles of the transverse processes of the third, fourth, and fifth cervical vertebræ and, ascending obliquely with a medial inclination, is inserted by a narrow tendon into the tubercle on the anterior arch of the atlas.

The inferior oblique portion, the smallest part of the muscle, arises from the front of the bodies of the first two or three thoracic vertebræ; and, ascending obliquely in a lateral direction, is inserted into the anterior tubercles of the transverse processes of the fifth and sixth cervical vertebræ.

The vertical portion arises, below, from the front of the bodies of the upper three thoracic and lower three cervical vertebræ, and is inserted into the front of the bodies of the second, third, and fourth cervical vertebræ.

Neck

The neck is the part of the body, on many vertebrates, that separates the head from the torso. It contains blood vessels and nerves that supply structures in the head to the body. These in humans include part of the esophagus, the larynx, trachea, and thyroid gland, major blood vessels including the carotid arteries and jugular veins, and the top part of the spinal cord.

In anatomy, the neck is also called by its Latin names, cervix or collum, although when used alone, in context, the word cervix more often refers to the uterine cervix, the neck of the uterus. Thus the adjective cervical may refer either to the neck (as in cervical vertebrae or cervical lymph nodes) or to the uterine cervix (as in cervical cap or cervical cancer).

Pantosaurus

Pantosaurus ("all lizard") is an extinct genus of plesiosaur from the Late Jurassic (Oxfordian) of what is now Wyoming. It lived in what used to be the Sundance Sea. It was originally named Parasaurus ("near lizard") by Othniel Charles Marsh in reference to Plesiosaurus, but that name was preoccupied, and Marsh changed it. The species Muraenosaurus reedii is in fact a junior synonym of Pantosaurus. The holotype YPM 543 is a partial articulated skeleton, partially prepared to yield a distal humerus, four articulated carpals, a fragment of the coracoid, and several isolated cervical vertebrae from the Upper Member of the Sundance Formation. Other material includes USNM 536963, USNM 536965, UW 3, UW 5544 and UW 15938.

Pengornis

Pengornis is the largest known enantiornithine bird from the Early Cretaceous of northeast China. The name derives from "Peng", which refers to a mythological bird from Chinese folklore, and "-ornis", which means bird in Greek.

Pengornis is known from a single adult fossil, described by Zhou et al. in 2008. This holotype is in the collection of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing China. Its accession number is IVPP V15336. It was collected from the Jiufotang Formation, at Dapingfang, Chaoyang, Liaoning China. A second, juvenile specimen was described by Hu, Zhou, and O'Connor in 2014.

Pengornis shows characters of the humeral head, acromion, and anterior cervical vertebrae, that were previously known only in members of the Ornithurae. A phylogenetic analysis by Zhou et al. reduces to just three the number of characters that support enantiornithine monophyly. Thus, Pengornis supports the possibility that enantiornithines and Ornithurines may not be distinct clades.(see Apsaravis)

Phosphatodraco

Phosphatodraco (meaning "phosphate dragon", in reference to the phosphates of Morocco, the country where it was found) is a genus of azhdarchid pterodactyloid pterosaur from a late Maastrichtian-age Upper Cretaceous portion of the Oulad (or Qualad) Abdoun Phosphatic Basin, Grand Doui, near Khouribga, central Morocco.

Phosphatodraco is based on holotype OCP DEK/GE 111, found in 2000, which is composed of five associated, though disarticulated and compressed, damaged cervical vertebrae and a bone of unknown origin. The cervical vertebrae are thought to be a series from the fifth (the longest with a length of thirty centimeters) to the ninth. The individual to which the neck belonged would have had a wingspan of about five meters (16.4 feet). It is unusual among azhdarchids for having elongate vertebrae at the base of the neck (also with neural spines), interpreted as modified dorsal vertebrae; the neck is also one of the most complete known for azhdarchids. It was one of the last pterosaurs before the Cretaceous–Paleogene extinction event that finished off the group, and is the first azhdarchid found in northern Africa. P. mauritanicus is the type and only known species. The specific name refers to Mauretania.

Rabbit punch

A rabbit punch is a blow to the back of the head or to the base of the skull. It is considered especially dangerous because it can damage the cervical vertebrae and subsequently the spinal cord, which may lead to serious and irreparable spinal cord injury. A rabbit punch can also detach the victim's brain from the brain stem, which can kill instantly.

The punch's name is derived from the use of the technique by hunters to kill rabbits with a quick, sharp strike to the back of the head.

Semispinalis muscles

The semispinalis muscles are a group of three muscles belonging to the transversospinales. These are the semispinalis capitis, the semispinalis cervicis and the semispinalis thoracis.

The semispinalis capitis (complexus) is situated at the upper and back part of the neck, deep to the splenius, and medial to the longissimus cervicis and longissimus capitis. It arises by a series of tendons from the tips of the transverse processes of the upper six or seven thoracic and the seventh cervical vertebrae, and from the articular processes of the three cervical vertebrae above this (C4-C6).

The tendons, uniting, form a broad muscle, which passes upward, and is inserted between the superior and inferior nuchal lines of the occipital bone. It lies deep to the trapezius muscle and can be palpated as a firm round muscle mass just lateral to the cervical spinous processes.

The semispinalis cervicis (or semispinalis colli), arises by a series of tendinous and fleshy fibers from the transverse processes of the upper five or six thoracic vertebrae, and is inserted into the cervical spinous processes, from the axis to the fifth cervical vertebrae inclusive. The semispinalis cervicis is thicker than the semispinalis thoracis. The fasciculus connected with the axis is the largest, and is chiefly muscular in structure.

The semispinalis thoracis (or semispinalis dorsi) muscle consists of thin, narrow, fleshy fasciculi, interposed between tendons of considerable length. It arises by a series of small tendons from the transverse processes of the sixth to the tenth thoracic vertebrae, and is inserted, by tendons, into the spinous processes of the upper four thoracic and lower two cervical vertebrae.

The semispinalis muscles are innervated by the dorsal rami of the cervical spinal nerves.

Somphospondyli

Somphospondylans are an extinct clade of titanosauriform sauropods that lived throughout the world from the Late Jurassic through the Cretaceous. The group can be defined as "the most inclusive clade that includes Saltasaurus loricatus but excludes Brachiosaurus altithorax". Features found as diagnostic of this clade by Mannion et al. (2013) include the possession of at least 15 cervical vertebrae; a bevelled radius bone end; sacral vertebrae with camellate internal texture; convex posterior articular surfaces of middle to posterior caudal vertebrae; biconvex distal caudal vertebrae; humerus anterolateral corner "squared"; among multiple others.

Vertebra

In the vertebrate spinal column, each vertebra is an irregular bone with a complex structure composed of bone and some hyaline cartilage, the proportions of which vary according to the segment of the backbone and the species of vertebrate.

The basic configuration of a vertebra varies; the large part is the body, and the central part is the centrum. The upper and lower surfaces of the vertebra body give attachment to the intervertebral discs. The posterior part of a vertebra forms a vertebral arch, in eleven parts, consisting of two pedicles, two laminae, and seven processes. The laminae give attachment to the ligamenta flava (ligaments of the spine). There are vertebral notches formed from the shape of the pedicles, which form the intervertebral foramina when the vertebrae articulate. These foramina are the entry and exit conducts for the spinal nerves. The body of the vertebra and the vertebral arch form the vertebral foramen, the larger, central opening that accommodates the spinal canal, which encloses and protects the spinal cord.

Vertebrae articulate with each other to give strength and flexibility to the spinal column, and the shape at their back and front aspects determines the range of movement. Structurally, vertebrae are essentially alike across the vertebrate species, with the greatest difference seen between an aquatic animal and other vertebrate animals. As such, vertebrates take their name from the vertebrae that compose the vertebral column.

Vertebral column

The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordates has been replaced by a segmented series of bone: vertebrae separated by intervertebral discs. The vertebral column houses the spinal canal, a cavity that encloses and protects the spinal cord.

There are about 50,000 species of animals that have a vertebral column. The human vertebral column is one of the most-studied examples.

Vertebral vein

The vertebral vein is formed in the suboccipital triangle, from numerous small tributaries which spring from the internal vertebral venous plexuses and issue from the vertebral canal above the posterior arch of the atlas.

They unite with small veins from the deep muscles at the upper part of the back of the neck, and form a vessel which enters the foramen in the transverse process of the atlas, and descends, forming a dense plexus around the vertebral artery, in the canal formed by the foramina transversaria of the cervical vertebrae.

This plexus ends in a single trunk, which emerges from the foramen transversarium of the sixth cervical vertebra, and opens at the root of the neck into the back part of the innominate vein near its origin, its mouth being guarded by a pair of valves.

On the right side, it crosses the first part of the subclavian artery.

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