External occipital protuberance
Near the middle of the squamous part of occipital bone is the external occipital protuberance, the highest point of, referred to as the inion. The inion is the most prominent projection of the protuberance, located at the posterioinferior part of the human skull; the nuchal ligament and trapezius muscle attach to it. The inion is used as a landmark in the 10-20 system in electroencephalography recording. Extending laterally from it on either side is the superior nuchal line, above it is the faintly marked highest nuchal line. A study of 16th-century Anatolian remains showed that the external occipital protuberance statistically tends to be less pronounced in female remains. Internal occipital protuberance This article incorporates text in the public domain from page 185 of the 20th edition of Gray's Anatomy "Anatomy diagram: 34257.000-1". Roche Lexicon - illustrated navigator. Elsevier. Archived from the original on 2014-01-01. Http://www.upstate.edu/cdb/grossanat/hnsklatob1.shtml
YouTube is an American video-sharing website headquartered in San Bruno, California. Three former PayPal employees—Chad Hurley, Steve Chen, Jawed Karim—created the service in February 2005. Google bought the site in November 2006 for US$1.65 billion. YouTube allows users to upload, rate, add to playlists, comment on videos, subscribe to other users, it offers a wide variety of corporate media videos. Available content includes video clips, TV show clips, music videos and documentary films, audio recordings, movie trailers, live streams, other content such as video blogging, short original videos, educational videos. Most of the content on YouTube is uploaded by individuals, but media corporations including CBS, the BBC, Hulu offer some of their material via YouTube as part of the YouTube partnership program. Unregistered users can only watch videos on the site, while registered users are permitted to upload an unlimited number of videos and add comments to videos. Videos deemed inappropriate are available only to registered users affirming themselves to be at least 18 years old.
YouTube and its creators earn advertising revenue from Google AdSense, a program which targets ads according to site content and audience. The vast majority of its videos are free to view, but there are exceptions, including subscription-based premium channels, film rentals, as well as YouTube Music and YouTube Premium, subscription services offering premium and ad-free music streaming, ad-free access to all content, including exclusive content commissioned from notable personalities; as of February 2017, there were more than 400 hours of content uploaded to YouTube each minute, one billion hours of content being watched on YouTube every day. As of August 2018, the website is ranked as the second-most popular site in the world, according to Alexa Internet. YouTube has faced criticism over aspects of its operations, including its handling of copyrighted content contained within uploaded videos, its recommendation algorithms perpetuating videos that promote conspiracy theories and falsehoods, hosting videos ostensibly targeting children but containing violent and/or sexually suggestive content involving popular characters, videos of minors attracting pedophilic activities in their comment sections, fluctuating policies on the types of content, eligible to be monetized with advertising.
YouTube was founded by Chad Hurley, Steve Chen, Jawed Karim, who were all early employees of PayPal. Hurley had studied design at Indiana University of Pennsylvania, Chen and Karim studied computer science together at the University of Illinois at Urbana–Champaign. According to a story, repeated in the media and Chen developed the idea for YouTube during the early months of 2005, after they had experienced difficulty sharing videos, shot at a dinner party at Chen's apartment in San Francisco. Karim did not attend the party and denied that it had occurred, but Chen commented that the idea that YouTube was founded after a dinner party "was very strengthened by marketing ideas around creating a story, digestible". Karim said the inspiration for YouTube first came from Janet Jackson's role in the 2004 Super Bowl incident, when her breast was exposed during her performance, from the 2004 Indian Ocean tsunami. Karim could not find video clips of either event online, which led to the idea of a video sharing site.
Hurley and Chen said that the original idea for YouTube was a video version of an online dating service, had been influenced by the website Hot or Not. Difficulty in finding enough dating videos led to a change of plans, with the site's founders deciding to accept uploads of any type of video. YouTube began as a venture capital-funded technology startup from an $11.5 million investment by Sequoia Capital and an $8 million investment from Artis Capital Management between November 2005 and April 2006. YouTube's early headquarters were situated above a pizzeria and Japanese restaurant in San Mateo, California; the domain name www.youtube.com was activated on February 14, 2005, the website was developed over the subsequent months. The first YouTube video, titled Me at the zoo, shows co-founder Jawed Karim at the San Diego Zoo; the video was uploaded on April 23, 2005, can still be viewed on the site. YouTube offered the public a beta test of the site in May 2005; the first video to reach one million views was a Nike advertisement featuring Ronaldinho in November 2005.
Following a $3.5 million investment from Sequoia Capital in November, the site launched on December 15, 2005, by which time the site was receiving 8 million views a day. The site grew and, in July 2006, the company announced that more than 65,000 new videos were being uploaded every day, that the site was receiving 100 million video views per day. According to data published by market research company comScore, YouTube is the dominant provider of online video in the United States, with a market share of around 43% and more than 14 billion views of videos in May 2010. In May 2011, 48 hours of new videos were uploaded to the site every minute, which increased to 60 hours every minute in January 2012, 100 hours every minute in May 2013, 300 hours every minute in November 2014, 400 hours every minute in February 2017; as of January 2012, the site had 800 million unique users a month. It is estimated that in 2007 YouTube consumed as much bandwidth as the entire Internet in 2000. According to third-party web analytics providers and SimilarWeb, YouTube is the second-most visited website in the world, as of December 2016.
In human anatomy, the neurocranium known as the braincase, brainpan, or brain-pan is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the skullcap; the remainder of the skull is the facial skeleton. In comparative anatomy, neurocranium is sometimes used synonymously with endocranium or chondrocranium; the neurocranium is divided into two portions: the membranous part, consisting of flat bones, which surround the brain. In humans, the neurocranium is considered to include the following eight bones: 1 ethmoid bone 1 frontal bone 1 occipital bone 2 parietal bones 1 sphenoid bone 2 temporal bonesThe ossicles are not included as bones of the neurocranium. There may variably be extra sutural bones present. Below the neurocranium is a complex of openings and bones, including the foramen magnum which houses the neural spine; the auditory bullae, located in the same region, aid in hearing. The size of the neurocranium is variable among mammals.
The roof may contain ridges such as the temporal crests. The neurocranium arises from paraxial mesoderm. There is some contribution of ectomesenchyme. In Chondrichthyes and other cartilaginous vertebrates this portion of the cranium does not ossify; the neurocranium is formed by the endocranium, the lower portions of the cranial vault, the skull roof. These are not fused in fishes, a proper neurocranium is only found in land vertebrates. Evolutionarily, the human neurocranium has expanded from comprising the back part of the mammalian skull to being the upper part: during the evolutionary expansion of the brain, the neurocranium has overgrown the splanchnocranium; the upper-frontmost part of the cranium houses the evolutionarily newest part of the human brain, the frontal lobes. Cranial cavity
The cerebellum is a major feature of the hindbrain of all vertebrates. Although smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or larger. In humans, the cerebellum plays an important role in motor control, it may be involved in some cognitive functions such as attention and language as well as in regulating fear and pleasure responses, but its movement-related functions are the most solidly established. The human cerebellum does not initiate movement, but contributes to coordination and accurate timing: it receives input from sensory systems of the spinal cord and from other parts of the brain, integrates these inputs to fine-tune motor activity. Cerebellar damage produces disorders in fine movement, equilibrium and motor learning in humans. Anatomically, the human cerebellum has the appearance of a separate structure attached to the bottom of the brain, tucked underneath the cerebral hemispheres, its cortical surface is covered with finely spaced parallel grooves, in striking contrast to the broad irregular convolutions of the cerebral cortex.
These parallel grooves conceal the fact that the cerebellar cortex is a continuous thin layer of tissue folded in the style of an accordion. Within this thin layer are several types of neurons with a regular arrangement, the most important being Purkinje cells and granule cells; this complex neural organization gives rise to a massive signal-processing capability, but all of the output from the cerebellar cortex passes through a set of small deep nuclei lying in the white matter interior of the cerebellum. In addition to its direct role in motor control, the cerebellum is necessary for several types of motor learning, most notably learning to adjust to changes in sensorimotor relationships. Several theoretical models have been developed to explain sensorimotor calibration in terms of synaptic plasticity within the cerebellum; these models derive from those formulated by David Marr and James Albus, based on the observation that each cerebellar Purkinje cell receives two different types of input: one comprises thousands of weak inputs from the parallel fibers of the granule cells.
The basic concept of the Marr–Albus theory is that the climbing fiber serves as a "teaching signal", which induces a long-lasting change in the strength of parallel fiber inputs. Observations of long-term depression in parallel fiber inputs have provided support for theories of this type, but their validity remains controversial. At the level of gross anatomy, the cerebellum consists of a folded layer of cortex, with white matter underneath and a fluid-filled ventricle at the base. Four deep cerebellar nuclei are embedded in the white matter; each part of the cortex consists of the same small set of neuronal elements, laid out in a stereotyped geometry. At an intermediate level, the cerebellum and its auxiliary structures can be separated into several hundred or thousand independently functioning modules called "microzones" or "microcompartments"; the cerebellum is located in the posterior cranial fossa. The fourth ventricle and medulla are in front of the cerebellum, it is separated from the overlying cerebrum by a layer of leathery dura mater, the tentorium cerebelli.
Anatomists classify the cerebellum as part of the metencephalon, which includes the pons. Like the cerebral cortex, the cerebellum is divided into two hemispheres. A set of large folds is, by convention, used to divide the overall structure into 10 smaller "lobules"; because of its large number of tiny granule cells, the cerebellum contains more neurons than the total from the rest of the brain, but takes up only 10% of the total brain volume. The number of neurons in the cerebellum is related to the number of neurons in the neocortex. There are about 3.6 times as many neurons in the cerebellum as in the neocortex, a ratio, conserved across many different mammalian species. The unusual surface appearance of the cerebellum conceals the fact that most of its volume is made up of a tightly folded layer of gray matter: the cerebellar cortex; each ridge or gyrus in this layer is called a folium. It is estimated that, if the human cerebellar cortex were unfolded, it would give rise to a layer of neural tissue about 1 meter long and averaging 5 centimeters wide—a total surface area of about 500 square cm, packed within a volume of dimensions 6 cm × 5 cm × 10 cm.
Underneath the gray matter of the cortex lies white matter, made up of myelinated nerve fibers running to and from the cortex. Embedded within the white matter—which is sometimes called the arbor vitae because of its branched, tree-like appearance in cross-section—are four deep cerebellar nuclei, composed of gray matter. Connecting the cerebellum to different parts of the nervous system are three paired cerebellar peduncles; these are the superior cerebellar peduncle, the middle cerebellar peduncle and the inferior cerebellar peduncle, named by their position relative to the vermis. The superior cerebellar peduncle is an output to the cerebral cortex, carrying efferent fibers via thalamic nuclei to upper motor neurons in the cerebral cortex; the fibers arise from the deep cerebellar nuclei. The middle cerebellar peduncle is connected to the pons and receives all of its input from the pons from the pontine nuclei; the input to the pons is from the cerebral cortex and is relayed from the pontine nuclei via transverse pontine fibers to the cerebellum
Apical ligament of dens
The ligament of apex dentis is a ligament that spans between the second cervical vertebra in the neck and the skull. It lies as a fibrous cord in the triangular interval between the alar ligaments, which extends from the tip of the odontoid process on the axis to the anterior margin of the foramen magnum, being intimately blended with the deep portion of the anterior atlantooccipital membrane and superior crus of the transverse ligament of the atlas, it is regarded as a rudimentary intervertebral fibrocartilage, in it traces of the notochord may persist. This article incorporates text in the public domain from page 296 of the 20th edition of Gray's Anatomy "Anatomy diagram: 25420.000-1". Roche Lexicon - illustrated navigator. Elsevier. Archived from the original on 2014-01-01
Sahelanthropus tchadensis is an extinct species of the Hominini and is the ancestor to Orrorin, dated to about 7 million years ago, during the Miocene epoch very close to the time of the chimpanzee–human divergence. Few specimens other than the partial skull, nicknamed Toumaï, are known. Existing fossils include a small cranium named Toumaï, five pieces of jaw, some teeth, making up a head that has a mixture of derived and primitive features; the braincase, being only 320 cm3 to 380 cm3 in volume, is similar to that of extant chimpanzees and is notably less than the approximate human volume of 1350 cm3. The teeth, brow ridges, facial structure differ markedly from those found in Homo sapiens. Cranial features show a flatter face, u-shaped dental arcade, small canines, an anterior foramen magnum, heavy brow ridges. No postcranial remains have been recovered; the only known skull suffered a large amount of distortion during the time of fossilisation and discovery, as the cranium is dorsoventrally flattened, the right side is depressed.
Sahelanthropus tchadensis may have walked on two legs. However, because no postcranial remains have been discovered, it is not known definitively whether Sahelanthropus was indeed bipedal, although claims for an anteriorly placed foramen magnum suggests that this may have been the case. Upon examination of the foramen magnum in the primary study, the lead author speculated that a bipedal gait "would not be unreasonable" based on basicranial morphology similar to more recent hominins; some palaeontologists have disputed this interpretation, stating that the basicranium, as well as dentition and facial features, do not represent adaptations unique to the hominin clade, nor indicative of bipedalism. Further, according to recent information, what might be a femur of a hominid was discovered near the cranium—but which has not been published nor accounted for. Fifteen years after the discovery of the fossil, the anthropologist Roberto Macchiarelli—professor at the University of Poitiers and the Museum of Natural History of Paris—suspects Michel Brunet and his laboratory in Poitiers of blocking information about a femur found close to the skull.
That the laboratory would have delayed identification may question the bipedalism of Toumaï. The fossils were discovered in the Djurab Desert of Chad by a team of four led by a Frenchman, Alain Beauvilain, three Chadians, Adoum Mahamat, Djimdoumalbaye Ahounta, Gongdibé Fanoné, members of the Mission paleoanthropologique Franco-tchadienne led by Michel Brunet. All known material of Sahelanthropus was found between July 2001 and March 2002 at three sites: TM 247, TM 266, which yielded most of the material, including a cranium and a femur, TM 292; the discoverers claimed that S. tchadensis is the oldest-known human ancestor after the split of the human line from that of chimpanzees. The bones were found far from most previous hominin fossil finds, which are from Eastern and Southern Africa. However, an Australopithecus bahrelghazali mandible was found in Chad by Mamelbaye Tomalta and Alain Beauvilain, Michel Brunet and Aladji H. E. Moutaye as early as 1995. With the sexual dimorphism known to have existed in early hominins, the difference between Ardipithecus and Sahelanthropus may not be large enough to warrant a separate species for the latter.
Sahelanthropus may represent a common ancestor of humans and chimpanzees, though no consensus has been reached yet by the scientific community. The original placement of this species as a human ancestor but not a chimpanzee ancestor would complicate the picture of human phylogeny. In particular, if Toumaï is indeed a direct human ancestor its facial features bring into doubt the status of Australopithecus whose thickened brow ridges were reported to be similar to those of some fossil hominins, where the brow ridge morphology of Sahelanthropus differs from that observed in all australopithecines, most fossil hominins and extant humans. Another possibility is that Toumaï is related to both humans and chimpanzees, but is the ancestor of neither. Brigitte Senut and Martin Pickford, the discoverers of Orrorin tugenensis, suggested that the features of S. tchadensis are consistent with a female proto-gorilla. If this claim is upheld the find would lose none of its significance, because at present few chimpanzee or gorilla ancestors have been found anywhere in Africa.
Thus if S. tchadensis is an ancestral relative of the chimpanzees or gorillas it represents the earliest known member of their lineage. And S. tchadensis does indicate that the last common ancestor of humans and chimpanzees is unlikely to resemble extant chimpanzees, as had been supposed by some paleontologists. A further possibility, highlighted by research published in 2012, is that the human–chimpanzee split is earlier than thought, with a possible range of 7 to 13 million years ago, based on slower than thought changes between generations in human DNA. Indeed, some researchers consider suggestions that Sahelanthropus is too early to be a human ancestor to have evaporated. Sediment isotope analysis of cosmogenic atoms in the fossil yielded an age of about 7 million years. In this case, the fossils were found exposed in loose sand. In fact, Toumaï may have been reburied in the r
Posterior spinal artery
The posterior spinal artery arises from the vertebral artery in 25% of humans or the posterior inferior cerebellar artery in 75% of humans, adjacent to the medulla oblongata. It supplies the white posterior columns of the spinal cord, it passes posteriorly to descend the medulla passing in front of the posterior roots of the spinal nerves. Along its course it is reinforced by a succession of segmental or radiculopial branches, which enter the vertebral canal through the intervertebral foramina, forming a plexus called the vasocorona with the anterior verterbral arteries. Below the medulla spinalis and upper cervical spine, the posterior spinal arteries are rather discontinuous. At the level of the conus medullaris, the posterior spinals are more seen as distinct arteries, communicating with the anterior spinal artery to form a characteristic "basket" which angiographically defines the caudal extent of the spinal cord and its transition to the cauda equina. Branches from the posterior spinal arteries form a free anastomosis around the posterior roots of the spinal nerves, communicate, by means of tortuous transverse branches, with the vessels of the opposite side.
Close to its origin each posterior spinal artery gives off an ascending branch, which ends ipsilaterally near the fourth ventricle. The posterior spinal artery can originate from the posterior inferior cerebellar artery, rather than the vertebral. Most cranially, the posterior spinal artery supplies the dorsal column of the closed medulla containing fasiculus gracilis, gracile nucleus, fasciculus cuneatus, cuneate nucleus. At the spinal cord level, the posterior spinal artery supplies the dorsal and lateral column as well as the peripheral part of the anterior and central column. Disruption of blood supply to this particular artery in the medulla would result in a number of sensory deficits. If occlusion occurs above the level of sensory decussation, it would affect the proprioception and two-point discrimination of the contralateral side of the body; when occurs below the level of sensory decussation, however, it would cause the effect on the ipsilateral side instead. At the upper cervical spinal cord where the posterior spinal artery supplies some parts of the ventral column, lesion might extend to the anterolateral system and triggers the loss of pain and temperature sensation on the contralateral side of the body.
This article incorporates text in the public domain from page 579 of the 20th edition of Gray's Anatomy Diagram at nih.gov Image at anaesthesiauk.com