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The germ theory of disease, also called the pathogenic theory of medicine, is a theory that proposes that microorganisms are the cause of many diseases. Although highly controversial when first proposed, it is now a cornerstone of modern medicine and clinical microbiology, leading to such important innovations as antibiotics and hygienic practices.
The ancient historical view was that disease was spontaneously generated instead of being created by microorganisms which grow by reproduction. The Atharvaveda, a sacred text ofHinduism, is one of the earliest ancient texts dealing with medicine. It identifies the causes of disease as living causative agents such as the yatudhÄ�nya, the kimÄ«di, the ká¹›imi and the durá¹‡ama. The atharvÄ�ns seek to kill them with a variety of drugs in order to counter the disease (see XIX.34.9). One of the earliest Western references to this latter theory appears in On Agriculture by Marcus Terentius Varro (published in 36 BC), wherein there is a warning about locating a homestead in the proximity of swamps:
In The Canon of Medicine(1025),AbÅ« AlÄ« ibn SÄ«nÄ� (Avicenna) stated that bodily secretion is contaminated by foul foreign earthly bodies before being infected. When the Black Deathbubonic plague reached al-Andalus in the 14th century, Ibn Khatima hypothesized that infectious diseases are caused by "minute bodies" which enter the human body and cause disease. Another 14th century Andalusian physician, Ibn al-Khatib, wrote a treatise called On the Plague, in which he stated:
"The existence of contagion is established by experience, investigation, the evidence of the senses and trustworthy reports. These facts constitute a sound argument. The fact of infection becomes clear to the investigator who notices how he who establishes contact with the afflicted gets the disease, whereas he who is not in contact remains safe, and how transmission is affected through garments, vessels and earrings."
Girolamo Fracastoro proposed in 1546 that epidemic diseases are caused by transferable seed-like entities that could transmit infection by direct or indirect contact or even without contact over long distances. The Italian Agostino Bassi is often credited with having stated the germ theory of disease for the first time, based on his observations on the lethal and epidemic muscardine disease of silkworms. In 1835 he specifically blamed the deaths of the insects on a contagious, living agent, that was visible to the naked eye as powdery spore masses; this microscopic fungus was subsequently called Beauveria bassianain his honor.
Microorganisms were first directly observed by Anton van Leeuwenhoek, who is considered the father of microbiology. Building on Leeuwenhoek's work, physician Nicolas Andry argued in 1700 that microorganisms he called "worms" were responsible for smallpox and other diseases. Ignaz Semmelweis was a Hungarianobstetrician working at Vienna's Allgemeines Krankenhaus in 1847, when he noticed the dramatically high incidence of death from puerperal fever among women who delivered at the hospital with the help of the doctors and medical students. Births attended by the midwives were relatively safe. Investigating further, Semmelweis made the connection between puerperal fever and examinations of delivering women by doctors, and further realized that these physicians had usually come directly from autopsies. Asserting that puerperal fever was a contagious disease and that matter from autopsies were implicated in its development, Semmelweis made doctors wash their hands with chlorinated lime water before examining pregnant women, thereby reducing mortality from childbirth to less than 2% at his hospital. Nevertheless, he and his theories were viciously attacked by most of the Viennese medical establishment.
John Snow contributed to the formation of the germ theory when he traced the source of the 1854 cholera outbreak in Soho, London. The statistical analysis of the affected cases showed that the outbreak was not consistent with the miasma theory which was prevalent at the time. Contrary to the miasma model, he identified drinking water as the vessel for transmission of the disease. He found that cases occurred in the homes which obtained their water from the Broad Street pump, which was at the geographical center of the outbreak.
Italian physician Francesco Redi provided early evidence against spontaneous generation. He devised an experiment in 1668 where he used three jars. He placed a meat loaf in each of the three jars. He had one of the jars open, another one tightly sealed, and the last one covered with gauze. After a few d
The color opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner. The three types of cones (L for long, M for medium and S for short) have some overlap in the wavelengths of light to which they respond, so it is more efficient for the visual system to record differences between the responses of cones, rather than each type of cone's individual response. The opponent color theory suggests that there are three opponent channels: red versus green, blue versus yellow, and black versus white (the latter type is achromatic and detects light-dark variation, or luminance). Responses to one color of an opponent channel are antagonistic to those to the other color. That is, since one color produces an excitatory effect and the other produces an inhibitory effect, the opponent colors are never perceived at the same time (the visual system cannot be simultaneously excited and inhibited).
While the trichromatic theory defines the way the retina of the eye allows the visual system to detect color with three types of cones, the opponent process theory accounts for mechanisms that receive and process information from cones. Though the trichromatic and opponent processes theories were initially thought to be at odds, it later came to be understood that the mechanisms responsible for the opponent process receive signals from the three types of cones and process them at a more complex level.
Besides the cones, which detect light entering the eye, the biological basis of the opponent theory involves two other types of cells: bipolar cells, and ganglion cells. Information from the cones is passed to the bipolar cells in the retina, which may be the cells in the opponent process that transform the information from cones. The information is then passed to ganglion cells, of which there are two major classes: magnocellular, or large-cell layers, and parvocellular, or small-cell layers. Parvocellular cells, or P cells, handle the majority of information about color, and fall into two groups: one that processes information about differences between firing of L and M cones, and one that processes differences between S cones and a combined signal from both L and M cones. The first subtype of cells are responsible for processing red-green differences,and the second process blue-yellow differences. P cells also transmit information about intensity of light (how much of it there is) due to their receptive fields.
Johann Wolfgang von Goethe first studied the physiological effect of opposed colors in his Theory of Coloursin 1810. Goethe arranged his color wheel symmetrically, "for the colours diametrically opposed to each other in this diagram are those which reciprocally evoke each other in the eye. Thus, yellow demands purple; orange, blue; red, green; and vice versa: thus again all intermediate gradations reciprocally evoke each other."
Ewald Hering proposed opponent color theory in 1892. He thought that the colors red, yellow, green, and blue are special in that any other color can be described as a mix of them, and that they exist in opposite pairs. That is, either red or green is perceived and never greenish-red; although yellow is a mixture of red and green in the RGB color theory, the eye does not perceive it as such.
In 1957, Hurvich and Jameson provided quantitative data for Hering's color opponency theory. Their method was called "hue cancellation". Hue cancellation experiments start with a color (e.g. yellow) and attempt to determine how much of the opponent color (e.g. blue) of one of the starting color's components must be added to eliminate any hint of that component from the starting color (Wolfe, Kluender, & Levi, 2009).
Griggs expanded the concept to reflect a wide range of opponent processes for biological systems in this book Biological Relativity (c) 1967.
In 1970, Solomon expanded Hurvich's general neurological opponent process model to explain emotion, drug addiction, and work motivation.
Subjective color and new colors
Reddish green and yellowish blue
Under normal circumstances, there is no hue one could describe as a mixture of opponent hues; that is, as a hue looking "redgreen" or "yellowblue". However, in 1983 Crane and Piantanida carried out an experiment under special viewing conditions in which red and green stripes (or blue and yellow stripes) were placed adjacent to each other and the image held in the same position relative to the viewer's eyes (using an eye tracker to compensate for minor muscle movements). Under such conditions, the borders between the stripes seem to disappear and the colors flowed into each other, making it apparently possible to override the opponency mechanisms and, for a moment, get some people to perceive novel colors. :
- "[s]ome observers indicated that although they were aware that what they were viewing was a color (that is, the field was not achromatic), they were unable to name or describe the color. One of these observers was an artist with a large color vocabulary. Other observers of the novel hues described the first stimulus as a reddish-green."
However, some subjects in the Crane and Piantanida study merely reported seeing hallucinatory textures, such as blue specks on a yellow backdrop. A possible explanation is that the study did not control for variations in the perceived luminance of the colors from subject to subject (two colors are equiluminant for an observer when rapidly alternating between the colors produces the least impression of flickering). To investigate thi
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision (adjectival form: visual, optical, or ocular). The various physiological components involved in vision are referred to collectively as the visual system, and are the focus of much research in psychology, cognitive science, neuroscience, and molecular biology.
The visual system in humans allows individuals to assimilate information from the environment. The act of seeing starts when the lens of the eye focuses an image of its surroundings onto a light-sensitive membrane in the back of the eye, called the retina. The retina is actually part of the brain that is isolated to serve as a transducer for the conversion of patterns of light into neuronal signals. The lens of the eye focuses light on the photoreceptive cells of the retina, which detect the photons of light and respond by producing neural impulses. These signals are processed in a hierarchical fashion by different parts of the brain, from the retina upstream to central ganglia ie the brain.
Note that up til now the above paragraph could apply to octopi, molluscs, worms, insects and things more primitive; anything with a more concentrated nervous system and better eyes than say a jellyfish. However, the following applies to mammals generally and birds (in modified form): The retina in these more complex animals sends fibers (the optic nerve) to the lateral geniculate nucleus, to the primary and secondary visual cortex of the brain. Signals from the retina can also travel directly from the retina to the Superior colliculus.
Study of visual perception
The major problem in visual perception is that what people see is not simply a translation of retinal stimuli (i.e., the image on the retina). Thus people interested in perception have long struggled to explain what visual processing does to create what we actually see.
Early studies on visual perception
There were two major ancient Greek schools, providing a primitive explanation of how vision is carried out in the body.
The first was the "emission theory" which maintained that vision occurs when rays emanate from the eyes and are intercepted by visual objects. If we saw an object directly it was by 'means of rays' coming out of the eyes and again falling on the object. A refracted image was, however, seen by 'means of rays' as well, which came out of the eyes, traversed through the air, and after refraction, fell on the visible object which was sighted as the result of the movement of the rays from the eye. This theory was championed by scholars like Euclid and Ptolemy and their followers.
The second school advocated the so called 'intro-mission' approach which sees vision as coming from something entering the eyes representative of the object. With its main propagators Aristotle, Galen and their followers, this theory seems to have some contact with modern theories of what vision really is, but it remained only a speculation lacking any experimental foundation.
Both schools of thought relied upon the principle that "like is only known by like," and thus upon the notion that the eye was composed of some "internal fire" which interacted with the "external fire" of visible light and made vision possible. Plato makes this assertion in his dialogue, Timaeus; as does Aristotle, in his De Sensu.
Leonardo DaVinci (1452â€“1519) was the first to recognize the special optical qualities of the eye. He wrote "The function of the human eye ... was described by a large number of authors in a certain way. But I found it to be completely different." His main experimental finding was that there is only a distinct and clear vision at the line of sight, the optical line that ends at the fovea. Although he did not use these words literally he actually is the father of the modern distinction between foveal and peripheral vision.
Hermann von Helmholtz is often credited with the first study of visual perception in modern times. Helmholtz examined the human eye and concluded that it was, optically, rather poor. The poor quality information gathered via the eye seemed to him
Alexander Graham Bell was born on March 3, 1847, in Edinburgh. His father, Alexander Melville Bell, was an expert in vocal physiology and elocution; his grandfather, Alexander Bell, was an elocution professor. After studying at the University of Edinburgh and University College, London, Bell became his father's assistant. He taught the deaf to talk by adopting his father's system of visible speech (illustrations of speaking positions of the lips and tongue). In London he studied Hermann Ludwig von Helmholtz's experiments with tuning forks and magnets to produce complex sounds. In 1865 Bell made scientific studies of the resonance of the mouth while speaking. In 1870 the Bells moved to Brantford, Ontario, Canada, to preserve Alexander's health. He went to Boston in 1871 to teach at Sarah Fuller's School for the Deaf, the first such school in the world. He also tutored private students, including Helen Keller. As professor of vocal physiology and speech at Boston University in 1873, he initiated conventions for teachers of the deaf. Throughout his life he continued to educate the deaf, and he founded the American Association to Promote the Teaching of Speech to the Deaf. From 1873 to 1876 Bell experimented with a phonautograph, a multiple telegraph, and an electric speaking telegraph (the telephone). Funds came from the fathers of two of his pupils; one of these men, Gardiner Hubbard, had a deaf daughter, Mabel, who later became Bell's wife. To help deaf children, Bell experimented in the summer of 1874 with a human ear and attached bones, a tympanum, magnets, and smoked glass. He conceived the theory of the telephone: an electric current can be made to change intensity precisely as air density varies during sound production. Unlike the telegraph's use of intermittent current, the telephone requires continuous current with varying intensity. That same year he invented a harmonic telegraph, to transmit several messages simultaneously over one wire, and a telephonic-telegraphic receiver. Trying to reproduce the human voice electrically, he became expert with electric wave transmission. Bell supplied the ideas; Thomas Watson made and assembled the equipment. Working with tuned reeds and magnets to synchronize a receiving instrument with a sender, they transmitted a musical note on June 2, 1875. Bell's telephone receiver and transmitter were identical: a thin disk in front of an electromagnet. On Feb. 14, 1876, Bell's attorney filed for a patent. The exact hour was not recorded, but on that same day Elisha Gray filed his caveat (intention to invent) for a telephone. The U.S. Patent Office granted Bell the patent for the "electric speaking telephone" on March 7. It was the most valuable single patent ever issued, and it opened a new age in communication technology. Bell continued his experiments to improve the telephone's quality. By accident, Bell sent the first sentence, "Watson, come here; I want you, " on March 10, 1876. The first demonstration occurred at the American Academy of Arts and Sciences convention in Boston 2 months later. Bell's display at the Philadelphia Centennial Exposition a month later gained more publicity, and Emperor Dom Pedro of Brazil ordered 100 telephones for his country. The telephone, accorded only 18 words in the official catalog of the exposition, suddenly became the "star" attraction. Repeated demonstrations overcame public skepticism. The first reciprocal outdoor conversation was between Boston and Cambridge, Mass., by Bell and Watson on Oct. 9, 1876. In 1877 the first telephone was installed in a private home; a conversation was conducted between Boston and New York, using telegraph lines; in May, the first switchboard, devised by E. T. Holmes in Boston, was a burglar alarm connecting five banks; and in July the first organization to commercialize the invention, the Bell Telephone Company, was formed. That year, while on his honeymoon, Bell introduced the telephone to England and France. The first commercial switchboard was set up in New Haven, Conn., in 1878, and Bell's first subsidiary, the New England Telephone Company, was organized that year. Switchboards were improved by Charles Scribner, with more than 500 inventions. Thomas Cornish, a Philadelphia electrician, had a switchboard for eight customers and published a one-page directory in 1878. Other inventors had been at work. Between 1867 and 1873 Professor Elisha Gray (of Oberlin College) invented an "automatic self-adjusting telegraph relay, " installed it in hotels, and made telegraph printers and repeaters. He tried to perfect a speaking telephone from his harmonic (multiple-current) telegraph. The Gray and Batton Manufacturing Company of Chicago developed into the Western Electric Company. Another competitor was Professor Amos E. Dolbear, who insisted that Bell's telephone was only an improvement on an 1860 invention by Johann Reis, a German, who had experimented with pigs' ear membranes and may have made a telephone. Dolbear's own instrument, operating by "make and break" current, could transmit pitch but not voice quality. In 1879 Western Union, with its American Speaking Telephone Company, ignored Bell's patents and hired Thomas Edison, along with Dolbear and Gray, as inventors and improvers. Later that year Bell and Western Union formed a joint company, with the latter getting 20 percent for providing wires, circuits, and equipment. Theodore Vail, organizer of Bell Telephone Company, consolidated six companies in 1881. The modern transmitter evolved mainly from the work of Emile Berliner and Edison in 1877 and Francis Blake in 1878. Blake's transmitter was later sold to Bell for stock. The claims of other inventors were contested. Daniel Drawbaugh, from rural Pennsylvania, with little formal schooling, almost won a legal battle with Bell in 1884 but was defeated by a 4 to 3 vote in the Supreme Court. The claim by this "Edison of the Cumberland Valley" was the most exciting (and futile) litigation over telephone patents. Altogether, the Bell Company was involved in 587 lawsuits, of which 5 went to the Supreme Court; Bell won every case. A convincing argument was that no competitor claimed originality until 17 months after Bell's patent. Also, at the 1876 Philadelphia Exposition, eminent electrical scientists, especially Lord Kelvin, the world's foremost authority, had declared it to be "new." Professors, scientists, and researchers defended Bell, pointing to his lifelong study of the ear and his books and lectures on speech mechanics. The Bell Company built the first long-distance line in 1884, connecting Boston and New York. The American Telephone and Telegraph Company was organized by Bell and others in 1885 to operate other long-distance lines. By 1889, when insulation was perfected, there were 11, 000 miles of underground wires in New York City. The Volta Laboratory was started by Bell in Washington, D.C., with the Volta Prize money (50, 000 francs, about $10, 000) awarded by France for his invention. At the laboratory he and associates worked on various projects during the 1880s, including the photophone, induction balance, audiometer, and phonograph improvements. The photophone transmitted speech by light, using a primitive photoelectric cell. The induction balance (electric probe) located metal in the body. The audiometer indicated Bell's continued interest in deafness. The first successful phonograph record, a shellac cylinder, as well as wax disks and cylinders, was produced. The Columbia Gramophone Company exploited Bell's phonograph records. With the profits Bell established the Volta Bureau in Washington to study deafness. Other activities took much time. The magazine Science (later the official organ of the American Association for the Advancement of Science) was founded in 1880 because of Bell's efforts. He made numerous addresses and published many monographs. As National Geographic Society president from 1896 to 1904, he fostered the success of the society and its publications. In 1898 he became a regent of the Smithsonian Institution. He was also involved in sheep breeding, hydrodynamics, and aviation projects. Aviation was Bel
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Answers:Robert Hooke - The first person to see cells, he was looking at cork and noted that he saw "a great many boxes. (1665) Anton van Leeuwenhock - Observed living cells in pond water, which he called "animalcules" (1673) Theodore Schwann - zoologist who observed that the tissues of animals had cells (1839) Mattias Schleiden - botonist, observed that the tissues of plants contained cells ( 1845) Rudolf Virchow - also reported that every living thing is made of up vital units, known as cells. He also predicted that cells come from other cells. (1850 ) The Cell Theory 1. Every living organism is made of one or more cellss. 2. The cell is the basic unit of structure and function. It is the smallest unit that can perform life functions. 3. All cells arise from pre-existing cells.
Answers:I'll answer 1 of many because I don't feel like typing such a long list. 13) cancer cells have abnormal cell growth and can metastasize or spread to different tissues.
Answers:1. cells 2. Robert Hooke 3. 4. Anton van Leeuwenhoek 5. Cell Theory 6. Theodor Schwann, Matthias Jakob Schleiden, and Rudolf Virchow. 7. Cytologists 8. All cells come from pre-existing cells by division. (exception: the first cell) 8. All known living things are made up of cells. 8. The cell is the fundamental unit of structure and function in living things. a. protection against foreign elements, regulated transfer of materials and waste b. it would die out since all of the "instructions" to develop proteins are all found in the nucleus 1. nucleus and its DNA 2. protection, regulation, transport 3. prokaryotic = nucleoid region 3. eukaryotic = true nucleus 4. biomolecules