只有人类才有下巴？揭秘关于人体的25个惊人事实

人类的身体堪称这个世界上最神秘、最完美的杰作。科学家探索了几千年，也没能将人体完全搞清楚。你知道人类是唯一有下巴的动物吗？你知道人类为什么会起鸡皮疙瘩吗？你知道拇指有自己的脉搏吗？关于人体的25个惊人事实，跟我们一起了解一下。

1. It’s possible to brush your teeth too aggressively. Doing so can wear down enamel and make teeth sensitive to hot and cold foods.

刷牙时有可能会过于剧烈。这么做会磨损牙釉质，使得牙齿对冷热食物敏感。

2. Goose bumps evolved to make our ancestors’ hair stand up, making them appear more threatening to predators.

我们的祖先之所以会进化出鸡皮疙瘩，是为了令自己的毛发竖起来，以此对掠食者更具威胁性。

3. Wisdom teeth serve no purpose. They’re left over from hundreds of thousands of years ago. As early humans’ brains grew bigger, it reduced space in the mouth, crowding out this third set of molars.

智齿没有任何用处。它们只是几十万年前的遗留产物。由于早期人类的大脑逐渐增大，进而缩减了口腔内的空间，从而挤掉了第三大臼齿。

molar[\'molɚ]: n. 臼齿，磨牙

4. Scientists aren\'t exactly sure why we yawn, but it may help regulate body temperature.

科学家们并不完全确定我们打哈欠的原因，但打哈欠可能有助于调节体温。

5. Your fingernails don’t actually grow after you’re dead.

你死后你的指甲不会继续长。

6. If they were laid end to end, all of the blood vessels in the human body would encircle the Earth four times.

如果把人体内的所有血管都连成一条直线，其长度可环绕地球四圈。

7. Humans are the only animals with chins.

人类是唯一有下巴的动物。

8. As you breathe, most of the air is going in and out of one nostril. Every few hours, the workload shifts to the other nostril.

当你呼吸时，大部分空气都会只通过一个鼻孔进出。每隔几个小时，会换到另一个鼻孔。

9. Blood makes up about 8 percent of your total body weight.

血液占你总体重的8％左右。

10. The human nose can detect about 1 trillion smells.

人类的鼻子可以检测到大约1万亿种气味。

11. You have two kidneys, but only one is necessary to live.

虽然你有两个肾脏，但只需要一个肾就能活下来。

12. Belly buttons grow special hairs to catch lint.

你的肚脐会长出特殊毛发以捕捉衣物中的棉绒纤维。

lint[lɪnt]: n. 线头；棉绒

13. The satisfying sound of cracking your knuckles comes from gas bubbles bursting in your joints.

掰手指时所听到的让你畅快的“咔咔”响声来自于关节中的气泡破裂声。

14. Skin is the body’s largest organ and can comprise 15 percent of a person’s total weight.

皮肤是人体最大的器官，可占人体总重量的15％。

15. Thumbs have their own pulse.

拇指有自己的脉搏。

16. Your tongue is made up of eight interwoven muscles, similar in structure to an elephant’s trunk or an octopus’s tentacle.

你的舌头由八块交织的肌肉组成，其结构与大象的长鼻或章鱼的触手相似。

trunk[trʌŋk]: n. 象鼻

17. On a genetic level, all human beings are more than 99 percent identical.

所有人类的基因有超过99％是相同的。

18. The foot is one of the most ticklish parts of the body.

脚是身体最怕痒的部位之一。

19. Extraocular muscles in the eye are the body’s fastest muscles. They allow both of your eyes to flick in the same direction in a single 50-millisecond movement.

眼外肌是人体中移动速度最快的肌肉。它们使得你的双眼可在50毫秒的运动中扫向相同的方向。

extraocular[,ɛkstrə\'ɑkjəlɚ]: adj. 眼外的

20. A surgical procedure called a selective amygdalohippocampectomy removes half of the brain’s amygdala—and with it, the patient’s sense of fear.

一种被称为“选择性杏仁核海马切除术”的外科手术可切除大脑中的一半杏仁核，从而消除掉患者的恐惧感。

21. The pineal gland, which secretes the hormone melatonin, got its name from its shape, which resembles a pine nut.

分泌褪黑激素的松果体之所以叫这个名字，是因为其形状看上去类似于松果。

22. Hair grows fast—about 6 inches per year. The only thing in the body that grows faster is bone marrow.

头发生长速度很快：每年约6英寸（15厘米）。而身体中唯一比头发长得更快的是：骨髓。

23. No one really knows what fingerprints are for, but they might help wick water away from our hands, prevent blisters, or improve touch.

没有人真正知道指纹的作用是什么，但它们可能有助于从手上吸走水份、防止水泡或改善触感。

24. The heart beats more than 3 billion times in the average human lifespan.

以平均寿命来计算，人的一生中心脏可跳动超过30亿次。

25. Blushing is caused by a rush of adrenaline.

脸红是由肾上腺素激增引起的。