Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, the processes by which they change over time. Geology can include the study of the solid features of any terrestrial planet or natural satellite such as Mars or the Moon. Modern geology overlaps all other earth sciences, including hydrology and the atmospheric sciences, so is treated as one major aspect of integrated earth system science and planetary science. Geology describes the structure of the Earth on and beneath its surface, the processes that have shaped that structure, it provides tools to determine the relative and absolute ages of rocks found in a given location, to describe the histories of those rocks. By combining these tools, geologists are able to chronicle the geological history of the Earth as a whole, to demonstrate the age of the Earth. Geology provides the primary evidence for plate tectonics, the evolutionary history of life, the Earth's past climates. Geologists use a wide variety of methods to understand the Earth's structure and evolution, including field work, rock description, geophysical techniques, chemical analysis, physical experiments, numerical modelling.
In practical terms, geology is important for mineral and hydrocarbon exploration and exploitation, evaluating water resources, understanding of natural hazards, the remediation of environmental problems, providing insights into past climate change. Geology is a major academic discipline, it plays an important role in geotechnical engineering; the majority of geological data comes from research on solid Earth materials. These fall into one of two categories: rock and unlithified material; the majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three major types of rock: igneous and metamorphic; the rock cycle illustrates the relationships among them. When a rock solidifies or crystallizes from melt, it is an igneous rock; this rock can be weathered and eroded redeposited and lithified into a sedimentary rock. It can be turned into a metamorphic rock by heat and pressure that change its mineral content, resulting in a characteristic fabric.
All three types may melt again, when this happens, new magma is formed, from which an igneous rock may once more solidify. To study all three types of rock, geologists evaluate the minerals; each mineral has distinct physical properties, there are many tests to determine each of them. The specimens can be tested for: Luster: Measurement of the amount of light reflected from the surface. Luster is broken into nonmetallic. Color: Minerals are grouped by their color. Diagnostic but impurities can change a mineral’s color. Streak: Performed by scratching the sample on a porcelain plate; the color of the streak can help name the mineral. Hardness: The resistance of a mineral to scratch. Breakage pattern: A mineral can either show fracture or cleavage, the former being breakage of uneven surfaces and the latter a breakage along spaced parallel planes. Specific gravity: the weight of a specific volume of a mineral. Effervescence: Involves dripping hydrochloric acid on the mineral to test for fizzing. Magnetism: Involves using a magnet to test for magnetism.
Taste: Minerals can have a distinctive taste, like halite. Smell: Minerals can have a distinctive odor. For example, sulfur smells like rotten eggs. Geologists study unlithified materials, which come from more recent deposits; these materials are superficial deposits. This study is known as Quaternary geology, after the Quaternary period of geologic history. However, unlithified material does not only include sediments. Magmas and lavas are the original unlithified source of all igneous rocks; the active flow of molten rock is studied in volcanology, igneous petrology aims to determine the history of igneous rocks from their final crystallization to their original molten source. In the 1960s, it was discovered that the Earth's lithosphere, which includes the crust and rigid uppermost portion of the upper mantle, is separated into tectonic plates that move across the plastically deforming, upper mantle, called the asthenosphere; this theory is supported by several types of observations, including seafloor spreading and the global distribution of mountain terrain and seismicity.
There is an intimate coupling between the movement of the plates on the surface and the convection of the mantle. Thus, oceanic plates and the adjoining mantle convection currents always move in the same direction – because the oceanic lithosphere is the rigid upper thermal boundary layer of the convecting mantle; this coupling between rigid plates moving on the surface of the Earth and the convecting mantle is called plate tectonics. The development of plate tectonics has provided a physical basis for many observations of the solid Earth. Long linear regions of geologic features are explained as plate boundaries. For example: Mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, are seen as divergent boundaries, where two plates move apart. Arcs of volcanoes and earthquakes are theorized as convergent boundaries, where one plate subducts, or moves, under another. Transform boundaries, such as the San Andreas Fault system, resulted in widespread powerful earthquakes.
Plate tectonics has provided a mechan
The cerebral arteries describe three main pairs of arteries and their branches, which perfuse the cerebrum of the brain. The three main arteries are the: Anterior cerebral artery Middle cerebral artery Posterior cerebral artery Both the ACA and MCA originate from the cerebral portion of internal carotid artery, while PCA branches from the intersection of the posterior communicating artery and the anterior portion of the basilar artery; the three pairs of arteries are linked via the anterior communicating artery and the posterior communicating arteries. All three arteries send out arteries that perforate brain in the medial central portions prior to branching and bifurcating further; the arteries are divided into different segments from 1–4 or 5 to denote how far the level of the branch with the lower numbers denoting vessels closer to the source artery. Though the arteries branching off these vessels retain some aspect of constancy in terms of size and position, a great amount of variety in topography, position and prominence exists
Circle of Willis
The circle of Willis is a circulatory anastomosis that supplies blood to the brain and surrounding structures. It is named after an English physician; the circle of Willis is a part of the cerebral circulation and is composed of the following arteries: Anterior cerebral artery Anterior communicating artery Internal carotid artery Posterior cerebral artery Posterior communicating artery The middle cerebral arteries, supplying the brain, are not considered part of the circle. The left and right internal carotid arteries arise from the left and right common carotid arteries; the posterior communicating artery is given off as a branch of the internal carotid artery just before it divides into its terminal branches - the anterior and middle cerebral arteries. The anterior cerebral artery forms the anterolateral portion of the circle of Willis, while the middle cerebral artery does not contribute to the circle; the right and left posterior cerebral arteries arise from the basilar artery, formed by the left and right vertebral arteries.
The vertebral arteries arise from the subclavian arteries. The anterior communicating artery connects the two anterior cerebral arteries and could be said to arise from either the left or right side. All arteries involved give off central branches; the central branches supply the interior of the circle of Willis, more the Interpeduncular fossa. The cortical branches are named for the area. Since they do not directly affect the circle of Willis, they are not dealt with here. Considerable anatomic variation exists in the circle of Willis. Based on a study of 1413 brains, the classic anatomy of the circle is only seen in 34.5% of cases. In one common variation the proximal part of the posterior cerebral artery is narrow and its ipsilateral posterior communicating artery is large, so the internal carotid artery supplies the posterior cerebrum. In another variation the anterior communicating artery is a large vessel, such that a single internal carotid supplies both anterior cerebral arteries; the arrangement of the brain's arteries into the circle of Willis creates redundancy for collateral circulation in the cerebral circulation.
If one part of the circle becomes blocked or narrowed or one of the arteries supplying the circle is blocked or narrowed, blood flow from the other blood vessels can preserve the cerebral perfusion well enough to avoid the symptoms of ischemia. The redundancies that the circle of Willis introduce can lead to reduced cerebral perfusion. In subclavian steal syndrome, blood is "stolen" from the circle of Willis to preserve blood flow to the upper limb. Subclavian steal syndrome results from a proximal stenosis of the subclavian artery, an artery supplied by the aorta, the same blood vessel that feeds the circle of Willis via the vertebral artery. Cerebral circulation Leptomeningeal collateral circulation
Superior epigastric artery
In human anatomy, superior epigastric artery refers to a blood vessel that carries oxygenated blood and arises from the internal thoracic artery. It anastomoses with the inferior epigastric artery at the umbilicus and supplies the anterior part of the abdominal wall and some of the diaphragm. Along its course, it is accompanied by a named vein, the superior epigastric vein; the superior epigastric arteries, inferior epigastric arteries, internal thoracic arteries and left subclavian artery and right subclavian artery / brachiocephalic are collateral vessels to the thoracic aorta and abdominal aorta. If the abdominal aorta develops a significant stenosis and/or blockage, this collateral pathway may develop sufficiently, over time, to supply blood to the lower limbs. A congenitally narrowed aorta, due to coarctation, is associated with a significant enlargement of the internal thoracic and epigastric arteries. Terms for anatomical location Anatomy photo:18:07-0103 at the SUNY Downstate Medical Center - "Thoracic wall: Branches of the Internal Thoracic Artery" Anatomy figure: 35:04-03 at Human Anatomy Online, SUNY Downstate Medical Center - "Incisions and the contents of the rectus sheath.
" Gray's s156 - External iliac artery
A leaf is an organ of a vascular plant and is the principal lateral appendage of the stem. The leaves and stem together form the shoot. Leaves are collectively referred to as foliage, as in "autumn foliage". A leaf is a thin, dorsiventrally flattened organ borne above ground and specialized for photosynthesis. In most leaves, the primary photosynthetic tissue, the palisade mesophyll, is located on the upper side of the blade or lamina of the leaf but in some species, including the mature foliage of Eucalyptus, palisade mesophyll is present on both sides and the leaves are said to be isobilateral. Most leaves have distinct upper surface and lower surface that differ in colour, the number of stomata, the amount and structure of epicuticular wax and other features. Leaves can have many different shapes and textures; the broad, flat leaves with complex venation of flowering plants are known as megaphylls and the species that bear them, the majority, as broad-leaved or megaphyllous plants. In the clubmosses, with different evolutionary origins, the leaves are simple and are known as microphylls.
Some leaves, such as bulb scales, are not above ground. In many aquatic species the leaves are submerged in water. Succulent plants have thick juicy leaves, but some leaves are without major photosynthetic function and may be dead at maturity, as in some cataphylls and spines. Furthermore, several kinds of leaf-like structures found in vascular plants are not homologous with them. Examples include flattened plant stems called phylloclades and cladodes, flattened leaf stems called phyllodes which differ from leaves both in their structure and origin; some structures of non-vascular plants function much like leaves. Examples include the phyllids of liverworts. Leaves are the most important organs of most vascular plants. Green plants are autotrophic, meaning that they do not obtain food from other living things but instead create their own food by photosynthesis, they capture the energy in sunlight and use it to make simple sugars, such as glucose and sucrose, from carbon dioxide and water. The sugars are stored as starch, further processed by chemical synthesis into more complex organic molecules such as proteins or cellulose, the basic structural material in plant cell walls, or metabolised by cellular respiration to provide chemical energy to run cellular processes.
The leaves draw water from the ground in the transpiration stream through a vascular conducting system known as xylem and obtain carbon dioxide from the atmosphere by diffusion through openings called stomata in the outer covering layer of the leaf, while leaves are orientated to maximise their exposure to sunlight. Once sugar has been synthesized, it needs to be transported to areas of active growth such as the plant shoots and roots. Vascular plants transport sucrose in a special tissue called the phloem; the phloem and xylem are parallel to each other but the transport of materials is in opposite directions. Within the leaf these vascular systems branch to form veins which supply as much of the leaf as possible, ensuring that cells carrying out photosynthesis are close to the transportation system. Leaves are broad and thin, thereby maximising the surface area directly exposed to light and enabling the light to penetrate the tissues and reach the chloroplasts, thus promoting photosynthesis.
They are arranged on the plant so as to expose their surfaces to light as efficiently as possible without shading each other, but there are many exceptions and complications. For instance plants adapted to windy conditions may have pendent leaves, such as in many willows and eucalyptss; the flat, or laminar, shape maximises thermal contact with the surrounding air, promoting cooling. Functionally, in addition to carrying out photosynthesis, the leaf is the principal site of transpiration, providing the energy required to draw the transpiration stream up from the roots, guttation. Many gymnosperms have thin needle-like or scale-like leaves that can be advantageous in cold climates with frequent snow and frost; these are interpreted as reduced from megaphyllous leaves of their Devonian ancestors. Some leaf forms are adapted to modulate the amount of light they absorb to avoid or mitigate excessive heat, ultraviolet damage, or desiccation, or to sacrifice light-absorption efficiency in favour of protection from herbivory.
For xerophytes the major constraint drought. Some window plants such as Fenestraria species and some Haworthia species such as Haworthia tesselata and Haworthia truncata are examples of xerophytes. and Bulbine mesembryanthemoides. Leaves function to store chemical energy and water and may become specialised organs serving other functions, such as tendrils of peas and other legumes, the protective spines of cacti and the insect traps in carnivorous plants such as Nepenthes and Sarracenia. Leaves are the fundamental structural units from which cones are constructed in gymnosperms and from which flowers are constructed in flowering plants; the internal organisation of most kinds of leaves has evolved to maximise exposure of the photosynthetic organelles, the chloroplasts, to light and to increase the absorption of carbon dioxide while at the same time controlling water loss. Their surfaces are waterproofed by the plant cuticle and gas exchange between the mesophyll cells and the atmosphere is controlled by minute openings called stomata which open or close to regulate the rate exchange of carbon dioxide and water vapour into
In cell biology, the nucleus is a membrane-bound organelle found in eukaryotic cells. Eukaryotes have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, a few others including osteoclasts have many; the cell nucleus contains all of the cell's genome, except for a small fraction of mitochondrial DNA, organized as multiple long linear DNA molecules in a complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these chromosomes are structured in such a way to promote cell function; the nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression—the nucleus is, the control center of the cell. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm, the nuclear matrix, a network within the nucleus that adds mechanical support, much like the cytoskeleton, which supports the cell as a whole.
Because the nuclear envelope is impermeable to large molecules, nuclear pores are required to regulate nuclear transport of molecules across the envelope. The pores cross both nuclear membranes, providing a channel through which larger molecules must be transported by carrier proteins while allowing free movement of small molecules and ions. Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes. Although the interior of the nucleus does not contain any membrane-bound subcompartments, its contents are not uniform, a number of nuclear bodies exist, made up of unique proteins, RNA molecules, particular parts of the chromosomes; the best-known of these is the nucleolus, involved in the assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA; the nucleus was the first organelle to be discovered. What is most the oldest preserved drawing dates back to the early microscopist Antonie van Leeuwenhoek.
He observed the nucleus, in the red blood cells of salmon. Unlike mammalian red blood cells, those of other vertebrates still contain nuclei; the nucleus was described by Franz Bauer in 1804 and in more detail in 1831 by Scottish botanist Robert Brown in a talk at the Linnean Society of London. Brown was studying orchids under the microscope when he observed an opaque area, which he called the "areola" or "nucleus", in the cells of the flower's outer layer, he did not suggest a potential function. In 1838, Matthias Schleiden proposed that the nucleus plays a role in generating cells, thus he introduced the name "cytoblast", he believed that he had observed new cells assembling around "cytoblasts". Franz Meyen was a strong opponent of this view, having described cells multiplying by division and believing that many cells would have no nuclei; the idea that cells can be generated de novo, by the "cytoblast" or otherwise, contradicted work by Robert Remak and Rudolf Virchow who decisively propagated the new paradigm that cells are generated by cells.
The function of the nucleus remained unclear. Between 1877 and 1878, Oscar Hertwig published several studies on the fertilization of sea urchin eggs, showing that the nucleus of the sperm enters the oocyte and fuses with its nucleus; this was the first time. This was in contradiction to Ernst Haeckel's theory that the complete phylogeny of a species would be repeated during embryonic development, including generation of the first nucleated cell from a "monerula", a structureless mass of primordial mucus. Therefore, the necessity of the sperm nucleus for fertilization was discussed for quite some time. However, Hertwig confirmed his observation in other animal groups, including amphibians and molluscs. Eduard Strasburger produced the same results for plants in 1884; this paved the way to assign the nucleus an important role in heredity. In 1873, August Weismann postulated the equivalence of the maternal and paternal germ cells for heredity; the function of the nucleus as carrier of genetic information became clear only after mitosis was discovered and the Mendelian rules were rediscovered at the beginning of the 20th century.
The nucleus is the largest organelle in animal cells. In mammalian cells, the average diameter of the nucleus is 6 micrometres, which occupies about 10% of the total cell volume; the contents of the nucleus are held in the nucleoplasm similar to the cytoplasm in the rest of the cell. The fluid component of this is termed the nucleosol, similar to the cytosol in the cytoplasm. In most types of granulocyte, a white blood cell, the nucleus is lobated and can be bi-lobed, tri-lobed or multi-lobed; the nuclear envelope, otherwise known as nuclear membrane, consists of two cellular membranes, an inner and an outer membrane, arranged parallel to one another and separated by 10 to 50 nanometres. The nuclear envelope encloses the nucleus and separates the cell's genetic material from the surrounding cytoplasm, serving as a barrier to prevent macromolecules from diffusing between the nucleoplasm and the cytoplasm; the outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum, is studded with ribosomes.
The space between the membranes is called the perinuclear space and is continuous with the RER lumen. Nuclear pores, which provide aqueous cha
In biology, a septum is a wall, dividing a cavity or structure into smaller ones. Interatrial septum, the wall of tissue, a sectional part of the left and right atria of the heart Interventricular septum, the wall separating the left and right ventricles of the heart Lingual septum, a vertical layer of fibrous tissue that separates the halves of the tongue Nasal septum: the cartilage wall separating the nostrils of the nose Alveolar septum: the thin wall which separates the alveoli from each other in the lungs Orbital septum, a palpabral ligament in the upper and lower eyelids Septum pellucidum or septum lucidum, a thin structure separating two fluid pockets in the brain Uterine septum, a malformation of the uterus Vaginal septum, a lateral or transverse partition inside the vagina Intermuscular septa separating the muscles of the arms and legsHistological septa are seen throughout most tissues of the body where they are needed to stiffen soft cellular tissue, they provide planes of ingress for small blood vessels.
Because the dense collagen fibres of a septum extend out into the softer adjacent tissues, microscopic fibrous septa are less defined than the macroscopic types of septa listed above. In rare instances, a septum is a cross-wall, thus it divides a structure into smaller parts. The Septum is the boundary formed between dividing cells in the course of cell division. A partition dividing filamentous hyphae into discrete cells in fungi. A partition that separates the locules of a fruit, anther, or sporangium. A coral septum is one of the radial calcareous plates in the corallites of a coral. Annelids have septa. Many shelled organisms have septa subdividing their shell chamber, including rhizopods and gastropods, the latter serving as a defence against shell-boring predators. A rubber septum is an engineered membrane that permits transfer of a substance without contact with air using a syringe with needle