A speculum is a medical tool for investigating body orifices, with a form dependent on the orifice for which it is designed. In old texts, the speculum may be referred to as a diopter or dioptra. Like an endoscope, a speculum allows a view inside the body. Vaginal and anal specula were used by the ancient Greeks and Romans, speculum artifacts have been found in Pompeii. A vaginal speculum, developed by J. Marion Sims, consists of a hollow cylinder with a rounded end, divided into two hinged parts, somewhat like the beak of a duck; the speculum is inserted into the vagina to dilate it for examination of the cervix. Specula were made of stainless steel, sterilized after use. However, many those used in emergency departments and doctor's offices, are now made of plastic, are disposable, single-use items; those used in surgical suites are still made of metal. Specula come in a variety of shapes based on their purpose, a variety of sizes; the best-known speculum is the bivalved vaginal speculum. A cylindrical-shaped speculum, introduced in 2001, the dilating vaginal speculum invented by Clemens van der Weegen, inflates the vagina with filtered air.
The device has two main functions: a) to take a normal Pap smear with a cervical brush or a cytology brush. It has a facility to attach a digital camera for viewing and recording. A specialized form of vaginal speculum is the weighted speculum, which consists of a broad half tube, bent at about a 90 degree angle, with the channel of the tube on the exterior side of the angle. One end of the tube has a spherical metal weight surrounding the channel of the speculum. A weighted speculum is placed in the vagina during vaginal surgery with the patient in the lithotomy position; the weight frees the surgeon's hands for other tasks. A vaginal speculum is used in fertility treatments artificial insemination, allows the vaginal cavity to be opened and observed thereby facilitating the deposit of semen into the vagina. Cylindrical shapeFerguson Glass speculum Veda-scope One blade Two blades Three blades Vaginal specula are used for anal surgery, although several other forms of anal specula exist. One form, the anoscope, resembles a tube.
When the anoscope is inserted into the anus, the insert dilates the anus to the diameter of the tube. The insert is removed, leaving the tube to allow examination of the lower rectum and anus; this style of anal speculum is one of the oldest designs for surgical instruments still in use, with examples dating back many centuries. The sigmoidoscope can be further advanced into the lower intestinal tract and requires an endoscopic set-up. Tubal shapeAniscopeOne bladeCzernyTwo bladesThree bladesAlan Park Cook Mathieu Nasal specula have two flat blades with handle; the instrument is hinged so that when the handles are squeezed together the blades spread laterally, allowing examination. Additionally, the Thudichum nasal speculum is used in the outpatient examination of the nose. Ear or aural specula resemble a funnel, come in a variety of sizes. In veterinary medicine, a McPherson Speculum can be used for oral examination; the speculum helps avoid biting injuries. Speculum is used for Speculum Play under BDSM for spreading the anus.
Endoscope Colposcope Vaginal dilator
In biomechanics, balance is an ability to maintain the line of gravity of a body within the base of support with minimal postural sway. Sway is the horizontal movement of the centre of gravity when a person is standing still. A certain amount of sway is essential and inevitable due to small perturbations within the body or from external triggers. An increase in sway is not an indicator of dysfunctional balance so much as it is an indicator of decreased sensorimotor control. Maintaining balance requires coordination of input from multiple sensory systems including the vestibular and visual systems. Vestibular system: sense organs that regulate equilibrium. There are environmental factors that can affect balance such as light conditions, floor surface changes, alcohol and ear infection. There are balance impairments associated with aging. Age-related decline in the ability of the above systems to receive and integrate sensory information contributes to poor balance in older adults; as a result, the elderly are at an increased risk of falls.
In fact, one in three adults aged 65 and over will fall each year. In the case of an individual standing upright, the limit of stability is defined as the amount of postural sway at which balance is lost and corrective action is required. Body sway can occur in all planes of motion, which make it an difficult ability to rehabilitate. There is strong evidence in research showing that deficits in postural balance is related to the control of medial-lateral stability and an increased risk of falling. To remain balanced, a person standing must be able to keep the vertical projection of their center of mass within their base of support, resulting in little medial-lateral or anterior-posterior sway. Ankle sprains are one of the most occurring injuries among athletes and physically active people; the most common residual disability post ankle sprain is instability along with body sway. Mechanical instability includes insufficient stabilizing structures and mobility that exceed physiological limits. Functional instability involves a feeling of giving way of the ankle.
Nearly 40 % of patients with ankle sprains suffer from an increase in body sway. Injury to the ankle causes impaired postural control. Individuals with muscular weakness, occult instability, decreased postural control are more susceptible to ankle injury than those with better postural control. Balance can be affected in individuals with neurological conditions. People who suffer a stroke or spinal cord injury for example, can struggle with this ability. Impaired balance is associated with future function and recovery after a stroke, is the strongest predictor of falls. Another population where balance is affected is Parkinson's disease patients. A study done by Nardone and Schieppati showed that individuals with Parkinson's disease problems in balance have been related to a reduced limit of stability and an impaired production of anticipatory motor strategies and abnormal calibration. Balance can be negatively affected in a normal population through fatigue in the musculature surrounding the ankles and hips.
Studies have found, that muscle fatigue around the hips and knees have a greater effect on postural stability. It is thought that muscle fatigue leads to a decreased ability to contract with the correct amount of force or accuracy; as a result and kinesthetic feedback from joints are altered so that conscious joint awareness may be negatively effected. Since balance is a key predictor of recovery and is required in so many of our activities of daily living, it is introduced into treatment plans by physiotherapists and occupational therapists when dealing with geriatrics, patients with neurological conditions, or others whom they have determined it to be beneficial. Balance training in stroke patients has been supported in the literature. Methods used and proven to be effective for this population include sitting or standing balance practice with various progressions including reaching, variations in base of support, use of tilt boards, gait training varying speed, stair climbing exercises. Another method to improve balance is perturbation training, an external force applied to a person's center of mass in an attempt to move it from the base of support.
The type of training should be determined by a physiotherapist and will depend on the nature and severity of the stroke, stage of recovery, the patient's abilities and impairments after the stroke. Populations such as the elderly, children with neuromuscular diseases, those with motor deficits such as chronic ankle instability have all been studied and balance training has been shown to result in improvements in postural sway and improved “one-legged stance balance” in these groups; the effects of balance training can be measured by
The Weber test is a quick screening test for hearing. It can detect unilateral sensorineural hearing loss; the test is named after Ernst Heinrich Weber. Conductive hearing ability is mediated by the middle ear composed of the ossicles: incus, stapes. Sensorineural hearing ability is mediated by the inner ear composed of the cochlea with its internal basilar membrane and attached cochlear nerve; the outer ear consisting of the pinna, ear canal, ear drum or tympanic membrane transmits sounds to the middle ear but does not contribute to the conduction or sensorineural hearing ability save for hearing transmissions limited by cerumen impaction. The Weber test has had its value as a screening test questioned in the literature; the Weber and the Rinne test are performed together with the results of each combined to determine the location and nature of any hearing losses detected. In the Weber test a vibrating tuning fork is placed in the middle of the forehead, above the upper lip under the nose over the teeth, or on top of the head equi-distant from the patient's ears on top of thin skin in contact with the bone.
The patient is asked to report. A normal weber test has a patient reporting the sound heard in both sides. In an affected patient, if the defective ear hears the Weber tuning fork louder, the finding indicates a conductive hearing loss in the defective ear. In an affected patient, if the normal ear hears the tuning fork sound better, there is sensorineural hearing loss on the other ear. However, the aforegoing presumes one knows in advance which ear is defective and, normal and the testing is being done to characterize the type, conductive or sensorineural, of hearing loss, occurring. In the case where the patient is unaware or has acclimated to their hearing loss, the clinician has to use the Rinne test in conjunction with the Weber to characterize and localize any deficits; that is, an abnormal Weber test is only able to tell the clinician that there is a conductive loss in the ear which hears better or that there is a sensorineural loss in the ear which does not hear as well. For the Rinne test, a vibrating tuning fork is placed on the mastoid process behind each ear until sound is no longer heard.
Without re-striking the fork, the fork is quickly placed just outside the ear with the patient asked to report when the sound caused by the vibration is no longer heard. A normal or positive Rinne test is when sound is still heard when the tuning fork is moved to air near the ear, indicating that AC is equal or greater than. Therefore, AC > BC. In conductive hearing loss, bone conduction is better than air or BC > AC, a negative Rinne, the patient will report that they do not hear the fork once it is moved. The Rinne test is not ideal for distinguishing sensorineural hearing loss, as both sensorineural hearing loss and normal hearing report a positive Rinne test. In a normal patient, the Weber tuning fork sound is heard loudly in both ears, with no one ear hearing the sound louder than the other. A patient with symmetrical hearing loss will hear the Weber tuning fork sound well, with diagnostic utility only in asymmetric hearing losses. In a patient with hearing loss, the Weber tuning fork sound is heard louder in one ear than the other.
This clinical finding should be confirmed by repeating the procedure and having the patient occlude one ear with a finger. The results of both tests are noted and compared accordingly below to localize and characterize the nature of any detected hearing losses. Note: the Weber and Rinne are screening tests that are not replacements for formal audiometry hearing tests. A patient with a unilateral conductive hearing loss would hear the tuning fork loudest in the affected ear; this is because the ear with the conductive hearing loss is only receiving input from the bone conduction and no air conduction, the sound is perceived as louder in that ear. This finding is because the conduction problem of the middle ear masks the ambient noise of the room, while the well-functioning inner ear picks the sound up via the bones of the skull, causing it to be perceived as a louder sound in the affected ear. Another theory, however, is based on the occlusion effect described by Tonndorf et al. in 1966. Lower frequency sounds.
If an occlusion is present, the sound cannot escape and appears louder on the ear with the conductive hearing loss. Conductive hearing loss can be mimicked by plugging one ear with a finger and performing the Rinne and Weber tests, which will help clarify the above. Humming a constant note and plugging one ear is a good way to mimic the findings of the Weber test in conductive hearing loss; the simulation of the Weber test is the basis for the Bing test. If air conduction is intact on both sides, the patient will report a quieter sound in the ear with
Hearing, or auditory perception, is the ability to perceive sounds by detecting vibrations, changes in the pressure of the surrounding medium through time, through an organ such as the ear. The academic field concerned with hearing is auditory science. Sound may be heard through liquid, or gaseous matter, it is one of the traditional five senses. In humans and other vertebrates, hearing is performed by the auditory system: mechanical waves, known as vibrations are detected by the ear and transduced into nerve impulses that are perceived by the brain. Like touch, audition requires sensitivity to the movement of molecules in the world outside the organism. Both hearing and touch are types of mechanosensation. There are three main components of the human ear: the outer ear, the middle ear, the inner ear; the outer ear includes the pinna, the visible part of the ear, as well as the ear canal which terminates at the eardrum called the tympanic membrane. The pinna serves to focus sound waves through the ear canal toward the eardrum.
Because of the asymmetrical character of the outer ear of most mammals, sound is filtered differently on its way into the ear depending on what vertical location it is coming from. This gives these animals the ability to localize sound vertically; the eardrum is an airtight membrane, when sound waves arrive there, they cause it to vibrate following the waveform of the sound. The middle ear consists of a small air-filled chamber, located medial to the eardrum. Within this chamber are the three smallest bones in the body, known collectively as the ossicles which include the malleus and stapes, they aid in the transmission of the vibrations from the eardrum into the cochlea. The purpose of the middle ear ossicles is to overcome the impedance mismatch between air waves and cochlear waves, by providing impedance matching. Located in the middle ear are the stapedius muscle and tensor tympani muscle, which protect the hearing mechanism through a stiffening reflex; the stapes transmits sound waves to the inner ear through the oval window, a flexible membrane separating the air-filled middle ear from the fluid-filled inner ear.
The round window, another flexible membrane, allows for the smooth displacement of the inner ear fluid caused by the entering sound waves. The inner ear consists of the cochlea, a spiral-shaped, fluid-filled tube, it is divided lengthwise by the organ of Corti, the main organ of mechanical to neural transduction. Inside the organ of Corti is the basilar membrane, a structure that vibrates when waves from the middle ear propagate through the cochlear fluid – endolymph; the basilar membrane is tonotopic, so that each frequency has a characteristic place of resonance along it. Characteristic frequencies are high at the basal entrance to the cochlea, low at the apex. Basilar membrane motion causes depolarization of the hair cells, specialized auditory receptors located within the organ of Corti. While the hair cells do not produce action potentials themselves, they release neurotransmitter at synapses with the fibers of the auditory nerve, which does produce action potentials. In this way, the patterns of oscillations on the basilar membrane are converted to spatiotemporal patterns of firings which transmit information about the sound to the brainstem.
The sound information from the cochlea travels via the auditory nerve to the cochlear nucleus in the brainstem. From there, the signals are projected to the inferior colliculus in the midbrain tectum; the inferior colliculus integrates auditory input with limited input from other parts of the brain and is involved in subconscious reflexes such as the auditory startle response. The inferior colliculus in turn projects to the medial geniculate nucleus, a part of the thalamus where sound information is relayed to the primary auditory cortex in the temporal lobe. Sound is believed to first become consciously experienced at the primary auditory cortex. Around the primary auditory cortex lies Wernickes area, a cortical area involved in interpreting sounds, necessary to understand spoken words. Disturbances at any of these levels can cause hearing problems if the disturbance is bilateral. In some instances it can lead to auditory hallucinations or more complex difficulties in perceiving sound. Hearing can be measured by behavioral tests using an audiometer.
Electrophysiological tests of hearing can provide accurate measurements of hearing thresholds in unconscious subjects. Such tests include auditory brainstem evoked potentials, otoacoustic emissions and electrocochleography. Technical advances in these tests have allowed hearing screening for infants to become widespread; the hearing structures of many species have defense mechanisms against injury. For example, the muscles of the middle ear in many mammals contract reflexively in reaction to loud sounds which may otherwise injure the hearing ability of the organism. There are several different types of hearing loss: Conductive hearing loss, sensorineural hearing loss and mixed types. Conductive hearing loss Sensorineural hearing loss Mixed hearing lossThere are defined degrees of hearing loss: Mild hearing loss - People with mild hearing loss have difficulties keeping up with conversations in noisy surroundings; the most quiet sounds that people with mild hearing loss can hear with their better ear are between 25 and 40 dB HL.
Moderate hearing loss - People with moderate hearing loss have difficulty keeping up with conversations when they are not using a hearing aid. On average, the most quiet sounds heard by
AC power plugs and sockets
AC power plugs and sockets connect electric equipment to the alternating current power supply in buildings and at other sites. Electrical plugs and sockets differ from one another in voltage and current rating, shape and connector type. Different standard systems of plugs and sockets are used around the world. Plugs and sockets for portable appliances became available in the 1880s, to replace connections to light sockets with wall-mounted outlets. A proliferation of types developed for both protection from electrical injury. Today there are about 20 types in common use around the world, many obsolete socket types are found in older buildings. Coordination of technical standards has allowed some types of plug to be used across large regions to facilitate trade in electrical appliances, for the convenience of travellers and consumers of imported electrical goods; some multi-standard sockets allow use of several types of plug. A plug is the movable connector attached to an electrically operated device, the socket is fixed on equipment or a building structure and connected to an energised electrical circuit.
The plug is a male connector with protruding pins that match the openings and female contacts in a socket. Some plugs have female contacts; some plugs have built-in fuses for safety. To reduce the risk of electric shock and socket systems have safety features in addition to the recessed contacts of the energised socket; these may include plugs with insulated sleeves, recessed sockets, or automatic shutters to block socket apertures when a plug is removed. A socket may be surrounded by a decorative or protective cover which may be integral with the socket. Single-phase sockets have two current-carrying connections to the power supply circuit, may have a third pin for a safety connection to earth ground. Depending on the supply system, one or both current-carrying connections may have significant voltage to earth ground; when commercial electric power was first introduced in the 1880s, it was used for lighting. Other portable appliances were connected to light-bulb sockets; as early as 1885 a two-pin plug and wall socket format was available on the British market.
By about 1910 the first three-pin earthed. Over time other safety improvements were introduced to the market; the earliest national standard for plug and wall socket forms was set in 1915. Designs of plugs and sockets have developed to reduce the risk of electric shock and fire. Plugs are shaped to prevent finger contact with live parts, sockets may be recessed; some types can include fuses and switches. Shutters on the socket prevents foreign objects from contacting live contacts; the first shuttered socket was introduced by British manufacturer Crompton, in 1893. Electrical insulation of the pin shanks to reduce live contact exposure was added to some designs, as early as 1905. A third contact for a connection to earth is intended to protect against insulation failure of the connected device; some early unearthed plug and socket types were revised to include an earthing pin or phased out in favour of earthed types. The plug is designed so that the earth ground contact connects before the energized circuit contacts.
The assigned IEC appliance classis governed by the requirement for earthing or equivalent protection. Class I equipment requires an earth contact in the plug and socket, while Class II equipment is unearthed and protects the user with double insulation. Where a "neutral" conductor exists in supply wiring, polarization of the plug can improve safety by preserving the distinction in the equipment. For example, appliances may ensure that switches interrupt the line side of the circuit, or can connect the shell of a screw-base lampholder to neutral to reduce electric shock hazard. In some designs, polarized plugs cannot be mated with non-polarized sockets. Wiring systems where both circuit conductors have a significant potential with respect to earth do not benefit from polarized plugs. "Universal" or "multi-standard" sockets are intended to accommodate plugs of various types. In some jurisdictions, they violate safety standards for sockets. Safety advocates, the United States Army, a manufacturer of sockets point out a number of safety issues with universal socket and adapters, including voltage mismatch, exposure of live pins, lack of proper earth ground connection, or lack of protection from overload or short circuit.
Universal sockets may not meet technical standards for durability, plug retention force, temperature rise of components, or other performance requirements, as they are outside the scope of national and international technical standards. A technical standard may include compatibility of a socket with more than one form of plug; the Thai dual socket is specified in figure 4 of TIS 166-2549 and is designed to accept Thai plugs, Type A, B and C plugs. Chinese dual sockets have both an unearthed socket complying with figure 5 of GB 1002-2008, an earthed socket complying with figure 4 of GB 1002-2008; the exception is that both Thai and Chinese dual sockets accept 120 V rated plugs causing an electrical incompatibility because both states use a 220 V residential voltage. Plugs and power cords have a rated current assigned to them by the manufacturer. Using a plug or power cord, inappropriate for the load may be a safety hazard. For example, high-current equipment can cause a f
A magnifying glass is a convex lens, used to produce a magnified image of an object. The lens is mounted in a frame with a handle. A magnifying glass can be used to focus light, such as to concentrate the sun's radiation to create a hot spot at the focus for fire starting. A sheet magnifier consists of many narrow concentric ring-shaped lenses, such that the combination acts as a single lens but is much thinner; this arrangement is known as a Fresnel lens. The magnifying glass is an icon of detective fiction that of Sherlock Holmes. "The evidence indicates that the use of lenses was widespread throughout the Middle East and the Mediterranean basin over several millennia". The earliest explicit written evidence of a magnifying device is a joke in Aristophanes's The Clouds from 424 BC, where magnifying lenses to start kindling were sold in a pharmacy, Pliny the Elder's "lens", a glass globe filled with water, used to cauterize wounds.. A convex lens used for forming a magnified image was described in the Book of Optics by Ibn al-Haytham in 1021.
After the book was translated during the Latin translations of the 12th century, Roger Bacon described the properties of a magnifying glass in 13th-century England. This was followed by the development of eyeglasses in 13th-century Italy; the magnification of a magnifying glass depends upon where it is placed between the user's eye and the object being viewed, the total distance between them. The magnifying power is equivalent to angular magnification; the magnifying power is the ratio of the sizes of the images formed on the user's retina with and without the lens. For the "without" case, it is assumed that the user would bring the object as close to one eye as possible without it becoming blurry; this point, known as the near point, varies with age. In a young child, it can be as close as 5 cm, while, in an elderly person it may be as far as one or two metres. Magnifiers are characterized using a "standard" value of 0.25 m. The highest magnifying power is obtained by putting the lens close to one eye and moving the eye and the lens together to obtain the best focus.
The object will typically be close to the lens. The magnifying power obtained in this condition is MP0 = Φ + 1, where Φ is the optical power in dioptres, the factor of 0.25 m represents the assumed near point. This value of the magnifying power is the one used to characterize magnifiers, it is denoted "m×", where m = MP0. This is sometimes called the total power of the magnifier. However, magnifiers are not always used as described above because it is more comfortable to put the magnifier close to the object; the eye can be a larger distance away, a good image can be obtained easily. The magnifying power in this case is MP = Φ. A typical magnifying glass might have a focal length of 25 cm, corresponding to an optical power of 4 dioptres; such a magnifier would be sold as a "2×" magnifier. In actual use, an observer with "typical" eyes would obtain a magnifying power between 1 and 2, depending on where lens is held. Magnifying glasses have low magnifying power: 2×–6×, with the lower-power types being much more common.
At higher magnifications, the image quality of a simple magnifying glass becomes poor due to optical aberrations spherical aberration. When more magnification or a better image is required, other types of hand magnifier are used. A Coddington magnifier provides higher magnification with improved image quality. Better images can be obtained with a multiple-lens magnifier, such as a Hastings triplet. High power magnifiers are sometimes mounted in a conical holder with no handle; this is called a loupe. Such magnifiers can reach up to about 30×, at these magnifications the aperture of the magnifier becomes small and it must be placed close to both the object and the eye. For more convenient use or for magnification beyond about 30×, one must instead use a microscope; the magnifying glass is used as a symbolic representation for the ability to search or zoom in computer software and websites. Burning-glass Dome magnifier Reading stone Glasses Macro lens Screen magnifier Graphoscope Aspheric lens
The vestibulo-ocular reflex is a reflex, where activation of the vestibular system causes eye movement. This reflex functions to stabilize images on the retinas during head movement by producing eye movements in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, vice versa. Since slight head movement is present all the time, VOR is necessary for stabilizing vision: patients whose VOR is impaired find it difficult to read using print, because they cannot stabilize the eyes during small head tremors, because damage to the VOR can cause vestibular nystagmus; the VOR does not depend on visual input. It can be elicited by caloric stimulation of the inner ear, works in total darkness or when the eyes are closed. However, in the presence of light, the fixation reflex is added to the movement. In other animals, the organs that coordinate balance and motor coordination do not operate independently from the organs that control the eyes.
A fish, for instance, moves its eyes by reflex. Humans have semicircular canals, neck muscle "stretch" receptors, the utricle. Though the semicircular canals cause most of the reflexes which are responsive to acceleration, the maintaining of balance is mediated by the stretch of neck muscles and the pull of gravity on the utricle of the inner ear; the VOR has both translational aspects. When the head rotates about any axis distant visual images are stabilized by rotating the eyes about the same axis, but in the opposite direction; when the head translates, for example during walking, the visual fixation point is maintained by rotating gaze direction in the opposite direction, by an amount that depends on distance. The VOR is driven by signals from the vestibular apparatus in the inner ear; the semicircular canals detect head rotation and drive the rotational VOR, whereas the otoliths detect head translation and drive the translational VOR. The main "direct path" neural circuit for the horizontal rotational VOR is simple.
It starts in the vestibular system, where semicircular canals get activated by head rotation and send their impulses via the vestibular nerve through the vestibular ganglion and end in the vestibular nuclei in the brainstem. From these nuclei, fibers cross to the contralateral cranial nerve VI nucleus. There they synapse with 2 additional pathways. One pathway projects directly to the lateral rectus of the eye via the abducens nerve. Another nerve tract projects from the abducens nucleus by the medial longitudinal fasciculus to the contralateral oculomotor nucleus, which contains motorneurons that drive eye muscle activity activating the medial rectus muscle of the eye through the oculomotor nerve. Another pathway directly projects from the vestibular nucleus through the ascending tract of Dieters to the ipsilateral medial rectus motoneuron. In addition there are inhibitory vestibular pathways to the ipsilateral abducens nucleus; however no direct vestibular neuron to medial rectus motoneuron pathway exists.
Similar pathways exist for the vertical and torsional components of the VOR. In addition to these direct pathways, which drive the velocity of eye rotation, there is an indirect pathway that builds up the position signal needed to prevent the eye from rolling back to center when the head stops moving; this pathway is important when the head is moving because here position signals dominate over velocity signals. David A. Robinson discovered that the eye muscles require this dual velocity-position drive, proposed that it must arise in the brain by mathematically integrating the velocity signal and sending the resulting position signal to the motoneurons. Robinson was correct: the'neural integrator' for horizontal eye position was found in the nucleus prepositus hypoglossi in the medulla, the neural integrator for vertical and torsional eye positions was found in the interstitial nucleus of Cajal in the midbrain; the same neural integrators generate eye position for other conjugate eye movements such as saccades and smooth pursuit.
For instance, if the head is turned clockwise as seen from above excitatory impulses are sent from the semicircular canal on the right side via the vestibular nerve through Scarpa's ganglion and end in the right vestibular nuclei in the brainstem. From this nuclei excitatory fibres cross to the left abducens nucleus. There they stimulate the lateral rectus of the left eye via the abducens nerve. In addition, by the medial longitudinal fasciculus and oculomotor nuclei, they activate the medial rectus muscles on the right eye; as a result, both eyes will turn counter-clockwise. Furthermore, some neurons from the right vestibular nucleus directly stimulate the right medial rectus motoneurons, inhibits the right abducens nucleus; the vestibulo-ocular reflex needs to be fast: for clear vision, head movement must be compensated immediately. To achieve clear vision, signals from the semicircular canals are sent as directly as possible to the eye muscles: the connection involves only three neurons, is correspondingly called the three neuron arc.
Using these direct connections, eye movements lag the head movements by less than 10 ms, thus the vestibulo-ocular reflex is one of the fastest reflexes in the human body. During head-free pursuit of moving targets, the VOR is counterproductive to the goal of reducing retinal offset. Research indicates that th