Part 3

Eyes and sight Your eyes are responsible for your sense of sight, allowing you to see what is going on around you. They collect light from objects and process the information to create images of what you are looking at. Lens This clear disk focuses light onto the retina. Pupil Light enters the eye through this hole in the iris. Cornea This dome-shaped clear layer covers the front of the eye. Iris The colored part of the eye is a ring of muscle that controls the size of the pupil. Sclera A tough protective layer forms the "white" of the eye. Retina A layer of light-sensitive receptor cells collects the light coming in and turns it into signals to be sent to the brain. Optic nerve This nerve carries signals from the retina to the brain. Inside the eye Light enters the eye through the pupil. The light is focused onto the retina at the back of the eye and the information is sent to the brain. The pupil changes in size based on the lighting conditions. It gets bigger in dim light and smaller in bright light. Tears wash away grit, so crying cleans your eyes! Just like your fingerprint, your iris is completely unique to you! SEEING IN 3D Look straight ahead and notice how much you can see without turning your head. This is your visual field— it is really large! This is because both eyes pick up light separately and send slightly different signals to the brain. The brain combines the signals to create a single 3D image of what you are looking at. Eye muscles Six tiny muscles move the eyeballs in different directions. They work together to make sure the movements are smooth. If one of the muscles doesn't work properly, it can result in blurred vision.

眼睛与视觉 你的眼睛负责视觉，能让你看到周围发生的一切。 它们收集物体发出的光线，处理信息后生成你所看事物的影像。 晶状体 这块透明的圆盘会把光线聚焦到视网膜上。 瞳孔 光线穿过虹膜上的这个小孔进入眼睛。 角膜 这层圆顶状的透明层覆盖在眼球前端。 虹膜 眼睛里带颜色的部分是一圈肌肉，负责控制瞳孔的大小。 巩膜 一层坚韧的保护层构成了眼睛的「眼白」。 视网膜 这一层布满感光细胞，它收集进入眼睛的光线，再将其转换成信号发送给大脑。 视神经 这条神经会把视网膜的信号传递给大脑。 眼睛内部 光线通过瞳孔进入眼睛。 光线聚焦在眼底的视网膜上，相关信息随后会发送给大脑。 瞳孔会根据光照条件改变大小。 它在昏暗光线下变大，在明亮光线下变小。 泪水能冲走沙粒，所以哭可以清洁你的眼睛哦！ 就像你的指纹一样，每个人的虹膜都是独一无二的！ 立体视觉 眼睛直视前方，不用转头看看你能看到多大范围。 这就是你的视野——它真的非常大！ 这是因为两只眼睛各自接收光线，传给大脑的信号略有不同。 大脑把这些信号整合起来，就生成了你看到的立体图像。 眼肌 六块细小的肌肉带动眼球向不同方向转动。 它们互相配合，保证眼球转动顺滑自然。 如果其中一块肌肉运作失常，就会导致视力模糊。

Nerve A large nerve carries information about what you are hearing from receptors in the cochlea to the brain. Ossicles Tiny bones—called the malleus, incus, and stapes—vibrate to pass sound waves into the inner ear. Ear canal This tunnel directs sounds heard toward the middle ear. It contains hair and wax. Cochlea This is full of fluid that vibrates when it picks up the sound waves. Receptors pick up these vibrations. Ear drum The tympanic membrane, or ear drum, vibrates to push sound waves along to the ossicles. Outer ear flap This flap, called the pinna, is made of cartilage. It collects all the sound waves around you. Ears and hearing Your ears don't just help you hear, they also help you balance. Ears have three parts. The outer ear is the part you can see, but ears are much bigger than that—inside your head are the middle ear and the inner ear. Hearing sounds Sound waves travel through the air as vibrations. They are picked up by the outer ear and pass through the middle ear, into the inner ear. Here, a nerve sends the vibrations to the brain as electrical signals. The brain then figures out the sounds that are being heard. Hearing aids A hearing aid is a device that can help you to hear better. It makes sounds louder and clearer. Early hearing aids looked like trumpets, but some are now so small that you can hardly see them. EAR WAX Ear wax is normal and healthy! Made by glands in the ear canal, it traps any dust and dirt, and carries it out of the ear. This protects your ears from infections. Ear wax comes out on its own, so you shouldn't try to remove it yourself. FEELING DIZZY Semicircular tubes inside your ears contain fluid to help you balance. The fluid moves when your body moves, so your brain is aware of your body's position. If you spin around and then stop, the fluid continues to move around for a while, so the brain gets confused and you feel dizzy!

神经 一根粗大的神经会把你听到的信息，从耳蜗的感受器传递给大脑。 听小骨 三块细小的骨头分别叫锤骨、砧骨和镫骨，它们通过振动把声波传入内耳。 耳道 这条通道把接收到的声音传向中耳。 耳道里长有耳毛和耳垢。 耳蜗 耳蜗里充满液体，接收到声波后液体就会振动起来。 感受器会接收这些振动。 鼓膜 鼓膜也叫耳膜，它会振动，把声波传递给听小骨。 耳郭 这块外翻的组织叫耳郭，由软骨构成。 它负责收集你周围所有的声波。 耳朵与听觉 你的耳朵不只能帮你听声音，还能帮你保持平衡。 耳朵分为三个部分。 外耳是你能看到的部分，但耳朵其实比这大得多——中耳和内耳都长在你的头颅内。 听见声音 声波以振动的形式在空气中传播。 它们被外耳接收后，穿过中耳，进入内耳。 在这里，神经会把振动转换成电信号传递给大脑。 接着大脑就能识别出你听到的是什么声音了。 助听器 助听器是一种能帮助你听得更清楚的装置。 它能让声音变大、变清晰。 早期的助听器长得像喇叭，现在有些助听器小到几乎看不见。 耳垢 耳垢是正常又健康的！ 它由耳道里的腺体分泌，能困住灰尘和脏东西，把它们带出耳朵。 这能保护耳朵，避免感染。 耳垢会自己排出体外，所以你不用自己动手清理它。 头晕 耳朵里的半规管充满液体，帮你维持平衡。 身体移动时液体会跟着晃动，大脑就能感知到身体的位置。 如果你原地转圈再停下，液体还会继续晃动一会儿，大脑就会被弄糊涂，你也就感觉头晕了！

Taste and smell These senses are linked together. You can't taste how tasty food is unless you can also smell it. Your senses of taste and smell can also keep you safe—for example, by warning you when food has gone bad. Smell When you sniff, smells go up the nostrils and enter the nose, where they bounce around. This makes the smells stronger before they hit the olfactory bulb, which is where smell receptors are found, high up in the nasal cavity. The nose can detect around one trillion different smells! Sniffing brings more smell particles into the nose! Taste The tongue has a lot of little bumps on its surface called papillae, which have taste buds on them. Eight tongue muscles help move food around in your mouth, and bring it in contact with the taste buds. Taste buds Taste receptors found inside the taste buds send information back to the brain so that it can figure out what you are tasting. Types of taste We can smell countless different smells, but we can only taste five basic flavors. These are sweet, sour, salty, bitter, and umami (savory).

味觉和嗅觉 这两种感官是相互关联的。 要是闻不到气味，你就尝不出食物有多美味哦。 味觉和嗅觉还能保护你的安全——比如食物坏掉的时候，它们会提醒你。 嗅觉 当你吸气闻味时，气味会顺着鼻孔进入鼻子，在里面打转。 这会让气味变得更浓郁，之后才会接触到嗅球，嗅球就是嗅觉感受器所在的位置，在鼻腔的上方。 鼻子大约能分辨出一万亿种不同的气味！ 吸气闻味能把更多气味微粒带进鼻子哦！ 味觉 舌头表面有很多小小的凸起，叫做舌乳头，舌乳头上长着味蕾。 舌头的八条肌肉能帮着搅动嘴里的食物，让食物接触到味蕾。 味蕾 藏在味蕾里的味觉感受器会把信息传回大脑，大脑就能分辨出你尝到的是什么味道了。 味道的种类 我们能闻出数不清的不同气味，但只能尝出五种基本味道。 它们分别是甜、酸、咸、苦和鲜（也就是鲜味）。

Teeth The main job of teeth is to break down the food you eat. Saliva (spit) produced in your mouth kills bacteria and helps keep your teeth healthy. Everyone has two sets of teeth in their life: first baby teeth and then adult teeth. Different jobs There are four types of teeth—each is shaped differently since they all have a different job to do. See if you can notice the straight, pointy, and flat teeth in your mouth. Inside a tooth The part of the tooth you can see is called the crown. The part hidden inside the gums is called the root. Each tooth is filled with nerves and blood vessels that keep it healthy. Incisors These straight-edged teeth at the front of the mouth help you bite food and cut it into pieces. Canines Canines are really pointy and sharp. They grip and pierce food, and tear it into smaller shreds. BABY TEETH A baby's first teeth, called milk teeth, begin to grow at around six months old. A child has a full set of 20 milk teeth by the time they are three years old. These start to fall out around the age of six, and are replaced with 32 adult teeth. Enamel is the hardest substance in the body and protects the crown of the tooth. Dentin makes up most of the tooth and is very hard. Gums surround each tooth and protect the delicate roots. Pulp contains a lot of blood vessels and nerves. The nerves allow you to feel pain in your teeth. Teeth are held in the skull by the jawbones. Premolars Smaller than the molars, these teeth help to crush food down further. Molars These big and chunky teeth help to grind food into mush that is small enough to swallow.

牙齿 牙齿最主要的工作，就是磨碎你吃进去的食物。 你嘴里产生的唾液（也就是口水）能杀死细菌，帮你保持牙齿健康。 人的一生一共会长两副牙齿：先长乳牙，之后再长恒牙。 不同分工 牙齿一共分为四种，每种形状都不一样，因为它们要做的工作不同。 你可以试试找找自己嘴里平平的、尖尖的、扁扁的牙齿哦。 牙齿的内部 我们能看见的牙齿部分叫做牙冠。 藏在牙龈里面的部分叫做牙根。 每颗牙齿里都布满神经和血管，维持牙齿的健康。 切牙 这些长在嘴巴前方、边缘平整的牙齿，帮你咬下食物，再把它切成小块。 尖牙 尖牙长得尖尖的，十分锋利。 它们负责抓住、刺穿食物，再把食物撕成小碎片。 乳牙 宝宝的第一副牙齿叫做奶牙，大概在宝宝六个月大的时候开始萌出。 小朋友长到三岁时，就能长齐整整20颗奶牙了。 奶牙大概六岁左右开始脱落，之后会被32颗恒牙替换掉。 牙釉质是人体最坚硬的物质，它保护着牙齿的牙冠。 牙本质构成了牙齿的大部分，质地非常坚硬。 牙龈包裹着每一颗牙齿，保护娇嫩的牙根。 牙髓里有很多血管和神经。 神经能让你感觉到牙齿的疼痛。 牙齿由颌骨固定在头骨上。 前磨牙 前磨牙比磨牙小，它们能帮着把食物压得更碎。 磨牙 这些又大又厚实的磨牙会把食物磨成细碎的糊糊，就可以咽下去了。

The journey of food Food contains nutrients, which give us energy and repair broken cells. When you eat, your food is broken down so that the nutrients can be absorbed into your blood and used by the body. This is called digestion. Digesting food Food is digested in a very long tube that starts at your mouth and ends at your anus. It can take from one to three days for a meal to pass through the digestive system. When you chew, your teeth cut food into smaller pieces. The food is mixed with saliva, or spit, which contains chemicals that break the pieces down further. It is then swallowed and moves down the esophagus to the stomach. Some food, like sweet corn, is hard to digest and might come out whole in your poop! Liver The body's largest internal organ processes and stores nutrients from food. Pancreas Pancreatic juice is made here. It contains chemicals called enzymes, which help break down food in the small intestine. Gall bladder This small organ releases bile, which breaks down fat. The stomach muscles help squash and squeeze food down. Here, the food is also mixed with acid to break it down further. The folds of your stomach stretch out as you fill it with food! The small intestine breaks down the partly digested food into simple nutrients, which are absorbed into the blood. The small intestine isn't that small! It is around 20–23 ft (6–7 m) in length. The large intestine absorbs water and the last bits of nutrients. Any food waste that is left over is pushed out of the large intestine as poop.

食物的旅途 食物里含有营养，它们能给我们提供能量，还能修复受损的细胞。 你吃东西的时候，食物会被分解，这样营养就能被血液吸收，供身体使用了。 这个过程叫做消化。 消化食物 食物在一条很长的管道里被消化，管道从嘴巴开始，到肛门结束。 一顿饭完全通过消化系统，需要花一到三天的时间哦。 你咀嚼的时候，牙齿会把食物切成更小的碎块。 食物会和唾液也就是口水混合，唾液里的化学物质会把食物碎块分解得更细。 之后食物会被咽下去，顺着食道进到胃里。 有些食物，比如甜玉米，很难消化，可能会完整地跟着便便排出来哦！ 肝脏 它是人体最大的内脏器官，负责处理和储存食物里的营养。 胰腺 胰液就是在这里产生的。 胰液里含有叫做酶的化学物质，能帮着分解小肠里的食物。 胆囊 这个小小的器官会释放胆汁，胆汁用来分解脂肪。 胃的肌肉会不断挤压、揉碎食物。 在这里，食物还会和胃酸混合，被分解得更细。 你吃下食物把胃填满的时候，胃上的褶皱就会伸展开哦！ 小肠会把初步消化的食物分解成简单的营养物质，这些营养会被吸收进血液里。 小肠其实一点都不小哦！ 它的长度大约有6到7米呢。 大肠会吸收食物里剩下的水分和最后一点营养。 剩下没用的食物残渣会变成粪便，被推出大肠排出体外。

Kidneys The Kidneys do a lot of important jobs. They filter around 4 fl. oz (120 ml) of blood every minute. They also help control blood pressure and produce a hormone—a chemical messenger—to make new red blood cells. Removing waste Each kidney has millions of tiny filtering units called nephrons. As the blood passes through these tubes, waste products are separated, and trickle down a tube called the ureter to the bladder. Cleaning the blood Most people have two kidneys that filter blood to keep it clean. Waste substances and extra water the body doesn't need are removed and stored in the bladder. These then leave the body as urine. WHAT IS URINE MADE OF? Urine, or pee, is mostly unwanted water, but it also contains the waste chemical urea— made when the body breaks down protein, other substances called electrolytes, and extra salt. Urea can break down into the chemical ammonia, which makes pee smelly. PEE IS MORE THAN 90 PERCENT WATER. Colorful pee Sometimes your urine can change color based on what you eat—beets can turn pee pink, while some food dyes can cause green or blue urine! Chemicals in pee sometimes harden into crystals, called kidney stones, which can make you sick. Bladder An adult's empty bladder is roughly the size of a pear. But when it's really full of pee, it is more like the size of a grapefruit! Urine leaves the body through a tube called the urethra.

肾脏 肾脏肩负着很多重要的工作。 它们每分钟大约过滤4液量盎司 （120毫升）的血液。 它们还能帮助控制血压，还会生成一种激素——也就是化学信使，来制造新的红细胞。 排出废物 每个肾脏都有数百万个叫做肾单位的微小过滤单位。 血液流过这些细管时，废物会被分离出来，顺着名为输尿管的管子流进膀胱。 清洁血液 大多数人都有两个肾脏，它们过滤血液，让血液保持洁净。 身体不需要的废物和多余水分会被分离出来，储存在膀胱中。 之后这些废物会以尿液的形式排出体外。 尿液由什么组成？ 尿液，也就是尿尿，大部分是身体不需要的水分，同时它还含有废物尿素——这是身体分解蛋白质时产生的，还有叫做电解质的其他物质，以及多余的盐分。 尿素会分解为化学物质氨气，这就是尿尿有异味的原因。 尿液中超过90%都是水。 五颜六色的尿尿 有时候你的尿液颜色会随你吃的食物改变——甜菜会让尿尿变粉，有些食用色素会让尿液变成绿色或蓝色！ 尿液里的化学物质有时候会变硬形成结晶，也就是肾结石，它会让你生病。 膀胱 成年人排空的膀胱差不多和一颗梨一样大。 但当它装满尿尿的时候，大小差不多就和一个葡萄柚一样大啦！ 尿液通过名为尿道的管子排出体外。