They carry impulses from the skin receptors to the spinal cord and from the spinal cord to the muscles. Because of Winn Dixie. There is a noteworthy point that the roots of seventh to tenth nerves first run inside the neural canal of the vertebral column for some distance to form a horse-tail shaped structure, the cauda equina. From the truncus arteriosus mixed blood goes to three arches carotid, systemic and pulmocutaneous and ultimately reaches all the parts of the body. Active Ingredient Screening Test for Plants. However, a single positive study performed according to good scientific principles and with statistically and biologically significant positive results may justify classification.
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Igneous intrusions such as batholiths , laccoliths , dikes , and sills , push upwards into the overlying rock, and crystallize as they intrude. Deformation typically occurs as a result of horizontal shortening, horizontal extension , or side-to-side strike-slip motion. These structural regimes broadly relate to convergent boundaries , divergent boundaries , and transform boundaries , respectively, between tectonic plates. Earth is estimated to have formed 4. Initially molten, the outer layer of the Earth cooled, resulting in the solid crust.
Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor , most or all of which came from ice delivered by comets , produced the oceans and other water sources. Continents formed, then broke up and reformed as the surface of Earth reshaped over hundreds of millions of years, occasionally combining to make a supercontinent.
During the Neoproterozoic era, freezing temperatures covered much of the Earth in glaciers and ice sheets. Since the Cambrian explosion there have been five distinctly identifiable mass extinctions. Several million years ago, a species of small African ape gained the ability to stand upright. The present era is classified as part of a mass extinction event , the Holocene extinction event, the fastest ever to have occurred.
The Earth's atmosphere is a key factor in sustaining the ecosystem. The thin layer of gases that envelops the Earth is held in place by gravity.
Air is mostly nitrogen , oxygen , water vapor , with much smaller amounts of carbon dioxide, argon, etc. The atmospheric pressure declines steadily with altitude. The ozone layer plays an important role in depleting the amount of ultraviolet UV radiation that reaches the surface. The atmosphere also retains heat during the night, thereby reducing the daily temperature extremes. Terrestrial weather occurs almost exclusively in the lower part of the atmosphere , and serves as a convective system for redistributing heat.
These currents help to moderate the differences in temperature between winter and summer in the temperate zones. Also, without the redistributions of heat energy by the ocean currents and atmosphere, the tropics would be much hotter, and the polar regions much colder.
Weather can have both beneficial and harmful effects. Extremes in weather, such as tornadoes or hurricanes and cyclones , can expend large amounts of energy along their paths, and produce devastation. Surface vegetation has evolved a dependence on the seasonal variation of the weather, and sudden changes lasting only a few years can have a dramatic effect, both on the vegetation and on the animals which depend on its growth for their food.
Climate is a measure of the long-term trends in the weather. Various factors are known to influence the climate , including ocean currents, surface albedo , greenhouse gases , variations in the solar luminosity, and changes to the Earth's orbit. Based on historical records, the Earth is known to have undergone drastic climate changes in the past, including ice ages.
The climate of a region depends on a number of factors, especially latitude. A latitudinal band of the surface with similar climatic attributes forms a climate region. There are a number of such regions, ranging from the tropical climate at the equator to the polar climate in the northern and southern extremes.
Weather is also influenced by the seasons, which result from the Earth 's axis being tilted relative to its orbital plane. Thus, at any given time during the summer or winter, one part of the Earth is more directly exposed to the rays of the sun. This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the northern and southern hemispheres experience opposite seasons. Weather is a chaotic system that is readily modified by small changes to the environment , so accurate weather forecasting is limited to only a few days.
Water is a chemical substance that is composed of hydrogen and oxygen and is vital for all known forms of life. Additionally, a minute amount of the Earth's water is contained within biological bodies and manufactured products. An ocean is a major body of saline water , and a principal component of the hydrosphere.
More than half of this area is over 3, meters 9, feet deep. Average oceanic salinity is around 35 parts per thousand ppt 3. Though generally recognized as several 'separate' oceans, these waters comprise one global, interconnected body of salt water often referred to as the World Ocean or global ocean. The major oceanic divisions are defined in part by the continents , various archipelagos , and other criteria: Smaller regions of the oceans are called seas, gulfs , bays and other names.
There are also salt lakes , which are smaller bodies of landlocked saltwater that are not interconnected with the World Ocean. A lake from Latin lacus is a terrain feature or physical feature , a body of liquid on the surface of a world that is localized to the bottom of basin another type of landform or terrain feature; that is, it is not global and moves slowly if it moves at all.
On Earth, a body of water is considered a lake when it is inland, not part of the ocean, is larger and deeper than a pond, and is fed by a river. It is not known if Titan's lakes are fed by rivers, though Titan's surface is carved by numerous river beds.
Natural lakes on Earth are generally found in mountainous areas, rift zones , and areas with ongoing or recent glaciation. Other lakes are found in endorheic basins or along the courses of mature rivers.
In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the last Ice Age.
All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them.
A pond is a body of standing water , either natural or man-made, that is usually smaller than a lake. A wide variety of man-made bodies of water are classified as ponds, including water gardens designed for aesthetic ornamentation, fish ponds designed for commercial fish breeding, and solar ponds designed to store thermal energy.
Ponds and lakes are distinguished from streams via current speed. While currents in streams are easily observed, ponds and lakes possess thermally driven micro-currents and moderate wind driven currents.
These features distinguish a pond from many other aquatic terrain features, such as stream pools and tide pools. A river is a natural watercourse ,  usually freshwater , flowing toward an ocean, a lake, a sea or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including stream, creek, brook, rivulet, and rill; there is no general rule that defines what can be called a river.
Many names for small rivers are specific to geographic location; one example is Burn in Scotland and North-east England. Sometimes a river is said to be larger than a creek, but this is not always the case, due to vagueness in the language. Water within a river is generally collected from precipitation through surface runoff , groundwater recharge, springs , and the release of stored water in natural ice and snowpacks i.
A stream is a flowing body of water with a current , confined within a bed and stream banks. In the United States, a stream is classified as a watercourse less than 60 feet 18 metres wide. Streams are important as conduits in the water cycle , instruments in groundwater recharge , and they serve as corridors for fish and wildlife migration.
The biological habitat in the immediate vicinity of a stream is called a riparian zone. Given the status of the ongoing Holocene extinction , streams play an important corridor role in connecting fragmented habitats and thus in conserving biodiversity. The study of streams and waterways in general involves many branches of inter-disciplinary natural science and engineering, including hydrology , fluvial geomorphology , aquatic ecology , fish biology , riparian ecology , and others.
Ecosystems are composed of a variety of abiotic and biotic components that function in an interrelated way. Variations of these factors will initiate dynamic modifications to the ecosystem. Some of the more important components are: Central to the ecosystem concept is the idea that living organisms interact with every other element in their local environment. Eugene Odum, a founder of ecology, stated: A smaller unit of size is called a microecosystem.
For example, a microsystem can be a stone and all the life under it. A macroecosystem might involve a whole ecoregion , with its drainage basin. Wilderness is generally defined as areas that have not been significantly modified by human activity. Wilderness areas can be found in preserves, estates, farms, conservation preserves, ranches, national forests , national parks , and even in urban areas along rivers, gulches, or otherwise undeveloped areas.
Wilderness areas and protected parks are considered important for the survival of certain species , ecological studies, conservation , and solitude. Some nature writers believe wilderness areas are vital for the human spirit and creativity,  and some ecologists consider wilderness areas to be an integral part of the Earth's self-sustaining natural ecosystem the biosphere.
They may also preserve historic genetic traits and that they provide habitat for wild flora and fauna that may be difficult or impossible to recreate in zoos , arboretums , or laboratories. Although there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life is characterized by organization , metabolism , growth , adaptation , response to stimuli , and reproduction.
Properties common to terrestrial organisms plants, animals, fungi , protists , archaea , and bacteria are that they are cellular, carbon-and-water-based with complex organization, having a metabolism, a capacity to grow, respond to stimuli, and reproduce.
An entity with these properties is generally considered life. However, not every definition of life considers all of these properties to be essential.
Human-made analogs of life may also be considered to be life. From the broadest geophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere rocks , hydrosphere water , and atmosphere air. Over nine-tenths of the total biomass on Earth is plant life, on which animal life depends very heavily for its existence.
The origin of life on Earth is not well understood, but it is known to have occurred at least 3. Once life had appeared, the process of evolution by natural selection resulted in the development of ever-more diverse life forms.
Species that were unable to adapt to the changing environment and competition from other life forms became extinct. However, the fossil record retains evidence of many of these older species.
Current fossil and DNA evidence shows that all existing species can trace a continual ancestry back to the first primitive life forms. When basic forms of plant life developed the process of photosynthesis the sun's energy could be harvested to create conditions which allowed for more complex life forms.
The incorporation of smaller cells within larger ones resulted in the development of yet more complex cells called eukaryotes. With the ozone layer absorbing harmful ultraviolet radiation , life colonized the surface of Earth. The first form of life to develop on the Earth were microbes, and they remained the only form of life until about a billion years ago when multi-cellular organisms began to appear.
They include Bacteria, Fungi , Archaea , and Protista. These life forms are found in almost every location on the Earth where there is liquid water, including in the Earth's interior. The combination of a high mutation rate and a horizontal gene transfer  ability makes them highly adaptable, and able to survive in new environments, including outer space. However, some microorganisms are pathogenic and can post health risk to other organisms. Originally Aristotle divided all living things between plants, which generally do not move fast enough for humans to notice, and animals.
In Linnaeus ' system, these became the kingdoms Vegetabilia later Plantae and Animalia. Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the fungi and several groups of algae were removed to new kingdoms.
However, these are still often considered plants in many contexts. Bacterial life is sometimes included in flora,   and some classifications use the term bacterial flora separately from plant flora. Among the many ways of classifying plants are by regional floras , which, depending on the purpose of study, can also include fossil flora , remnants of plant life from a previous era. People in many regions and countries take great pride in their individual arrays of characteristic flora, which can vary widely across the globe due to differences in climate and terrain.
Regional floras commonly are divided into categories such as native flora and agricultural and garden flora , the lastly mentioned of which are intentionally grown and cultivated. Some types of "native flora" actually have been introduced centuries ago by people migrating from one region or continent to another, and become an integral part of the native, or natural flora of the place to which they were introduced.
This is an example of how human interaction with nature can blur the boundary of what is considered nature. Another category of plant has historically been carved out for weeds. Though the term has fallen into disfavor among botanists as a formal way to categorize "useless" plants, the informal use of the word "weeds" to describe those plants that are deemed worthy of elimination is illustrative of the general tendency of people and societies to seek to alter or shape the course of nature.
Similarly, animals are often categorized in ways such as domestic , farm animals , wild animals , pests , etc. Animals as a category have several characteristics that generally set them apart from other living things. Animals are eukaryotic and usually multicellular although see Myxozoa , which separates them from bacteria, archaea , and most protists. They are heterotrophic , generally digesting food in an internal chamber, which separates them from plants and algae.
They are also distinguished from plants, algae, and fungi by lacking cell walls. With a few exceptions—most notably the two phyla consisting of sponges and placozoans —animals have bodies that are differentiated into tissues. These include muscles , which are able to contract and control locomotion, and a nervous system , which sends and processes signals.
There is also typically an internal digestive chamber. The eukaryotic cells possessed by all animals are surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form structures like shells , bones , and spicules , a framework upon which cells can move about and be reorganized during development and maturation, and which supports the complex anatomy required for mobility.
Although humans comprise only a minuscule proportion of the total living biomass on Earth, the human effect on nature is disproportionately large. Because of the extent of human influence, the boundaries between what humans regard as nature and "made environments" is not clear cut except at the extremes. Even at the extremes, the amount of natural environment that is free of discernible human influence is diminishing at an increasingly rapid pace.
The development of technology by the human race has allowed the greater exploitation of natural resources and has helped to alleviate some of the risk from natural hazards. In spite of this progress, however, the fate of human civilization remains closely linked to changes in the environment.
There exists a highly complex feedback loop between the use of advanced technology and changes to the environment that are only slowly becoming understood.
Humans have contributed to the extinction of many plants and animals. Humans employ nature for both leisure and economic activities. The acquisition of natural resources for industrial use remains a sizable component of the world's economic system.
Agriculture was first adopted around the 9th millennium BCE. Ranging from food production to energy, nature influences economic wealth. Although early humans gathered uncultivated plant materials for food and employed the medicinal properties of vegetation for healing,  most modern human use of plants is through agriculture.
The clearance of large tracts of land for crop growth has led to a significant reduction in the amount available of forestation and wetlands, resulting in the loss of habitat for many plant and animal species as well as increased erosion.
Beauty in nature has historically been a prevalent theme in art and books, filling large sections of libraries and bookstores. That nature has been depicted and celebrated by so much art, photography, poetry, and other literature shows the strength with which many people associate nature and beauty.
Reasons why this association exists, and what the association consists of, are studied by the branch of philosophy called aesthetics. Beyond certain basic characteristics that many philosophers agree about to explain what is seen as beautiful, the opinions are virtually endless.
An early tradition of landscape art began in China during the Tang Dynasty — The tradition of representing nature as it is became one of the aims of Chinese painting and was a significant influence in Asian art. Although natural wonders are celebrated in the Psalms and the Book of Job , wilderness portrayals in art became more prevalent in the s, especially in the works of the Romantic movement. British artists John Constable and J. Turner turned their attention to capturing the beauty of the natural world in their paintings.
Before that, paintings had been primarily of religious scenes or of human beings. William Wordsworth 's poetry described the wonder of the natural world, which had formerly been viewed as a threatening place. Increasingly the valuing of nature became an aspect of Western culture. A common classical idea of beautiful art involves the word mimesis , the imitation of nature. Also in the realm of ideas about beauty in nature is that the perfect is implied through perfect mathematical forms and more generally by patterns in nature.
As David Rothenburg writes, "The beautiful is the root of science and the goal of art, the highest possibility that humanity can ever hope to see". Some fields of science see nature as matter in motion, obeying certain laws of nature which science seeks to understand. Retroperitoneal organs in general do not have a complete covering of peritoneum , so they are fixed in location. Intraperitoneal organs are completely surrounded by peritoneum and are therefore mobile.
The average inner diameter of sections of the colon in centimeters with ranges in parentheses are cecum 8. The cecum is the first section of the colon and involved in the digestion, while the appendix which develops embryologically from it, is a structure of the colon, not involved in digestion and considered to be part of the gut-associated lymphoid tissue.
The function of the appendix is uncertain, but some sources believe that the appendix has a role in housing a sample of the colon's microflora, and is able to help to repopulate the colon with bacteria if the microflora has been damaged during the course of an immune reaction. The appendix has also been shown to have a high concentration of lymphatic cells. The ascending colon is the first of four sections of the large intestine.
It is connected to the small intestine by a section of bowel called the cecum. One of the main functions of the colon is to remove the water and other key nutrients from waste material and recycle it. As the waste material exits the small intestine through the ileocecal valve , it will move into the cecum and then to the ascending colon where this process of extraction starts.
The unwanted waste material is moved upwards toward the transverse colon by the action of peristalsis. The ascending colon is sometimes attached to the appendix via Gerlach's valve. In ruminants , the ascending colon is known as the spiral colon.
The transverse colon is the part of the colon from the hepatic flexure , also known as the right colic, the turn of the colon by the liver to the splenic flexure also known as the left colic, the turn of the colon by the spleen. The transverse colon hangs off the stomach , attached to it by a large fold of peritoneum called the greater omentum. On the posterior side, the transverse colon is connected to the posterior abdominal wall by a mesentery known as the transverse mesocolon.
The transverse colon is encased in peritoneum , and is therefore mobile unlike the parts of the colon immediately before and after it. The proximal two-thirds of the transverse colon is perfused by the middle colic artery , a branch of the superior mesenteric artery SMA , while the latter third is supplied by branches of the inferior mesenteric artery IMA.
The "watershed" area between these two blood supplies, which represents the embryologic division between the midgut and hindgut , is an area sensitive to ischemia. The descending colon is the part of the colon from the splenic flexure to the beginning of the sigmoid colon. One function of the descending colon in the digestive system is to store feces that will be emptied into the rectum. It is retroperitoneal in two-thirds of humans.
In the other third, it has a usually short mesentery. The descending colon is also called the distal gut , as it is further along the gastrointestinal tract than the proximal gut. Gut flora are very dense in this region. The sigmoid colon is the part of the large intestine after the descending colon and before the rectum. The name sigmoid means S-shaped see sigmoid ; cf.
The walls of the sigmoid colon are muscular, and contract to increase the pressure inside the colon, causing the stool to move into the rectum. The sigmoid colon is supplied with blood from several branches usually between 2 and 6 of the sigmoid arteries , a branch of the IMA. The IMA terminates as the superior rectal artery.
Sigmoidoscopy is a common diagnostic technique used to examine the sigmoid colon. The rectum is the last section of the large intestine. It holds the formed feces awaiting elimination via defecation. The taenia coli run the length of the large intestine. Because the taenia coli are shorter than the large bowel itself, the colon becomes sacculated , forming the haustra of the colon which are the shelf-like intraluminal projections.
Arterial supply to the colon comes from branches of the superior mesenteric artery SMA and inferior mesenteric artery IMA. Flow between these two systems communicates via a "marginal artery" that runs parallel to the colon for its entire length. Historically, it has been believed that the arc of Riolan, or the meandering mesenteric artery of Moskowitz , is a variable vessel connecting the proximal SMA to the proximal IMA that can be extremely important if either vessel is occluded.
However, recent studies conducted with improved imaging technology have questioned the actual existence of this vessel, with some experts calling for the abolition of the terms from future medical literature. Venous drainage usually mirrors colonic arterial supply, with the inferior mesenteric vein draining into the splenic vein , and the superior mesenteric vein joining the splenic vein to form the hepatic portal vein that then enters the liver.
Lymphatic drainage from the ascending colon and proximal two-thirds of the transverse colon is to the colic lymph nodes and the superior mesenteric lymph nodes , which drain into the cisterna chyli.
One variation on the normal anatomy of the colon occurs when extra loops form, resulting in a colon that is up to five metres longer than normal. This condition, referred to as redundant colon , typically has no direct major health consequences, though rarely volvulus occurs, resulting in obstruction and requiring immediate medical attention. The wall of the large intestine is lined with simple columnar epithelium with invaginations.
The invaginations are called the intestinal glands or colonic crypts. The colon crypts are shaped like microscopic thick walled test tubes with a central hole down the length of the tube the crypt lumen. Four tissue sections are shown here, two cut across the long axes of the crypts and two cut parallel to the long axes. In these images the cells have been stained by immunohistochemistry to show a brown-orange color if the cells produce a mitochondrial protein called cytochrome c oxidase subunit I CCOI.
The nuclei of the cells located at the outer edges of the cells lining the walls of the crypts are stained blue-gray with haematoxylin. As seen in panels C and D, crypts are about 75 to about cells long. Thus, by the images shown here, there are an average of about 1, to cells per colonic crypt.
Cells are produced at the crypt base and migrate upward along the crypt axis before being shed into the colonic lumen days later. As estimated from the image in panel A, there are about colonic crypts per square millimeter of the colonic epithelium.
In the four tissue sections shown here, many of the intestinal glands have cells with a mitochondrial DNA mutation in the CCOI gene and appear mostly white, with their main color being the blue-gray staining of the nuclei.
Crypts of the colon can reproduce by fission, as seen in panel C, where a crypt is fissioning to form two crypts, and in panel B where at least one crypt appears to be fissioning. About of the many thousands of protein coding genes expressed in the large intestine, some are specific to the mucous membrane in different regions and include CEACAM7. The large intestine absorbs water and any remaining absorbable nutrients from the food before sending the indigestible matter to the rectum.
The colon absorbs vitamins that are created by the colonic bacteria, such as vitamin K especially important as the daily ingestion of vitamin K is not normally enough to maintain adequate blood coagulation , thiamine and riboflavin. Chloride secretion increases in cystic fibrosis. Recycling of various nutrients takes place in colon. Examples include fermentation of carbohydrates, short chain fatty acids, and urea cycling. The appendix is attached to the inferior surface of the cecum, and contains a small amount of mucosa-associated lymphoid tissue which gives the appendix an undetermined role in immunity.
However, the appendix is known to be important in fetal life as it contains endocrine cells that release biogenic amines and peptide hormones important for homeostasis during early growth and development.
At this point some electrolytes like sodium , magnesium , and chloride are left as well as indigestible parts of ingested food e. As the chyme moves through the large intestine, most of the remaining water is removed, while the chyme is mixed with mucus and bacteria known as gut flora , and becomes feces.
The ascending colon receives fecal material as a liquid. The muscles of the colon then move the watery waste material forward and slowly absorb all the excess water, causing the stools to gradually solidify as they move along into the descending colon.
The bacteria break down some of the fiber for their own nourishment and create acetate , propionate , and butyrate as waste products, which in turn are used by the cell lining of the colon for nourishment. The large intestine  produces no digestive enzymes — chemical digestion is completed in the small intestine before the chyme reaches the large intestine.
The pH in the colon varies between 5. Water absorption at the colon typically proceeds against a transmucosal osmotic pressure gradient. The standing gradient osmosis is the reabsorption of water against the osmotic gradient in the intestines. Cells occupying the intestinal lining pump sodium ions into the intercellular space, raising the osmolarity of the intercellular fluid. This hypertonic fluid creates an osmotic pressure that drives water into the lateral intercellular spaces by osmosis via tight junctions and adjacent cells, which then in turn moves across the basement membrane and into the capillaries , while more sodium ions are pumped again into the intercellular fluid.
This allows the large intestine to absorb water despite the blood in capillaries being hypotonic compared to the fluid within the intestinal lumen. The large intestine houses over species of bacteria that perform a variety of functions, as well as fungi , protozoa , and archaea. Species diversity varies by geography and diet. This mass of mostly symbiotic microbes has recently been called the latest human organ to be "discovered" or in other words, the "forgotten organ".
The large intestine absorbs some of the products formed by the bacteria inhabiting this region. Undigested polysaccharides fiber are metabolized to short-chain fatty acids by bacteria in the large intestine and absorbed by passive diffusion. The bicarbonate that the large intestine secretes helps to neutralize the increased acidity resulting from the formation of these fatty acids.
These bacteria also produce large amounts of vitamins , especially vitamin K and biotin a B vitamin , for absorption into the blood. Although this source of vitamins, in general, provides only a small part of the daily requirement, it makes a significant contribution when dietary vitamin intake is low. An individual who depends on absorption of vitamins formed by bacteria in the large intestine may become vitamin-deficient if treated with antibiotics that inhibit the vitamin producing species of bacteria as well as the intended disease-causing bacteria.
Other bacterial products include gas flatus , which is a mixture of nitrogen and carbon dioxide , with small amounts of the gases hydrogen , methane , and hydrogen sulfide. Bacterial fermentation of undigested polysaccharides produces these. Some of the fecal odor is due to indoles , metabolized from the amino acid tryptophan. The normal flora is also essential in the development of certain tissues, including the cecum and lymphatics.