Visão

**Como é constituído e como funciona o olho humano?**

Os constituintes do olho humano, responsáveis pela visão, são:
 * **Córnea**;
 * **Pupila**;
 * **Cristalino**;
 * **Retina**;
 * **Coróide**;
 * **Nervo óptico**.

A **córnea** é uma membrana que __protege o olho__ e __ajuda a focar__ as imagens. Quando os raios  luminosos atravessam a córnea, sofrem uma **refracção** (mudança de direcção). A **pupila** é um __orifício de tamanho variável__, que **controla a entrada de luz** no olho: **dilata-se** quando existe **pouca luz** e **contrai-se** quando existe **muita luz**. O **cristalino ** é uma __<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">lente convergente __<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">, <span style="background: rgb(255, 255, 0) none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">situada <span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> atrás da córnea e da pupila, <span style="background: rgb(255, 255, 0) none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">que **<span style="background: rgb(255, 255, 0) none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">orienta a luz até à **<span style="color: rgb(0, 0, 0)">**<span style="background: rgb(255, 255, 0) none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">retina ** <span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">. Quando os raios luminosos atravessam o cristalino sofrem uma **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">refracção **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">, de modo a **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">convergirem na retina ** <span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">e a formarem **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">imagens focadas ** <span style="background: rgb(255, 255, 0) none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">(  <span style="background: rgb(255, 255, 0) none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">nítidas  <span style="background: rgb(255, 255, 0) none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">)  <span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">. <span style="font-family: Arial,sans-serif"><span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">O **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">cristalino **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> consegue **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">focar **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> rapidamente as imagens de **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">objectos próximos **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> ou **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">afastados **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> do olho ( <span style="background: rgb(255, 255, 0) none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">os raios luminosos convergem sempre na retina <span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">). Quando olhamos para **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">objectos próximos **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> de nós, os **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">músculos **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> do cristalino **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">descontraem-se **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">, para que este adquira uma forma praticamente esférica. Quando olhamos para **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">objectos afastados **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> de nós, os **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">músculos **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">contraem-se **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">, de modo a que este adquira uma forma mais longa e fina. <span style="font-family: Arial,sans-serif">A **retina** é a __s____<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">uperfície interna __<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> do olho, onde os raios luminosos vão **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">convergir **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> e formar as **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">imagens focadas **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">e **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> invertidas **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">. A retina é constituída por **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">células detectoras de luz **<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> ( __<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">cones __<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial"> e __<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">bastonetes __<span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">). O s **bastonetes** detectam os diferentes tipos de **luminosidade**. Os **cones** detectam as três **cores primárias da luz** (__vermelho__, __azul__ e __verde__). <span style="font-family: Arial,sans-serif">Estas células transformam as **imagens** (__sinais luminosos__) em **impulsos eléctricos** (__impulsos nervosos__), que são transportados pelo **nervo óptico** até ao **cérebro**, onde as imagens são novamente invertidas para ficarem direitas.

<span style="font-family: Arial,sans-serif"><span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">A **coróide** é um tecido com células que possuem **melanina** (proteína que funciona como um pigmento). A melanina **absorve parte da luz** que chega à retina, evitando a sua **reflexão**.

<span style="font-family: Arial,sans-serif">**Quais são os defeitos de visão e como se corrigem?**

<span style="font-family: Arial,sans-serif">Os defeitos de visão são: <span style="font-family: Arial,sans-serif">**Hipermetropia** ; <span style="font-family: Arial,sans-serif">**Miopia** ; <span style="font-family: Arial,sans-serif">**Presbiopia** ; <span style="font-family: Arial,sans-serif">**Astigmatismo**.

<span style="font-family: Arial,sans-serif"> Estes defeitos de visão são provocados por **alterações na curvatura do cristalino**, que **alteram a refracção da luz.** P odem ser corrigidos com **óculos**, **lentes de contacto** ou com uma **operação com laser**.

<span style="font-family: Arial,sans-serif">Uma pessoa com **hipermetropia** vê __mal ao perto__, porque o cristalino é **pouco curvo** e as imagens formam-se **atrás da retina**, ficando **desfocadas.** <span style="font-family: Arial,sans-serif">Para corrigir este defeito de visão utilizam-se **lentes convergentes** ou **convexas**, que obrigam os raios luminosos a **convergir na retina** e a formar uma imagem focada.

<span style="font-family: Arial,sans-serif">Uma pessoa com **miopia** vê __mal ao longe__, porque o cristalino é **muito curvo** e as imagens formam-se **à frente da retina**, ficando **desfocadas**. <span style="font-family: Arial,sans-serif">Para corrigir este defeito de visão utilizam-se **lentes divergentes** ou **concavas**, que obrigam os raios luminosos a **convergir na retina** e a formar uma imagem focada.

<span style="font-family: Arial,sans-serif">Uma pessoa com **presbiopia** (vista cansada) vê __mal ao perto__, porque o cristalino **perde elasticidade** e as imagens formam-se  **atrás da retina**  ,    ficando  **desfocadas**. A presbiopia surge geralmente depois dos 40 anos e tende a piorar com a idade. <span style="font-family: Arial,sans-serif">Para corrigir este defeito de visão utilizam-se **lentes convergentes**, que podem ser **bifocais** para permitir a focagem dos objectos ao longe.

<span style="font-family: Arial,sans-serif">Uma pessoa com **astigmatismo** vê __mal ao longe e ao perto__, porque a **curvatura** do cristalino altera-se e as imagens não ficam __focadas na posição correcta__. <span style="font-family: Arial,sans-serif">Para corrigir este defeito de visão utilizam-se **lentes cilíndricas**.

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<span style="font-family: Arial, sans-serif"><span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">Figura 1. Constituição do olho humano. <span style="font-family: Arial, sans-serif"><span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">Figura 2. Focagem pelo olho ao longe e ao perto, respectivamente. <span style="font-family: Arial, sans-serif"><span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">Figura 3. Formação de imagens na retina. <span style="font-family: Arial, sans-serif"><span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">Figura 4. Olho com hipermetropia e correcção com lente convergente. <span style="font-family: Arial, sans-serif"><span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">Figura 5. Olho com miopia e correcção com lente divergente. <span style="font-family: Arial, sans-serif"><span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">Visão nos animais
 * 1) <span style="font-family: Arial, sans-serif"><span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">Como é constituído e como funciona o olho humano?
 * 2) <span style="font-family: Arial, sans-serif"><span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">Como é que a luz chega ao cérebro?
 * 3) <span style="font-family: Arial, sans-serif"><span style="background: transparent none repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial">Quais são os defeitos de visão e como se corrigem?