Part 3

Eyesight The eyes are the organs that let us see. Light enters through the pupil and is detected by receptor cells at the back of the eye. These send a signal to the brain, which creates an image. 1 Light Light travels in tiny packets called photons. Photons hit receptors in the eye to generate electrical signals. 2 Cornea The cornea is the eye's clear covering. It protects the eye and lets light through. 3 Pupil The black circle in the centre of your eye is a hole through which light enters. It is controlled by the iris. 4 Iris This circular muscle is the coloured part of your eye. It makes the pupil bigger in the dark, to let in more light, and shrinks it in bright light. 5 Lens The clear lens changes shape to focus incoming photons onto the retina. 6 Retina Receptors that detect photons to create electrical signals are found in a sheet, called the retina, at the back of the eye. 7 Optic nerve Signals from the retina are passed to neurons whose axons run to the brain. 8 Thalamus The first stop for visual signals in the brain is the thalamus. From here, they are sent to the visual cortex. 9 Visual cortex The brain forms an internal image of the world when visual signals reach the visual cortex. Making pictures The visual cortex is divided into regions that look after different parts of vision. Some regions focus on colour, for example, others movement, and some combine the two images taken by your left and right eye. Seeing things The sense of vision is very important to humans. The eyes gather light and convert it into neural signals. The brain uses these to create a picture of the world. OPTICAL ILLUSION The brain sometimes gets it wrong. The different colours and shapes in the image to the right trick the brain so that it sees movement. This is called an optical illusion.

视力 眼睛是让我们看见东西的器官 光线穿过瞳孔，被眼球后部的感受器细胞捕捉 这些细胞向大脑发送信号，大脑会生成图像 1 光线 光线以被称作光子的微小能量包的形式传播 光子击中眼睛里的感受器，产生出电信号 2 角膜 角膜是覆盖在眼睛表面的透明组织 它保护眼睛，还能让光线穿过 3 瞳孔 你眼睛中央的黑色圆圈是孔洞，光线从这里进入眼睛 它的大小由虹膜控制 4 虹膜 这块环形肌肉就是你眼睛里带颜色的部分 在暗处它会让瞳孔变大，摄入更多光线，在亮处它会让瞳孔缩小 5 晶状体 透明的晶状体会改变形状，把入射的光子聚焦到视网膜上 6 视网膜 眼球后部有一层叫作视网膜的组织，这里布满了能探测光子、产生电信号的感受器 7 视神经 视网膜发出的信号会传递给神经元，这些神经元的轴突通向大脑 8 丘脑 视觉信号进入大脑后，第一站就是丘脑 信号从这里会被输送到视觉皮层 9 视觉皮层 当视觉信号到达视觉皮层，大脑就形成了对世界的内部图像 生成图像 视觉皮层分为不同区域，分别负责视觉的不同功能 比如有些区域负责识别颜色，有些负责识别运动，还有些区域会整合左右眼得到的两幅图像 看见事物 视觉对人类来说非常重要 眼睛收集光线，把它转换成神经信号 大脑利用这些信号生成出世界的图景 视错觉 大脑有时候也会出错 右图里不同的颜色和形状会误导大脑，让大脑误以为看到了运动 这就叫做视错觉

Listening Sound is detected by the ears. When sound vibrations travel into the ear they are turned into electrical signals by receptor cells, called hair cells. The brain turns these signals into sounds. 1 Sound waves Sound travels through the air as vibrations. 2 Eardrum This thin membrane vibrates when sound waves reach it. 3 Hammer, anvil, and stirrup In the air-filled middle part of the ear, there are three tiny bones that move when the eardrum vibrates. 4 Oval window The stirrup bone taps on the oval window, another membrane, to send vibrations into the fluid- filled inner ear, or cochlea. 5 Cochlea The inner ear is spiral-shaped and contains hair cells that change vibrations into electrical signals. 6 Nerve A bundle of axons carries signals from the cochlea to the brain. Hearing sounds Sounds, such as the singing of a bird, are vibrating waves of air. You detect them when they enter your ears. There, sounds are converted into signals that are sent to the brain. HAIR CELLS Hair cells are found inside the cochlea. Each cell is topped with tiny hair-like structures that bend when vibrations pass through the fluid inside the cochlea. This bending generates electrical signals that are then sent to the brain. Into the brain When sound messages enter the brain, it compares the timing of the signals from the right and left ears to work out which direction the sound came from. Hearing cortex You become aware of sounds when signals reach the hearing cortex on either side of the brain.

听觉 声音由耳朵感知 当声波振动进入耳朵，会被叫作毛细胞的感受器细胞转换成电信号 大脑再把这些信号转化为我们听到的声音 1 声波 声音以振动的形式在空气中传播 2 鼓膜 当声波到达这层薄膜，鼓膜就会振动 3 锤骨、砧骨和镫骨 在充满空气的中耳里，有三块细小的骨头，鼓膜振动时它们会跟着活动 4 卵圆窗 镫骨会敲击卵圆窗这层薄膜，把振动传入充满液体的内耳，也就是耳蜗 5 耳蜗 内耳是螺旋形状的，里面的毛细胞能把振动转换成电信号 6 神经 一束轴突会把耳蜗产生的信号传递给大脑 听到声音 声音比如鸟儿的歌唱，就是振动的空气波 它们进入耳朵后就会被我们感知 在耳朵里，声音会被转换成信号，再发送给大脑 毛细胞 毛细胞位于耳蜗内部 每个毛细胞顶端都有细小的毛状结构，当振动穿过耳蜗内的液体时，这些结构就会弯曲 这种弯曲会产生电信号，随后被发送到大脑 传入大脑 声音信号进入大脑后，大脑会对比左右耳信号的时间差，判断声音来自哪个方向 听觉皮层 当信号到达大脑两侧的听觉皮层，你就能感知到声音了

Making us move We need to move in order to do all sorts of things: to search for food, escape danger, to play, or to exercise. The brain makes the body move by controlling its muscles. Many muscles This is what the body looks like beneath the skin! Most muscles are attached to bones. The body moves when these muscles pull on the bones to move the skeleton. Bend When a signal is sent to contract your biceps, your arm bends. Bend or straighten Muscles mainly come in pairs, such as the biceps and triceps in your arms. They are controlled by the nervous system. Signals from the brain tell a muscle to shorten– or contract– to make you move. If there is no signal, the muscle stays relaxed. Straighten When a signal is sent to contract your triceps, your arm straightens. The human body contains roughly 700 muscles. MOVEMENT CORTEX Three areas of the cerebral cortex help control movement. They work with structures inside the brain to make you move. Muscle control To accurately throw a ball, you must move your arm, hand, and finger muscles all at the right time. The brain coordinates the different muscles, and tells them when to move and by how much. The brain constantly receives messages from the senses, and adjusts the muscles to throw the ball accurately. In less than a tenth of a second, signals arrive from the brain to a muscle.

让我们动起来 我们要动起来才能做各种各样的事：寻找食物、躲避危险、玩耍或是锻炼身体。 大脑通过控制肌肉，让身体动起来。 许许多多的肌肉 皮肤下面的身体就是这个样子哦！ 大部分肌肉都附着在骨骼上。 当这些肌肉拉动骨骼带动骨架活动时，身体就动起来啦。 弯曲 当收缩肱二头肌的信号发出后，你的胳膊就会弯曲。 弯曲或伸直 肌肉大多成对工作，比如你胳膊里的肱二头肌和肱三头肌。 它们都由神经系统控制。 来自大脑的信号会命令肌肉缩短——也就是收缩——让你动起来。 如果没有信号，肌肉就会保持放松状态。 伸直 当收缩肱三头肌的信号发出后，你的胳膊就会伸直。 人体大约一共有700块肌肉。 运动皮层 大脑皮层有三个区域能帮助控制运动。 它们和大脑内部的结构合作，让你能够活动。 控制肌肉 要准确投出一颗球，你得让胳膊、手掌和手指的肌肉都在正确的时机发力。 大脑会协调不同的肌肉，告诉它们什么时候动、该动多少。 大脑会不断接收来自感官的信息，调整肌肉动作，帮你把球投准。 不到十分之一秒，大脑发出的信号就能传到肌肉那里。

Relaxation If you're resting, your brain uses a part of the ANS called the parasympathetic nervous system to make your body relax. When there's no danger or anything to do, you save and store energy. Ready for anything The brain makes sure the body is ready for what it needs to do– whether that's relaxation or action. It does this using nerves that run to all parts of the body. These make up the autonomic nervous system (ANS). VOLUME CONTROL The more active you are, the more blood your body needs. Your heart always beats, but the ANS can speed it up or slow it down. Action When your body needs to get active, your brain uses a different part of the ANS, called the sympathetic nervous system. Digestion is switched off and the rest of the body gets ready to move. Fight or flight? When a person encounters danger, they need to decide very quickly if they should confront it or run away. The sympathetic nervous system prepares the body to do either. The brain also controls the release of certain chemicals, called hormones, which help prepare the body for action.

放松休息 当你休息时，大脑会通过自主神经系统（ANS）中名为副交感神经系统的部分，让身体放松下来。 当没有危险也没什么事情要做时，你的身体就会储存能量。 做好万全准备 大脑会让身体做好应对需求的准备——不管是要放松还是要活动。 它靠着通往全身各处的神经来完成这件事。 这些神经共同组成了自主神经系统（简称ANS）。 血量调节 你活动得越多，身体需要的血液就越多。 你的心脏一直在跳动，不过自主神经系统可以加快或者减慢心跳速度。 活动状态 当身体需要活动起来时，大脑会动用自主神经系统的另一部分，叫做交感神经系统。 消化功能会暂时关闭，身体其他部分做好活动准备。 战斗还是逃跑？ 当人遇到危险，需要极快做出决定：是对抗危险还是逃跑。 交感神经系统会帮身体做好应对这两种情况的准备。 大脑还会控制释放一种叫做激素的化学物质，帮助身体做好活动准备。

Knowing what you need! The brain knows when you are too hot or too cold, hungry or full, and what to do about it. Here, we look at how the brain recognizes that you're hungry and signals that you should eat. 1 Feeling hungry When your stomach is empty, it releases a hormone (chemical messenger) called ghrelin. Ghrelin travels in the blood to the brain, where it sticks to different neurons and makes you feel hungry. 2 Hunt for food When you're hungry, you want food. You might search, or ask, for it. Your brain helps you to notice anything that smells or looks like food. 3 Filling up When you find food, you eat it! As the stomach fills up, it stops producing ghrelin. Neurons attached to the stomach send messages to the brain, which tell it that the stomach is stretched. 4 Feeling full The intestines below the stomach release hormones that signal food has arrived there. These hormones tell the brain when you have eaten enough, so you feel full and stop eating... for a while. You absorb the food you have eaten, use up the energy from the food, and start to feel hungry again. The hypothalamus is the part of the brain that is most important for controlling hunger. NEGATIVE FEEDBACK When you are hungry, or full, the brain and body work together to change your behaviour, so that you stop feeling that way. Your body and brain use this "negative feedback" to keep you feeling just right.

了解你的需求！ 大脑知道你什么时候太热、什么时候太冷，什么时候饿、什么时候饱，还知道该怎么应对这些情况。 接下来，我们就来看看大脑怎么察觉你饿了，怎么发出要吃东西的信号。 1 感觉饥饿 当你的胃空了的时候，会释放一种叫做饥饿素的激素，也就是化学信使。 饥饿素跟着血液流到大脑，附着在不同的神经元上，让你感觉到饥饿。 2 寻找食物 当你饿了，你就会想要吃东西。 你可能会自己去找，或是向别人索要食物。 大脑会帮你留意任何闻起来、看起来像食物的东西。 3 吃饱进食 当你找到食物，就可以开吃啦！ 当胃被填满后，就会停止分泌饥饿素。 连接胃的神经元会给大脑发送消息，告诉大脑胃已经被撑开了。 4 感觉吃饱 胃下方的肠道会释放激素，发出食物已经到达这里的信号。 这些激素告诉大脑你已经吃够了，于是你会觉得饱，停下吃东西……过上一阵就又会饿了。 你吸收吃下的食物，用完食物里的能量，就会又感觉到饿了。 下丘脑是大脑中控制饥饿感最重要的部分。 负反馈调节 不管你是饿还是饱，大脑和身体都会一起合作改变你的状态，消除这种不适感。 你的身体和大脑靠这种"负反馈"让你的状态一直保持刚刚好。

Baby brains In the early stages of life, the brain and spinal cord are formed from a structure called the neural tube. The brain grows fast while the baby is in the womb, and it is about the size of a large orange at birth. 3 WEEKS A neural tube soon develops inside an embryo– the name given to an unborn baby between its second and eighth week. 6 WEEKS As the embryo grows, the front end of the neural tube grows fastest and divides into different parts. 9 WEEKS By nine weeks, sections of the neural tube have started to become the spinal cord, hindbrain, midbrain, and forebrain. The unborn baby is now known as a foetus. 13 WEEKS Over the following weeks, the forebrain grows the most. By 13 weeks, the brain looks similar in shape to an adult brain but much smaller. 25 WEEKS The surface of the brain is still smooth at 25 weeks, but folds are beginning to appear as it gets bigger. The midbrain is now inside the brain. 40 WEEKS Babies are usually born at about 40 weeks. A baby's brain looks like an adult's, but it's just one third of the size. It can't do much– but it can make the baby cry to get attention! At eight weeks old, a human embryo is the size of a raspberry! Making a brain A human brain forms as a baby grows inside its mother's womb. How it develops is controlled by instructions inside the baby's DNA and what the baby senses. SAVING SPACE The cerebral cortex that covers the brain is actually a big flat sheet of neurons, folded up to fit inside the skull. If you scrunch up a sheet of paper, you can see how the folds reduce the area it takes up.

宝宝的大脑 在生命早期，大脑和脊髓由一种叫做神经管的结构发育形成。 宝宝待在妈妈子宫里的时候，大脑就生长得很快，出生时差不多有一个大橙子那么大。 3周 神经管很快就在胚胎中发育出来——胚胎指的是怀孕第二到第八周的未出生宝宝。 6周 随着胚胎长大，神经管的前端生长速度最快，还分化成了不同的部分。 9周 到第九周，神经管的各个部分已经开始发育成脊髓、后脑、中脑和前脑了。 这个时候的未出生宝宝就被称为胎儿了。 13周 在之后的几周里，前脑生长得最快。 到13周，大脑的形状已经和成人的大脑很相似了，只是小很多。 25周 25周的时候，大脑表面还是光滑的，但随着它越长越大，褶皱开始慢慢出现了。 中脑此时已经生长到大脑内部了。 40周 宝宝一般会在怀孕约40周的时候出生。 新生儿的大脑看起来和成人的差不多，但体积只有成人的三分之一。 它现在还做不了太多事，但它能让宝宝用哭声获得关注呀！ 怀孕八周的人类胚胎，只有一颗树莓那么大！ 大脑的形成 宝宝在妈妈子宫里生长时，大脑也慢慢成型了。 大脑的发育既由宝宝DNA里的遗传指令控制，也受宝宝感知到的信息影响。 节省空间 覆盖在大脑外层的大脑皮层其实是一大片平整的神经元层，折叠起来才能塞进头骨里。 你把一张纸揉成团就能明白，褶皱是怎么缩小它占用的空间的。