Instruct: Given a question, retrieve passages that can help answer the question.
Query: How many vertebrae does a human have?
At birth the majority of humans have 33 separate vertebrae. However, during normal development several vertebrae fuse together, leaving a total of 24, in most cases. The confusion about whether or not there are 32-34 vertebrae stems from the fact that the two lowest vertebrae, the sacrum and the coccyx, are single bones made up of several smaller bones which have fused together. This is how the vertebrae are counted: 24 separate vertebrae and the sacrum, formed from 5 fused vertebrae and the coccyx, formed from 3-5 fused vertebrae. If you count the coccyx and sacrum each as one vertebra, then there are 26 vertebrae. If the fused vertebrae are all counted separately, then the total number of vertebrae comes to between 32 and 34.
A horse has an average total of 18 thoracic vertebrae, with 5 located in the withers. Each thoracic vertebra is also associated with a rib. A horse also has, on average, six lumbar vertebrae. Some breeds, such as the Arabian, will sometimes, but not always, have five lumbar vertebrae and 17 thoracic vertebrae. There appears to be little correlation between back length and number of vertebrae, as many horses with short backs do have the typical number of vertebrae. The length of each vertebra in the lumbar region seems to have the greater influence on the strength of the horse's back.
A recent suggestion, based upon observations that men and women have the same number of ribs, speculates that the bone was the baculum, a small structure found in the penis of many mammals, but not in humans.
A cervical rib in humans is an extra rib which arises from the seventh cervical vertebra. Sometimes known as "neck ribs", their presence is a congenital abnormality located above the normal first rib. A cervical rib is estimated to occur in 0.2% (1 in 500 people) to 0.5% of the population. People may have a cervical rib on the right, left or both sides.
Group D
The tail of "Dimetrodon" makes up a large portion of its total body length and includes around 50 caudal vertebrae. Tails were missing or incomplete in the first described skeletons of "Dimetrodon"; the only caudal vertebrae known were the eleven closest to the hip. Since these first few caudal vertebrae narrow rapidly as they progress farther from the hip, many paleontologists in the late nineteenth and early twentieth centuries thought that "Dimetrodon" had a very short tail. It was not until 1927 that a largely complete tail of "Dimetrodon" was described.
The stapes is the third bone of the three ossicles in the middle ear. The stapes is a stirrup-shaped bone, and the smallest in the human body. It rests on the oval window, to which it is connected by an annular ligament. The stapes is described as having a base, resting on the oval window, as well as a head that articulates with the incus. These are connected by anterior and posterior limbs (). The stapes articulates with the incus through the incudostapedial joint. The stapes is the smallest bone in the human body, and measures roughly 3 x 2.5mm, greater along the head-base span.
Group C
In most subsequent forms, many of these early ribs have been lost, and in living amphibians and reptiles, there is great variation in rib structure and number. For example, turtles have only eight pairs of ribs, which are developed into a bony or cartilaginous carapace and plastron, while snakes have numerous ribs running along the full length of their trunk. Frogs typically have no ribs, aside from a sacral pair, which form part of the pelvis.
Besides its small size, "D. milleri" differs from other species of "Dimetrodon" in that its neural spines are circular rather than figure-eight shaped in cross-section. Its vertebrae are also shorter in height relative to the rest of the skeleton than those of other "Dimetrodon" species. The skull is tall and the snout is short relative to the temporal region. A short vertebrae and tall skull are also seen in the species "D. booneorum", "D. limbatus" and "D. grandis", suggesting that "D. milleri" may be the first of an evolutionary progression between these species.
The stapes or stirrup is a bone in the middle ear of humans and other mammals which is involved in the conduction of sound vibrations to the inner ear. The stirrup-shaped small bone is on and transmits these to the oval window, medially. The stapes is the smallest and lightest named bone in the human body, and is so-called because of its resemblance to a stirrup ().
Andreas Vesalius (1514-1564), a Flemish anatomist, was at first a "Galenist" at the University of Paris. When he moved to Italy and entered the University of Padua, he began dissecting human bodies. He studied many details of human anatomy and found that Galen made some anatomical mistakes. For example, Galen wrote that the sternum has seven segments, but Vesalius found it has three segments. Galen wrote that the bone of the arm is the longest bone in the human body, but Vesalius found that the bone of the thigh is actually the longest bone in human body. At age 25 Vesalius realized that the anatomical knowledge of Galen was derived from animal anatomy and therefore Galen had never dissected a human body.
Two categories of blood groups, human-type and simian-type, have been found in apes and monkeys, and can be tested by methods established for grouping human blood.
The trapezoid bone (lesser multangular bone) is a carpal bone in tetrapods, including humans. It is the smallest bone in the distal row of carpal bones that give structure to the palm of the hand. It may be known by its wedge-shaped form, the broad end of the wedge constituting the dorsal, the narrow end the palmar surface; and by its having four articular facets touching each other, and separated by sharp edges. It is homologous with the "second distal carpal" of reptiles and amphibians.
Thewissen’s current work involves counting neurons in bowhead and beluga whale brain samples, to assess brain function, in Utqiagvik—formerly Barrow—Alaska. Though the size of the brains of sperm whales and killer whales are larger than those of any other organism, including humans, a better measure of brain function is to determine how many neurons there are in the brain. Suzana Herculano-Houzel has developed a method of counting of neurons of human and other animals' brains and the relation between the cerebral cortex area and thickness and number of cortical folds. Humans and other primates pack about twice the number of neurons in a cubed inch of brain as most other mammals.
In humans, the large intestine begins in the right iliac region of the pelvis, just at or below the waist, where it is joined to the end of the small intestine at the cecum, via the ileocecal valve. It then continues as the colon ascending the abdomen, across the width of the abdominal cavity as the transverse colon, and then descending to the rectum and its endpoint at the anal canal. Overall, in humans, the large intestine is about long, which is about one-fifth of the whole length of the gastrointestinal tract.
Flemish anatomist and physician Andreas Vesalius is often referred to as the founder of modern human anatomy for the publication of the seven-volume "De humani corporis fabrica" ("On the Structure of the Human Body") in 1543.
The stapedius is the smallest skeletal muscle in the human body. At just over one millimeter in length, its purpose is to stabilize the smallest bone in the body, the stapes.
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A number of partial fossil remains of "Secodontosaurus" have been identified from its characteristic long skull and jaws. The postcranial skeletal material from different individuals includes parts of the backbone with clear evidence of a tall sail very similar to that of "Dimetrodon". The limbs and tail are incomplete but probably resembled those of "Dimetrodon" as well. Like "Dimetrodon", "Secodontosaurus" would have had a short neck, robust body, short limbs, and a long tail. In light of such similarities, some skeletal remains with missing or fragmentary skulls that were previously identified as "Dimetrodon" may in fact belong to "Secodontosaurus". A key noncranial difference can be found in the axis neck vertebra, which has a tall and broad neural spine in "Dimetrodon" but has a lower neural spine in "Secodontosaurus".
The rectum is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction, the end of the sigmoid colon, at the level of the third sacral vertebra or the sacral promontory depending upon what definition is used. Its caliber is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla. It terminates at the level of the anorectal ring (the level of the puborectalis sling) or the dentate line, again depending upon which definition is used. In humans, the rectum is followed by the anal canal which is about long, before the gastrointestinal tract terminates at the anal verge. The word rectum comes from the Latin " ", meaning "straight intestine".
Cats have seven cervical vertebrae (as do most mammals); 13 thoracic vertebrae (humans have 12); seven lumbar vertebrae (humans have five); three sacral vertebrae (as do most mammals, but humans have five); and a variable number of caudal vertebrae in the tail (humans have only vestigial caudal vertebrae, fused into an internal coccyx). The extra lumbar and thoracic vertebrae account for the cat's spinal mobility and flexibility. Attached to the spine are 13 ribs, the shoulder, and the pelvis. Unlike human arms, cat forelimbs are attached to the shoulder by free-floating clavicle bones which allow them to pass their body through any space into which they can fit their head.
"Orthosuchus" has 24 vertebrae, and 8 of them are believed to be in the cervical region. All vertebrae appear to be amphicoelous. It does not have tall neural spine but the height of them decrease from head to tail. The vertebrae in the neck region are tightly bonded together. The ribs start at the twelfth vertebra, which supports the ilium. The tail consists of 8 vertebrae of different size and shape. The first one is in contact with the sacral ribs, three vertebrae at the far end of the tail are tiny in size, and the other four vertebrae form the middle of the tail. The ribs form antero-ventral and postero-dorsal flanges, which are for the muscle attachments.
In all mammals the thoracic vertebrae are connected to ribs and their bodies differ from the other regional vertebrae due to the presence of facets. Each vertebra has a facet on each side of the vertebral body which articulates with the head of a rib. There is also a facet on each of the transverse processes which articulates with the tubercle of a rib. The number of thoracic vertebrae varies considerably across the species. Most marsupials have thirteen, but koalas only have eleven. The norm is twelve to fifteen in mammals, (twelve in the human), though there are from eighteen to twenty in the horse, tapir, rhinoceros and elephant. In certain sloths there is an extreme number of twenty-five and at the other end only nine in the cetacean.
African apes have three and four lumbar vertebrae, (bonobos have longer spines with an additional vertebra) and humans normally five. This difference, and because the lumbar spines of the extinct "Nacholapithecus" (a Miocene hominoid with six lumbar vertebrae and no tail) are similar to those of early "Australopithecus" and early "Homo", it is assumed that the Chimpanzee-human last common ancestor also had a long vertebral column with a long lumbar region and that the reduction in the number of lumbar vertebrae evolved independently in each ape clade.