A body cavity is any space or compartment, or potential space , in an animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid.
79-606: Cephalorhyncha, Priapozoa Scalidophora is a group of marine pseudocoelomate ecdysozoans that was proposed on morphological grounds to unite three phyla : the Kinorhyncha , the Priapulida and the Loricifera . The three phyla have four characters in common — chitinous cuticle that is moulted, rings of scalids on the introvert, flosculi, and two rings of introvert retracts. The introvert and abdomen are separated by
158-628: A hernia . Spinal stenosis is a narrowing of the spinal canal which can occur in any region of the spine though less commonly in the thoracic region. The stenosis can constrict the spinal canal giving rise to a neurological deficit . Pain at the coccyx (tailbone) is known as coccydynia . Spinal cord injury is damage to the spinal cord that causes changes in its function, either temporary or permanent. Spinal cord injuries can be divided into categories: complete transection, hemisection, central spinal cord lesions, posterior spinal cord lesions, and anterior spinal cord lesions. Scalloping vertebrae
237-412: A central cavity, the central canal . Adjacent to each vertebra emerge spinal nerves . The spinal nerves provide sympathetic nervous supply to the body, with nerves emerging forming the sympathetic trunk and the splanchnic nerves . The spinal canal follows the different curves of the column; it is large and triangular in those parts of the column that enjoy the greatest freedom of movement, such as
316-435: A congenital malformation in which the heart is abnormally located outside the thorax. Another defect is gastroschisis, a congenital defect in the anterior abdominal wall through which the abdominal contents freely protrude. Another possibility is bladder exstrophy, in which part of the urinary bladder is present outside the body. In normal circumstances, the parietal mesoderm will form the parietal layer of serous membranes lining
395-408: A continuous, uninterrupted structure, the cranial and spinal cavities that house them are also continuous. The brain and spinal cord are protected by the bones of the skull and vertebral column and by cerebrospinal fluid, a colorless fluid produced by the brain, which cushions the brain and spinal cord within the dorsal body cavity. At the end of the third week of gestation , the neural tube , which
474-449: A curve, convex forward, that begins at the axis (second cervical vertebra) at the apex of the odontoid process or dens and ends at the middle of the second thoracic vertebra; it is the least marked of all the curves. This inward curve is known as a lordotic curve. The thoracic curve, concave forward, begins at the middle of the second and ends at the middle of the twelfth thoracic vertebra. Its most prominent point behind corresponds to
553-819: A distinct neck region in all groups, but in adult macroscopic priapulids it becomes rudimentary in Priapulus and is completely absent in Halicryptus . However, the monophyly of the Scalidophora was not supported by two molecular studies, where the position of the Loricifera was uncertain or as sister to the Panarthropoda . Both studies supported a reduced Scalidophora comprising the Kinorhyncha and Priapulida as sister phyla. Their closest relatives are
632-413: A neural arch, while the haemal arch is found underneath the centrum in the caudal (tail) vertebrae of fish , most reptiles , some birds, some dinosaurs and some mammals with long tails. The vertebral processes can either give the structure rigidity, help them articulate with ribs, or serve as muscle attachment points. Common types are transverse process, diapophyses, parapophyses, and zygapophyses (both
711-535: A region can vary but overall the number remains the same. In a human vertebral column, there are normally 33 vertebrae. The upper 24 pre-sacral vertebrae are articulating and separated from each other by intervertebral discs , and the lower nine are fused in adults, five in the sacrum and four in the coccyx , or tailbone . The articulating vertebrae are named according to their region of the spine. From top to bottom, there are 7 cervical vertebrae , 12 thoracic vertebrae and 5 lumbar vertebrae . The number of those in
790-456: A region can vary but overall the number remains the same. The number of those in the cervical region, however, is only rarely changed. The vertebrae of the cervical, thoracic, and lumbar spines are independent bones and generally quite similar. The vertebrae of the sacrum and coccyx are usually fused and unable to move independently. Two special vertebrae are the atlas and axis , on which the head rests. A typical vertebra consists of two parts:
869-399: A remnant of the notochord. Reptiles often retain the primitive intercentra, which are present as small crescent-shaped bony elements lying between the bodies of adjacent vertebrae; similar structures are often found in the caudal vertebrae of mammals. In the tail, these are attached to chevron-shaped bones called haemal arches , which attach below the base of the spine, and help to support
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#1732772753699948-438: A result of lifting the head and the lumbar curvature forms as a result of walking. The vertebral column surrounds the spinal cord which travels within the spinal canal , formed from a central hole within each vertebra . The spinal cord is part of the central nervous system that supplies nerves and receives information from the peripheral nervous system within the body. The spinal cord consists of grey and white matter and
1027-432: A segmented series of mineralized irregular bones (or sometimes, cartilages ) called vertebrae , separated by fibrocartilaginous intervertebral discs (the center of which is a notochord remnant). The dorsal portion of the vertebral column houses the spinal canal , an elongated cavity formed by alignment of the vertebral neural arches that encloses and protects the spinal cord , with spinal nerves exiting via
1106-473: Is a stub . You can help Misplaced Pages by expanding it . Body cavity The two largest human body cavities are the ventral body cavity , and the dorsal body cavity . In the dorsal body cavity the brain and spinal cord are located. The membranes that surround the central nervous system organs (the brain and the spinal cord , in the cranial and spinal cavities ) are the three meninges . The differently lined spaces contain different types of fluid. In
1185-430: Is a fold of one of the layers of the trilaminar germ disc , called the ectoderm , appears. This layer elevates and closes dorsally, while the gut tube rolls up and closes ventrally to create a "tube on top of a tube". The mesoderm , which is another layer of the trilaminar germ disc, holds the tubes together and the lateral plate mesoderm , the middle layer of the germ disc, splits to form a visceral layer associated with
1264-413: Is classed as a spinal disease or dorsopathy and includes the following abnormal curvatures: Individual vertebrae of the human vertebral column can be felt and used as surface anatomy , with reference points are taken from the middle of the vertebral body. This provides anatomical landmarks that can be used to guide procedures such as a lumbar puncture and also as vertical reference points to describe
1343-416: Is curved in several places, a result of human bipedal evolution . These curves increase the vertebral column's strength, flexibility, and ability to absorb shock, stabilising the body in upright position. When the load on the spine is increased, the curvatures increase in depth (become more curved) to accommodate the extra weight. They then spring back when the weight is removed. The upper cervical spine has
1422-437: Is lined by somatic and splanchnic lateral plate mesoderm, while the extraembryonic coelom is lined by extraembryonic mesoderm. The intraembryonic coelom is the only cavity that persists in the mammal at term, which is why its name is often contracted to simply coelomic cavity . Subdividing the coelomic cavity into compartments, for example, the pericardial cavity / pericardium , where the heart develops, simplifies discussion of
1501-432: Is shallow, and by the laminae and transverse processes in the thoracic region, where it is deep and broad; these grooves lodge the deep muscles of the back. Lateral to the spinous processes are the articular processes, and still more laterally the transverse processes. In the thoracic region, the transverse processes stand backward, on a plane considerably behind that of the same processes in the cervical and lumbar regions. In
1580-412: Is the increase in the concavity of the posterior vertebral body. It can be seen on lateral X-ray and sagittal views of CT and MRI scans. Its concavity is due to the increased pressure exerting on the vertebrae due to a mass. Internal spinal mass such as spinal astrocytoma , ependymoma , schwannoma , neurofibroma , and achondroplasia causes vertebrae scalloping. Excessive or abnormal spinal curvature
1659-521: The Arabian (breed) can have one less vertebrae and pair of ribs. This anomaly disappears in foals that are the product of an Arabian and another breed of horse. Vertebrae are defined by their location in the vertebral column. Cervical vertebrae are those in the neck area. With the exception of the two sloth genera ( Choloepus and Bradypus ) and the manatee genus, ( Trichechus ), all mammals have seven cervical vertebrae. In other vertebrates,
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#17327727536991738-446: The Panarthropoda , Nematoda and Nematomorpha . The two species in the genus Markuelia , known from fossilized embryos from the middle Cambrian , are thought to be stem scalidophorans. The group has also been considered a single group, Cephalorhyncha , with three classes. The group is named after the spines (scalids) covering the introvert (head that can be retracted into the trunk). This protostome -related article
1817-416: The anatomies of complex animals. Cavitation in the early embryo is the process of forming the blastocoel , the fluid-filled cavity defining the blastula stage in non-mammals, or the blastocyst in mammals. The dorsal (posterior) cavity and the ventral (anterior) cavity are the largest body compartments. The dorsal body cavity includes the cranial cavity , enclosed by the skull and contains
1896-470: The brain , and the spinal cavity , enclosed by the spine , and contains the spinal cord . The ventral body cavity includes the thoracic cavity , enclosed by the ribcage and contains the lungs and heart ; and the abdominopelvic cavity . The abdominopelvic cavity can be divided into the abdominal cavity , enclosed by the ribcage and pelvis and contains the kidneys , ureters , stomach , intestines , liver , gallbladder , and pancreas ; and
1975-433: The intervertebral discs . The notochord disappears in the sclerotome (vertebral body) segments but persists in the region of the intervertebral discs as the nucleus pulposus . The nucleus pulposus and the fibers of the anulus fibrosus make up the intervertebral disc. The primary curves (thoracic and sacral curvatures) form during fetal development. The secondary curves develop after birth. The cervical curvature forms as
2054-624: The intervertebral foramina to innervate each body segments . There are around 50,000 species of animals that have a vertebral column. The human spine is one of the most-studied examples, as the general structure of human vertebrae is fairly typical ( homologous ) of that found in other mammals , reptiles and birds . The shape of the vertebral body does, however, vary somewhat between different groups of living species. Individual vertebrae are named according to their corresponding body region ( neck , thorax , abdomen , pelvis or tail ). In clinical medicine , features on vertebrae (particularly
2133-412: The paraxial mesoderm that lies at the sides of the neural tube and they contain the precursors of spinal bone, the vertebrae ribs and some of the skull, as well as muscle, ligaments and skin. Somitogenesis and the subsequent distribution of somites is controlled by a clock and wavefront model acting in cells of the paraxial mesoderm. Soon after their formation, sclerotomes , which give rise to some of
2212-419: The pelvic cavity , enclosed by the pelvis and contains bladder , anus and reproductive system . The ventral cavity has two main subdivisions: the thoracic cavity and the abdominopelvic cavity. The thoracic cavity is the more superior subdivision of the ventral cavity, and is enclosed by the rib cage. The thoracic cavity contains the lungs surrounded by the pleural cavity , and the heart surrounded by
2291-413: The pericardial cavity , located in the mediastinum. The diaphragm forms the floor of the thoracic cavity and separates it from the more inferior abdominopelvic cavity. The abdominopelvic cavity is the largest cavity in the body occupying the entire lower half of the trunk. Although no membrane physically divides the abdominopelvic cavity, it can be useful to distinguish between the abdominal cavity, and
2370-436: The sacrum and coccyx are fused without a central foramen. The vertebral arch is formed by a ventral pair of pedicles and a dorsal pair of laminae , and supports seven processes , four articular , two transverse and one spinous , the latter also being known as the neural spine. The transverse and spinous processes and their associated ligaments serve as important attachment sites for back and paraspinal muscles and
2449-435: The spinal canal , a body cavity that contains the spinal cord . Because the vertebral column will outgrow the spinal cord during child development , by adulthood the spinal cord often ends at the upper lumbar spine (at around L1/L2 level), the lower ( caudal ) end of the spinal canal is occupied by a ponytail -like bundle of spinal nerves descriptively called cauda equina (from Latin " horse's tail " ), and
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2528-418: The spinal cavity . The cranial cavity is a large, bean-shaped cavity filling most of the upper skull where the brain is located. The spinal cavity is the very narrow, thread-like cavity running from the cranial cavity down the entire length of the spinal cord . In the dorsal cavity, the cranial cavity houses the brain , and the spinal cavity encloses the spinal cord. Just as the brain and spinal cord make up
2607-411: The spinal cord and the notochord . This column of tissue has a segmented appearance, with alternating areas of dense and less dense areas. As the sclerotome develops, it condenses further eventually developing into the vertebral body . Development of the appropriate shapes of the vertebral bodies is regulated by HOX genes . The less dense tissue that separates the sclerotome segments develop into
2686-420: The spinous process ) can be used as surface landmarks to guide medical procedures such as lumbar punctures and spinal anesthesia . There are also many different spinal diseases in humans that can affect both the bony vertebrae and the intervertebral discs, with kyphosis / scoliosis , ankylosing spondylitis , degenerative discs and spina bifida being recognizable examples. The number of vertebrae in
2765-438: The thoracolumbar fasciae . The spinous processes of the cervical and lumbar regions can be felt through the skin, and are important surface landmarks in clinical medicine . The four articular processes for two pairs of plane facet joints above and below each vertebra, articulating with those of the adjacent vertebrae and are joined by a thin portion of the neural arch called the pars interarticularis . The orientation of
2844-409: The vertebral body (or centrum ), which is ventral (or anterior , in the standard anatomical position ) and withstands axial structural load ; and the vertebral arch (also known as neural arch ), which is dorsal (or posterior ) and provides articulations and anchorages for ribs and core skeletal muscles . Together, these enclose the vertebral foramen , the series of which align to form
2923-493: The activity of nearby organs. The ventral cavity includes the thoracic and abdominopelvic cavities and their subdivisions. The dorsal cavity includes the cranial and spinal cavities. Organisms can be also classified according to the type of body cavity they possess, such as pseudocoelomates and protostome coelomates . In amniotes and some invertebrates , the coelom is the large cavity lined by mesothelium , an epithelium derived from mesoderm . Organs formed inside
3002-425: The adjacent vertebra to a degree less than a dislocation. Spondylolysis , also known as a pars defect, is a defect or fracture at the pars interarticularis of the vertebral arch. Spinal disc herniation , more commonly called a "slipped disc", is the result of a tear in the outer ring ( anulus fibrosus ) of the intervertebral disc , which lets some of the soft gel-like material, the nucleus pulposus , bulge out in
3081-416: The bone of the skull, the vertebrae and ribs, migrate, leaving the remainder of the somite now termed a dermamyotome behind. This then splits to give the myotomes which will form the muscles and dermatomes which will form the skin of the back. Sclerotomes become subdivided into an anterior and a posterior compartment. This subdivision plays a key role in the definitive patterning of vertebrae that form when
3160-427: The bony vertebral body. In most ray-finned fishes , including all teleosts , these two structures are fused with, and embedded within, a solid piece of bone superficially resembling the vertebral body of mammals. In living amphibians , there is simply a cylindrical piece of bone below the vertebral arch, with no trace of the separate elements present in the early tetrapods. In cartilaginous fish , such as sharks ,
3239-506: The centrum. Centra with flat ends are acoelous , like those in mammals. These flat ends of the centra are especially good at supporting and distributing compressive forces. Amphicoelous vertebra have centra with both ends concave. This shape is common in fish, where most motion is limited. Amphicoelous centra often are integrated with a full notochord . Procoelous vertebrae are anteriorly concave and posteriorly convex. They are found in frogs and modern reptiles. Opisthocoelous vertebrae are
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3318-423: The cervical and lumbar regions, and is small and rounded in the thoracic region, where motion is more limited. The spinal cord terminates in the conus medullaris and cauda equina . Spina bifida is a congenital disorder in which there is a defective closure of the vertebral arch. Sometimes the spinal meninges and also the spinal cord can protrude through this, and this is called spina bifida cystica . Where
3397-464: The cervical and upper part of the thoracic regions and gradually increasing in size to the last lumbar. They transmit the special spinal nerves and are situated between the transverse processes in the cervical region and in front of them, in the thoracic and lumbar regions. There are different ligaments involved in the holding together of the vertebrae in the column, and in the column's movement. The anterior and posterior longitudinal ligaments extend
3476-511: The cervical region, however, is only rarely changed, while that in the coccygeal region varies most. Excluding rare deviations, the total number of vertebrae ranges from 32 to 35. In about 10% of people, both the total number of pre-sacral vertebrae and the number of vertebrae in individual parts of the spine can vary. The most frequent deviations are: 11 (rarely 13) thoracic vertebrae, 4 or 6 lumbar vertebrae, 3 or 5 coccygeal vertebrae (rarely up to 7). There are numerous ligaments extending
3555-430: The cervical region, the transverse processes are placed in front of the articular processes, lateral to the pedicles and between the intervertebral foramina. In the thoracic region they are posterior to the pedicles, intervertebral foramina, and articular processes. In the lumbar region they are in front of the articular processes, but behind the intervertebral foramina. The sides of the vertebral column are separated from
3634-421: The child begins to walk. When viewed from in front, the width of the bodies of the vertebrae is seen to increase from the second cervical to the first thoracic; there is then a slight diminution in the next three vertebrae. Below this, there is again a gradual and progressive increase in width as low as the sacrovertebral angle. From this point there is a rapid diminution, to the apex of the coccyx. From behind,
3713-438: The coelom can freely move, grow, and develop independently of the body wall while fluid in the peritoneum cushions and protects them from shocks. Arthropods and most molluscs have a reduced (but still true) coelom, the hemocoel (of an open circulatory system ) and the smaller gonocoel (a cavity that contains the gonads ). Their hemocoel is often derived from the blastocoel . This Misplaced Pages entry incorporates text from
3792-429: The condition does not involve this protrusion it is known as spina bifida occulta . Sometimes all of the vertebral arches may remain incomplete. Another, though rare, congenital disease is Klippel–Feil syndrome , which is the fusion of any two of the cervical vertebrae. Spondylolisthesis is the forward displacement of a vertebra and retrolisthesis is a posterior displacement of one vertebral body with respect to
3871-402: The cranial zygapophyses and the caudal zygapophyses). The centrum of the vertebra can be classified based on the fusion of its elements. In temnospondyls , bones such as the spinous process , the pleurocentrum and the intercentrum are separate ossifications. Fused elements, however, classify a vertebra as having holospondyly. A vertebra can also be described in terms of the shape of the ends of
3950-403: The curvatures of the vertebral column. The articulating vertebrae are named according to their region of the spine. Vertebrae in these regions are essentially alike, with minor variation. These regions are called the cervical spine, thoracic spine, lumbar spine, sacrum, and coccyx. There are seven cervical vertebrae, twelve thoracic vertebrae, and five lumbar vertebrae. The number of vertebrae in
4029-447: The facet joints restricts the range of motion between the vertebrae. Underneath each pedicle is a small hole (enclosed by the pedicle of the vertebral below) called intervertebral foramen , which transmit the corresponding spinal nerve and dorsal root ganglion that exit the spinal canal. From top to bottom, the vertebrae are: For some medical purposes, adjacent vertebral regions may be considered together: The vertebral column
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#17327727536994108-508: The freely licensed Connexions [1] edition of Anatomy & Physiology [2] text-book by OpenStax College Vertebral column The vertebral column , also known as the spinal column , spine or backbone , is the core part of the axial skeleton in vertebrate animals . The vertebral column is the defining and eponymous characteristic of the vertebrate endoskeleton , where the notochord (an elastic collagen -wrapped glycoprotein rod) found in all chordates has been replaced by
4187-401: The gut and a parietal layer, which along with the overlying ectoderm, forms the lateral body wall. The space between the visceral and parietal layers of lateral plate mesoderm is the primitive body cavity. When the lateral body wall folds, it moves ventrally and fuses at the midline. The body cavity closes, except in the region of the connecting stalk. Here, the gut tube maintains an attachment to
4266-403: The gut as double layers of peritoneum. Mesenteries provide a pathway for vessels, nerves, and lymphatics to the organs. Initially, the gut tube from the caudal end of the foregut to the end of the hindgut is suspended from the dorsal body wall by dorsal mesentery. Ventral mesentery, derived from the septum transversum, exists only in the region of the terminal part of the esophagus, the stomach, and
4345-449: The length of the column, which include the anterior and posterior longitudinal ligaments at the front and back of the vertebral bodies , the ligamentum flavum in deep to the laminae , the interspinous and supraspinous ligaments between spinous processes , and the intertransverse ligaments between the transverse processes . The vertebrae in the human vertebral column is divided into different body regions , which correspond to
4424-432: The length of the vertebral column along the front and back of the vertebral bodies. The interspinous ligaments connect the adjoining spinous processes of the vertebrae. The supraspinous ligament extends the length of the spine running along the back of the spinous processes, from the sacrum to the seventh cervical vertebra . From there it is continuous with the nuchal ligament . The striking segmented pattern of
4503-403: The locations of other parts of human anatomy, such as the positions of organs . The general structure of vertebrae in other animals is largely the same as in humans. Individual vertebrae are composed of a centrum (body), arches protruding from the top and bottom of the centrum, and various processes projecting from the centrum and/or arches. An arch extending from the top of the centrum is called
4582-408: The lumbar region, by narrower intervals in the neck, and are closely approximated in the middle of the thoracic region. Occasionally one of these processes deviates a little from the median line — which can sometimes be indicative of a fracture or a displacement of the spine. On either side of the spinous processes is the vertebral groove formed by the laminae in the cervical and lumbar regions, where it
4661-405: The mechanisms involved in vertebral segmentation are conserved across vertebrates. In humans the first four somites are incorporated in the base of the occipital bone of the skull and the next 33 somites will form the vertebrae, ribs, muscles, ligaments and skin. The remaining posterior somites degenerate. During the fourth week of embryogenesis , the sclerotomes shift their position to surround
4740-415: The meninges for example the fluid is cerebrospinal fluid ; in the abdominal cavity the fluid contained in the peritoneum is a serous fluid . In amniotes and some invertebrates the peritoneum lines their largest body cavity called the coelom . Mammalian embryos develop two body cavities: the intraembryonic coelom and the extraembryonic coelom (or chorionic cavity ). The intraembryonic coelom
4819-423: The next vertebral body fits. Even these patterns are only generalisations, however, and there may be variation in form of the vertebrae along the length of the spine even within a single species. Some unusual variations include the saddle-shaped sockets between the cervical vertebrae of birds and the presence of a narrow hollow canal running down the centre of the vertebral bodies of geckos and tuataras , containing
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#17327727536994898-405: The number of cervical vertebrae can range from a single vertebra in amphibians to as many as 25 in swans or 76 in the extinct plesiosaur Elasmosaurus . The dorsal vertebrae range from the bottom of the neck to the top of the pelvis . Dorsal vertebrae attached to the ribs are called thoracic vertebrae, while those without ribs are called lumbar vertebrae. The sacral vertebrae are those in
4977-446: The opposite, possessing anterior convexity and posterior concavity. They are found in salamanders, and in some non-avian dinosaurs. Heterocoelous vertebrae have saddle -shaped articular surfaces. This type of configuration is seen in turtles that retract their necks, and birds, because it permits extensive lateral and vertical flexion motion without stretching the nerve cord too extensively or wringing it about its long axis. In horses,
5056-442: The outside (walls) of the peritoneal, pleural, and pericardial cavities. The visceral layer will form the visceral layer of the serous membranes covering the lungs, heart, and abdominal organs. These layers are continuous at the root of each organ as the organs lie in their respective cavities. The peritoneum , a serum membrane that forms the lining of the abdominal cavity, forms in the gut layers and in places mesenteries extend from
5135-422: The pelvic cavity. The abdominal cavity occupies the entire lower half of the trunk, anterior to the spine, and houses the organs of digestion. Just under the abdominal cavity, anterior to the buttocks, is the pelvic cavity. The pelvic cavity is funnel shaped, and is located inferior and anterior to the abdominal cavity, and houses the organs of reproduction. The dorsal body cavity contains the cranial cavity , and
5214-423: The pelvic region, and range from one in amphibians, to two in most birds and modern reptiles, or up to three to five in mammals. When multiple sacral vertebrae are fused into a single structure, it is called the sacrum. The synsacrum is a similar fused structure found in birds that is composed of the sacral, lumbar, and some of the thoracic and caudal vertebra, as well as the pelvic girdle . Caudal vertebrae compose
5293-418: The posterior part of one somite fuses to the anterior part of the consecutive somite during a process termed resegmentation. Disruption of the somitogenesis process in humans results in diseases such as congenital scoliosis. So far, the human homologues of three genes associated to the mouse segmentation clock, (MESP2, DLL3 and LFNG), have been shown to be mutated in cases of congenital scoliosis, suggesting that
5372-417: The posterior surface by the articular processes in the cervical and thoracic regions and by the transverse processes in the lumbar region. In the thoracic region, the sides of the bodies of the vertebrae are marked in the back by the facets for articulation with the heads of the ribs. More posteriorly are the intervertebral foramina, formed by the juxtaposition of the vertebral notches, oval in shape, smallest in
5451-542: The sacrovertebral articulation, and ends at the point of the coccyx ; its concavity is directed downward and forward as a kyphotic curve. The thoracic and sacral kyphotic curves are termed primary curves, because they are present in the fetus . The cervical and lumbar curves are compensatory , or secondary , and are developed after birth. The cervical curve forms when the infant is able to hold up its head (at three or four months) and sit upright (at nine months). The lumbar curve forms later from twelve to eighteen months, when
5530-401: The spine is established during embryogenesis when somites are rhythmically added to the posterior of the embryo. Somite formation begins around the third week when the embryo begins gastrulation and continues until all somites are formed. Their number varies between species: there are 42 to 44 somites in the human embryo and around 52 in the chick embryo. The somites are spheres, formed from
5609-446: The spinous process of the seventh thoracic vertebra. This curve is known as a kyphotic curve. The lumbar curve is more marked in the female than in the male; it begins at the middle of the last thoracic vertebra, and ends at the sacrovertebral angle. It is convex anteriorly, the convexity of the lower three vertebrae being much greater than that of the upper two. This curve is described as a lordotic curve. The sacral curve begins at
5688-416: The tail, and the final few can be fused into the pygostyle in birds, or into the coccygeal or tail bone in chimpanzees (and humans ). The vertebrae of lobe-finned fishes consist of three discrete bony elements. The vertebral arch surrounds the spinal cord, and is of broadly similar form to that found in most other vertebrates. Just beneath the arch lies a small plate-like pleurocentrum, which protects
5767-415: The tail. The general structure of human vertebrae is fairly typical of that found in other mammals , reptiles , and birds ( amniotes ). The shape of the vertebral body does, however, vary somewhat between different groups. In humans and other mammals, it typically has flat upper and lower surfaces, while in reptiles the anterior surface commonly has a concave socket into which the expanded convex face of
5846-565: The upper portion of the duodenum. These cavities contain and protect delicate internal organs, and the ventral cavity allows for significant changes in the size and shape of the organs as they perform their functions. Anatomical structures are often described in terms of the cavity in which they reside. The body maintains its internal organization by means of membranes, sheaths, and other structures that separate compartments. The lungs, heart, stomach, and intestines, for example, can expand and contract without distorting other tissues or disrupting
5925-463: The upper surface of the notochord , and below that, a larger arch-shaped intercentrum to protect the lower border. Both of these structures are embedded within a single cylindrical mass of cartilage. A similar arrangement was found in the primitive Labyrinthodonts , but in the evolutionary line that led to reptiles (and hence, also to mammals and birds), the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became
6004-449: The vertebrae consist of two cartilaginous tubes. The upper tube is formed from the vertebral arches, but also includes additional cartilaginous structures filling in the gaps between the vertebrae, and so enclosing the spinal cord in an essentially continuous sheath. The lower tube surrounds the notochord, and has a complex structure, often including multiple layers of calcification . Lampreys have vertebral arches, but nothing resembling
6083-409: The vertebral bodies found in all higher vertebrates . Even the arches are discontinuous, consisting of separate pieces of arch-shaped cartilage around the spinal cord in most parts of the body, changing to long strips of cartilage above and below in the tail region. Hagfishes lack a true vertebral column, and are therefore not properly considered vertebrates, but a few tiny neural arches are present in
6162-475: The vertebral column presents in the median line the spinous processes. In the cervical region (with the exception of the second and seventh vertebrae), these are short, horizontal, and bifid. In the upper part of the thoracic region they are directed obliquely downward; in the middle they are almost vertical, and in the lower part they are nearly horizontal. In the lumbar region they are nearly horizontal. The spinous processes are separated by considerable intervals in
6241-491: The yolk sac. The yolk sac is a membranous sac attached to the embryo, which provides nutrients and functions as the circulatory system of the very early embryo. The lateral body wall folds, pulling the amnion in with it so that the amnion surrounds the embryo and extends over the connecting stalk, which becomes the umbilical cord , which connects the fetus with the placenta . If the ventral body wall fails to close, ventral body wall defects can result, such as ectopia cordis ,
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