organization - that is to say, no particular chimpanzees keep >53 Nitinol permanently Nitinol is one of the most extraordinary metals to be discovered together. On the contrary, individuals move about at will, this alone or in century: A simple alloy of nickel and titanium, nitinol has small groups best described as bands, which sometimes form into some perplexing large properties. A metal with a memory, it can be made to remember aggregations. any shape
They leave their associates if they want to, and join up with new into which it is fashioned, returning to that shape whenever it is ones heated.
without conflict. \The general practice is best For example, a piece of nitinol wire bent to form a circle that is described as \ then
come, easy go\ although there are certain heated and quenched will remember this shape. It may then be group-forming bent or
tendencies. As a rule chimpanzees move about in one of four crumpled, but on reheating, will violently untwist, types of band: reforming its
adult males only; mothers and offspring and occasionally a few original shape. This remarkable ability is called Shape other Memory Effect
females; adults and adolescents of both sexes, but no mothers (SME); other alloys, such as brasses, are known to with young possess it to a
and representatives of all categories mixed together. The limited extent. No one fully understands SME, and nitinol composition of remains
bands may change a number of times during the particularly perplexing, for, whenever it performs this peculiar course of a day feat, it
as individuals wander off and groups split or combine with appears to be breaking the laws of thermodynamics by springing other groups. back into
On the other hand, certain individuals prefer one another's shape with greater force than was used to deform it in the first company. One of place. the researchers observed that four males often roamed together But not only is nitinol capable of remembering, it also has over a the
four-month period, and mothers often associated with ability to \If the heating-cooling-crumpling-reheating their older process is offsprings. carried out sufficiently often, and the metal is always crumpled
in 黑猩猩
黑猩猩最突出的一个特征是它们社会生活的灵活性，即缺乏exactly the same way, the nitinol will not only remember its 固定的组织形式。 它的这 一特征与狒狒的那种组织形态间original
shape, but gradually it learns to remember its crumpled form
的差别，在灵长动物中最为突出。
as well, and
由此可见灵长类动物中 适应性变化的多样性。 黑猩猩比狒
will begin to return to the same crumpled shape every time it is
狒更接近人类，或者说它与我们心愿的自我形象更为接 cooled.
近：自由自在，不落窠臼，不喜欢任何形式的约束并且往往Eventually, the metal will crumple and uncrumple, totally 魅力十足(在狒狒中魅力比较罕 见)。 通过在黑猩猩的自然栖unaided, in
response to changes in temperature and without any sign of 息地森林中对它们进行了 8
metal fatigue. 个月的观察，两名研究人员得出这
样结论：\我们注意到在黑猩猩的社会中似乎没有任何一种独Engineers have produced prototype engines that are driven by the
force of
立的社会基本单位，这一点令 我们很吃惊。
nitinol springing from one shape to another as it alternately
它们不仅没有'家庭'或'妻妾'组织，也没有'团队'组织，也就是encounters 说黑猩猩没有固 定地生活在一起。 hot and cold water. The energy from these remarkable engines 相反，每只黑猩猩随意流动，或是独自一人，或是作为最好is,
称之为团伙 的一员。 团伙有时会与其它团伙合并成大的聚however, not entirely free: heat energy is
required to produce 合体。 它们可以随意离开同伴，并与其它黑
猩猩组成新的团伙而不会产生任何纠纷。\把黑猩猩这一总的the temperature differences needed to run the engine. But the
optimum 特性称之为\来得容易去得快\是最恰当不过了。 但它们也
temperatures at which the metal reacts can be controlled by
有一定的群体倾向性。
altering the
通常，黑猩猩群的构成有以下 4 种：仅 有成年雄性； 母猩proportions of nickel to titanium; some alloys will even perform 猩及其子女而且偶尔有几只其它的雌性猩猩； 雄性和雌性的at room 成年和未成 temperature. The necessary temperature range between the warm 年黑猩猩但不包括有子女的雌性黑猩猩； 以及各种类型混杂and the
在一起。 一天之中一个团伙的
组成可能变化好几次，因为有的成员可能离开，而且群体可cold can be as little as twelve degrees centigrade.
能会与其它群体合并。 另一方 面，有些黑猩猩有自己喜欢镍钛合金镍钛诺是这个世纪所发现的最不寻常的金属之一，
作为镍和钛的简单的一种合 金，镍钛诺具有一些令人惊叹的的伙伴。
一名研究人员发现四只雄性黑猩猩在 4 个月中常常共 同游特征。
这是一种有记忆力的金属，人们可以使它记住它被 塑成的任荡，还有母亲们常与她们较年长的子女们在一起。
何形状，并在加热后恢复这一形状。 比如，一节镍钛诺线
圈在加热冷却后会记住 圆圈这形状。
随后它可能被弯折成其它形状，但一旦再次加热，就会迅速地自动恢复成最 初的圆圈状。 这种不寻常的能力被称为形状记忆效果(SME)。
其它一些合金如黄铜在一定
程度上也具有这种特性。 目前人们对 SME 这一特性尚缺乏透彻的认识，而镍钛诺尤其使
人惊奇，因为每当它展现这一惊人的功能时，似乎都违背了热力学原理。 因为它在恢复原 有形状时所释放的力比人们使它变形所施加的力大得多。 镍钛诺不仅有记忆力，还能\学习
\。 如果加热-冷却-弯曲-再加热这一过程重复一定次数，且每次冷却后它都被丝毫不差地弯
成同一形状，它不仅可以记住最初的形状，还能逐渐记住它被弯成的形状，并开始在每次冷 却时恢复这一形状。 最终，它会自动地随温度变化而弯曲和恢复这些形状，并且没有任何 疲劳迹象。 工程师们已制造出一些发动机样机，利用镍钛合金在交替遇热水和冷水时迅速
改变形状所产生的力做推动力。 然而这些神奇的发动机工作时并非完全不耗能，因为必须 有热能来制造温差才能使机器运转。
但人们可以通过改变合金中镍钛的比例来控制使它反 应的最佳温度。 有的镍钛合金甚至能在室温下做出反应，冷暖之间的最小温差只有 12℃。 >54 Treasure in Sunken Ships
Of the tens of thousands of ships on the ocean bottom, only a handful, less
than 1 percent, contain negotiable treasure, such as gold and jewels. Most
give us a different priceless treasure
-- history. A sunken ship lies in trust, preserved in the airless environment of the sea and those in deep water are especially well
protected. No dry land sites anywhere -- except perhaps Egyptian tombs --
are in a better state of preservation than a vessel deep in the ocean. A
sunken ship, therefore, can be a rare window through which a moment in time is glimpsed.
This is not to imply that sunken ships are always found intact. Most ships
break up on the way down, hit the bottom at about 100 miles per hour, and
become a chaotic, confusing jumble. I recall the chagrin of a novice diver
who, after surfacing from an underwater tour of a 400-foot ship, asked his
diving buddy, \ takes experience to actually know a
sunken ship when one sees it. But no matter what its condition on the
way down, a ship deteriorates much more slowly as it sinks deeper into
protective layers of sand and mud. Ancient vessels have been found in
remarkably good condition. In 1977 a group of marine archaeologists
excavating a 900-year-old wreck recovered engraved glassware. Greek coins,
bronze kettles, and amazingly, Greek jars containing seeds, almonds, and
lentils -- even a plate with chicken bones. 沉船中的宝藏 在数以万计的海底沉船中，只有极少数，不到百分之一的沉船上有可流 通的财富，如黄金和珠宝。 多数沉船提供给我们的是另一种无价的财富--历史。
沉船被海 洋中没有空气的环境保管起来，躺在深水中的船只被保存得尤其完好。 大概除了埃及金字 塔外，陆地上没有哪些遗迹比深海沉船保存得更完好了。
因此，每一只沉船便成为我们探 视历史的一扇难得的窗户。 但这并不是说沉船被发现时都完好无损。 大多数船只在下沉过 程中就已破碎，并以每小时 100
英里的速度撞击海底，因此成为乱糟糟的一团。 我至今仍 记得一位潜水新手造访一艘 400 英尺长的海底沉船后浮出水面时的懊丧神情。他问他的潜水
伙伴：\沉船在哪儿?\ 看到沉船时能知道这是一只沉船是需要经验的。 但不管船在下沉时
状况如何，当它下沉到海底具有保护作用的泥沙层后，船体的朽烂速度比在陆地上要慢得多。 人们曾在海底发现过保存异常完好的古船。 1977
年，一群海洋考古学家们在发掘一只有 900 年历史的沉船时，找到了雕花玻璃器皿、希腊硬币、青铜水壶，并令人吃惊地发现一些盛有
种子、杏仁和扁豆等东西的希腊罐子，甚至找到一个盛有鸡骨的盘子。
>55 Creating Colors
There are two ways to create colors in a photograph. One method, called
additive, starts with three basic colors and adds them together to produce
some other colors. The second method, called subtractive, starts with
white light (a mixture of all colors in the spectrum) and by taking away
some or all other colors leaves the one desired.
In the additive method separate colored lights are combined to produce
various other colors. The three additive primary colors are green, red and
blue (each proportions, about one third of the wavelengths in the total
spectrum). Mixed in varying proportions, they can produce all colors.
Green and red light mix to produce yellow, red and blue light mix to
produce magenta, green and blue mix to produce cyan. When equal parts
of all three of these primary colored beams of light overlap, the mixture
appears white to the eye.
In the subtractive process, colors are produced when dye (as in paint or
color photographic materials) absorbs some wavelengths and so passes on
only part of the spectrum. The subtractive primaries are cyan (a
blulish green), magenta (a purplish pink), and yellow; these are
the pigments or dyes that absorb red, green and blue wavelengths,
respectively, thus subtracting them from white light. These dye colors
are the complementary colors to the three additive primaries of red,
green and blue. Properly combined, the subtractive primaries can absorb
all colors of light, producing black. But, mixed in varying proportions
they too can produce any color in the spectrum.
Whether a particular color is obtained by adding colored lights together
or by subtracting some light from the total spectrum, the vitamins and other wonder foods. There are numerous result looks the unsubstantiated reports
same to the eye. The additive process was employed for early that natural vitamins are superior to synthetic ones, that color fertilized eggs
photography. But the subtractive method, while requiring are nutritionally superior to unfertilized eggs, that untreated complex chemical grains are
techniques, has turned out to be more practical and is the better than fumigated grains and the like. basis of all One thing that most organically grown food products seem to modern color films. have in common
conventionally grown foods. But 创造颜色有两种方法可以使照片具有色彩。 一种叫加色法，is that they cost more than
in many cases 通过把三种基本颜色相叠 加以产生其它色彩。
另一种叫减色法，通过去除白色(其实是光谱中所有颜色相混consumers are misled if they believe organic foods can
maintain
合的结果) 中的某些或其它所有色彩而把所需的色彩留下
health and provide better nutritional quality than
来。 在加色法中，不同颜色的光线混合以产
conventionally
生其它各种色彩。 加色法三原色是绿、红和蓝(其中每一种grown foods. So there is real cause for concern if consumers 占据总光谱中三分之一的波长)。 这三种色彩以不同比例相particularly 叠加可以产生所有色彩。 those with limited incomes, distrust the regular food supply and 绿光和红光叠加可产生黄色，红光与蓝 光叠加可产生品红buy only
色，绿光与蓝光叠加可产生青绿色。 当这三种原色光以相同expensive organic foods instead.
绿色食品绿色食品是最佳食物吗?有人认为绿色食品比通过比例叠加时， 眼睛所见的就成为白色。
而在减色法中，染料(比如在绘画颜料或彩色照相材料中)吸收常规方式生产和销售的食品 有更多的优越性，然而现在却有了 部分波长而只允许分光谱通过，从而产生各种色彩。 减人对此提出异议。
绿色食品--这一词的用法非常不统一-- 支持者往往声称绿色色法三原色是青绿色、品红(略带紫
色的粉红色)和黄色； 也就是可分别吸收红光、绿光和蓝光食品比其它食品更安全，营养价值更高。 北美的消费者们的色料或染料，从而去除白色光 中的这些色彩。这些染料色越来越关注
他们的日常饮食是否安全和有营养。 这一发展趋势是好的。 是加色法的三原色红、绿、蓝的相互补充。
即食物供应适当组合时，减色 法三原色能吸收光线中的所有色彩，产生然而这种关注多半是由一些武 断的说法引起的，
黑色。 但当它们以不同的比例叠加时，也能产生 光谱中所不安全或不能提供足够的营养。
尽管这些说法多数未被科学 证实，但是由于大量文章支持这有颜色。
不论某种颜色是通过把不同色彩的光线叠加还是把光谱中某一观点，使得普通群众难辨真伪。 因此那些认为只食用绿 些颜色的光 去除掉而产生的，它的视觉效果是一样的。 早色食品就能防治疾病或增进健康的观点便广为人知，并导致
一些不科学的做法和习惯。 公 期彩色摄影采用加色法。 但减色法尽管需要
复杂的化学技术，最终比前者更加实用，并成为现代彩色电众几乎每天都被\抗衰老\食品、新型维生素以及其它具有神
奇功效的食物广告所包围。 声 影的基础。
称天然维生素优于人造维生素，受精蛋比未受精蛋的营养价>56 The Organic Foods
Are organically grown foods the best food choices? The 值更高，未经薰蒸消毒处理的谷 物比经过处理的好等等诸如advantages claimed 此类的报道屡见不鲜，但没有一篇是经过证实的。
for such foods over conventionally grown and marketed 大多数绿色 食品有一共同点，即它们的价格高于以常规方food products are 式种植的食物。 但在许多情况下，如果消费 now being debated. Advocates of organic foods -- a term
者认为绿色食品能预防疾病且营养价值高于以常规方式生产
whose meaning
的食品，那么他们被误导了。如
varies greatly -- frequently proclaim that such products are safer
果消费者们，尤其是那些收入有限的消费者们，怀疑传统食and more
品而只购买更昂贵的绿色食品， 那就值得担忧了。 nutritious than others.
The growing interest of consumers in the safety and nutritional >57 Lighthouses
The first navigational lights in the New World quality of
the typical North American diet is a welcome development. were probably
lanterns hung at harbor entrances. The first lighthouse However, much of
this interest has been sparked by sweeping claims that the food was put up by
the Massachusetts Bay Colony in 1716 on Little Brewster Island supply is
unsafe or inadequate in meeting nutritional needs. Although at the
entrance to Boston Harbor. Paid for and maintained by \most of these
claims are not supported by scientific evidence, the dues\
on ships, the original beacon was blown up in 1776. By then preponderance of
written material advancing such claims makes it difficult for the there were only
a dozen or so true lighthouses in the colonies. Little over a general
public to separate fact from fiction. As a result, claims that century
later, there were 700 lighthouses. eating a
diet consisting entirely of organically grown foods prevents or The first light erected on the West Coast in the 1850's featured
the same cures
disease or provides other benefits to health have become widely basic New England design: a Cape Cod dwelling with the
tower rising from publicized
the center or standing close by. In New England and and form the basis for folklore.
Almost daily the public is besieged by claims for \elsewhere, though,
lighthouses reflected a variety of architectural styles. Since diets, new
most
stations in the Northeast were built on rocky eminences, Researchers have found that migrating animals use a variety of enormous towers inner
were not the rule. Some were made of stone and brick, others of compasses to help them navigate. Some steer by the position of wood or the Sun.
metal. Some stood on pilings or stilts; some were fastened to Others navigate by the stars. Some use the Sun as their guide rock with during the
iron rods. Farther south, from Maryland through theFlorida Keys, day and then switch to star navigation by night. One study the coast shows that the
was low and sandy. It was often necessary to build tall towers homing pigeon uses the Earth's magnetic fields as a guide in there - finding its
massive structures like the majestic Cape Hatteras, North way home and there are indications that various other animals Carolina, from insects lighthouse, which was lit in 1870. At to mollusks, can also make use of magnetic compasses. It is of course very 190 feet, it is the tallest brick lighthouse in the country. useful for a migrating bird to be able to switch to aNotwithstanding differences in appearance and magnetic compass construction, most when clouds cover the Sun; otherwise it would just have to land American lighthouses shared several features: a light, living and wait quarters and for the Sun to come out again.
sometimes a bell (or later, a foghorn).They also had something Even with the Sun or stars to steer by, the problems of navigation else in are more
common: a keeper and, usually, the keeper's family. The complicated than they might seem atfirst. For example, a worker keeper's honeybee
essential task was trimming the lantern wick in order to that has found a rich source of nectar and pollen flies rapidly maintain a home to the
steady, bright flame. The earliest keepers came from every hive to report. A naturalist has discovered that the bee scout walk of life - delivers her
they were seamen, farmers, mechanics, rough mill hands - and report through a complicated dance in the hive, in which she appointments tells the
were often handed out by local customs commissioners as other workers not only how far away the food is, but also what political plums. direction to
After the administration of lighthouses was taken over in fly in relation to the Sun. But the Sun does not stay in one 1852 by the place all United States Lighthouse Board, an agency of the Treasury day. As the workers start out to gather the food, the Sun Department, the may already keeper corps gradually became highly professional. have changed its position in the sky somewhat. In later trips 灯塔在新大陆上，最初的航标灯要算那些挂在港湾入口的提during the
day, the Sun will seem to move farther and farther toward the 灯了吧。 最初的灯塔是
1716 年由马萨诸塞湾殖民地在波士顿港入口一个叫小布罗west. Yet the
worker bees seem to have no trouble at all in finding the food
斯特的岛上建起的。 这最初的灯 塔是通过向船只征收\买光
source.
钱\来维持运作的，于 1776
Their inner clocks tell them just where the Sun will be and they
年被炸毁。 那时，在各殖民地，真正 的灯塔仅有十几个。 change
但过了一个世纪多一点，灯塔就增至七百多个了。 1850 their course correspondingly. 年，在西部 动物的罗盘
沿海建起的首座灯塔基本上沿袭了新英格兰灯塔的设计方研究人员已经发现迁徙性动物可以借助各种各样的体内罗盘案，即那种中心或周围升起塔楼的 典型的哥德角式房子。 来校正方向。 有些动物借 助太阳的位臵辨别方向，有些则新英格兰与其它一些地方的灯塔在建筑风格上是多样化的。 依靠星星。
在东北 部，灯塔大都建在石质的高地上，因此塔身不需要太还有的白天利用太阳，晚上利用星星。 有研究 发现信鸽以高。 它们有砖石结构的； 也有木材或 地磁场为向导来找到回家的路。 还有迹象表明许许多多其金属结构的，有的灯塔建在支架上，有的则用铁条固定在岩它生物，从昆虫到软 石上。 在更南方的地区，从马 体动物，也能利用这种磁场罗盘。 具有这种磁场罗盘，对里兰到弗罗里达礁群的海岸地势低平且多为沙质，因此高大候鸟来说当然非常有用，因为如
的塔身就很必要了。 例如 1870 年在北卡罗来纳的哈特拉果乌云遮日，它可以以磁场为向导继续飞行，否则它就只好角建的宏伟的大灯塔，高达 190 着陆，等待太阳重新出现。 即 使是借助太阳或星星导航，英尺，是全美最高的砖结构灯塔。 虽然美国的灯塔在外观和其中的学问也比它表面看起来的要复杂得多。
结构上存在一些差异，但它们确有些共同之处，比如都有灯例如，一只工蜂 找到了花圃蜜园，急忙飞回蜂房报信。博物标、有 住房、有时还有钟(后来改用雾角)。 学家发现这只蜂探在蜂房里跳了一段极为复杂的
此外，还有一个相同之处，那就是都有守塔人，通常 是守塔舞，通过这段舞她不但告诉其它工蜂食物有多远，而且报告人全家。 守塔人的基本工作就是修剪灯蕊，以便火焰明亮而了以太阳为参照的飞行路线。 但 太阳不会呆在天上不动。 稳定。 最早的守塔人来 当蜂群出发采食时，太阳可能已经多少改变了它在天空中的自各种行业：海员、农民、技工和干粗活的。 这个职位通位臵。 出发得越迟，太阳就越偏西，可这似乎没给寻找食物常是作为政治回报由地方海关官 员决定任命。 但在 1852 源的工蜂们带来任何不便。 它们体内
年后，财政部下属的美国灯塔管理委员会接管了所有灯塔，的时钟告诉他们太阳应在的位臵，据此它们相应地改变前进守塔人 这支队伍也逐渐趋向专业化了。 的方向。 >58 Animals' Compasses
>59 Muscles and Human 它们才能转动，从而拖动骨骼绕某个 轴旋转。 有几块肌肉Body It is in the joints of the human body that 的作用较特殊，例如隔膜肌。 movements of the bones 它带动肺部吸入空气。 呼吸基本上不 是骨骼运动带动的。 take place. The movement itself is caused by the pull of >60 Colds and Age sheets and A critical factor that plays a part in susceptibility to colds is cords of very tough tissue called muscle. Muscle tissue has age. A the special study done by the University of Michigan School of Public ability to shorten itself so that the bone on which it pulls has to Health revealed move. particulars that seem to hold true for the general population. When muscle tissue shortens, it also bunches up. Muscle Infants are tissue covers the most cold ridden group, averaging more than six colds in the body in sheets and bands that lie between the skin and the their first skeleton. years. Boys have more colds than girls up to age three. After the The bones are the framework of the body, but the muscles fill out age of the body three, girls are more susceptible than boys, and teenage girls shape. Most muscles extend from one bone to another. When the average muscle three colds a year to boys' two. The general incidence of colds between the bones shortens, one bone has to move. The point continues where the to decline into maturity. Elderly people who are in good health muscle is fastened to the unmoving bone is called the origin have as few of the as one or two colds annually. One exception is found muscle, whereas the point where the muscle is not fastened to among people in the bone their twenties, especially women, who show a rise in cold that is to be moved is called the insertion. Sometimes the infections, muscle is not because people in this age group are most likely to have young attached directly to the bone but to a tough, nonstretchable cord, children. or Adults who delay having children until their thirties and tendon, that is attached to the bone. Muscles do not push; they forties can only experience the same sudden increase in cold infections. The pull. To bend the arm at the elbow, the muscle at the front of the study also upper found that economics plays an important role. As income arm has to shorten and bunch up. To unbend the arm other increases, the muscles in the frequency at which colds are reported in the family decreases. back of the arm have to shorten. These two sets of muscles - the Families front and with the lowest income suffer about a third more colds than the back - are said to act in opposition to each other. When one families at the set is upper end. Lower income generally forces people to live in working, the other set is usually relaxed. But there are times more cramped when both of quarters than those typically occupied by wealthier people, and them work. Sometimes muscles are called upon to do more than crowding simply pull in increases the opportunities for the cold virus to travel from one direction. They may have to perform a turning motion. To be person to able to do person. Low income may also adversely influence diet. The this, the muscle must be attached to the bone at an angle. By degree to pulling, the which poor nutrition affects susceptibility to colds is not yet muscle can cause the bone to pivot. A few muscles have special clearly functions. established, but an inadequate diet is suspected of lowering The diaphragm, for example, forces the lungs to take in air. This resistance part of generally. breathing is not primarily a bone moving operation.
感冒与年龄
肌肉和人体 骨骼运动发生在人体的关节部位。这种运动是
年龄是人们是否易患感冒的一个重要因素。 密歇根大学公
由称为肌肉的片状或条状的 强韧组织拖动而引起的。
共卫生学院揭示了一些对人 们普遍适用的细节。
肌肉组织有一种特殊的收缩能力，可以带动与之相连的骨骼。
婴儿期是最易感冒的时期，从出生到一周岁平均感冒六次。
肌 肉在收缩的同时，也会聚成团。 肌肉以条状或片状形式
在三周岁 前，男孩比女孩感冒次数要多。 但三周岁后，女
布满全身，存在于皮肤与骨架之间。
孩就更易感冒了。 十几岁的女孩年平均感
骨骼是身体的支架，而肌肉则构成了人的体形。 大多数肌
冒三次，男孩则为两次。 一般说来，随着年龄的增长，感
肉接两块骨头，但当肌肉收缩时 只有一块骨头运动。
冒的次数也越来越少。 健康的老 年人每年只感冒一至两次
肌肉与不动骨相连的一端叫肌起端，与动骨相连的一端叫附
或更少。
着端。 有 些肌肉不与骨骼直接相连，而是连着附在骨骼上
但在二十至三十岁之间的人尤其是妇女出现了例外。 他 们
的不能收缩的带状组织或腱上。 肌肉不能
感冒的次数有所增加，其原因在于人们通常在这一时期带养
推动而只能拉动骨骼。 要从肘部弯曲手臂，就要收缩上臂
小孩。 那些晚育至三十几岁
正面的肌肉。 要伸直手臂，背面 的肌肉就得收缩。
或四十几岁才有孩子的人，在那一时期感冒次数也会突然增
这两组肌肉，正面的和背面的，称为相反运动肌肉。 当一
加。 研究还发现，经济状况与 感冒息息相关。 随着收入
组工作时，另 一组就休息。 但有时它们也一起工作。 有
的增加，家庭成员的感冒次数呈下降趋势。
时我们要求肌肉不仅仅做简单的单向拖动。
家境贫寒的人感冒次 数要比最富裕的人多三分之一。 低收
它 们需要转动。 但只有当肌肉与骨骼形成一定角度时，
入者被迫挤住在比富人居所狭小得多的空间里。 拥挤 的状