46.vision by
human vision like that of other 2 has evolved in an 3 environment. in the 4 complex world of a tropical forest, it is more important to see well that to develop an acute sense of smell. in the course of evolution members of the 1 line have acquired large eyes while the snout has shrunk to give the eye an unimpeded view. of mammals only humans and some primates enjoy color vision. the red flag is black to the bull. horses live in a monochrome world .light visible to human eyes however occupies only a very narrow band in the whole electromagnetic 5. ultraviolet rays are invisible to humans though ants and honeybees are sensitive to them. humans though ants and honeybees are sensitive to them. humans have no direct perception of 6 rays unlike the rattlesnake which has receptors 7 into 8 longer than 0.7 micron. the world would look 9 different if human eyes were sensitive to infrared radiation. then instead of the darkness of night, we would be able to move easily in a strange shadowless world where objects glowed with varying degrees of 10. but human eyes excel in other ways. they are in fact 11 discerning in color gradation. the color sensitivity of normal human vision is rarely surpassed even by sophisticated technical devices.
47 folk cultures
a folk culture is a small 12, 13, conservative, nearly self-sufficient group that is homogeneous in custom and race with a strong family or 14 structure and highly developed rituals. order is maintained through sanctions based in the religion or family and interpersonal. relationships are strong. tradition is 15, and change comes infrequently and slowly. there is 16 little division of 17 into 18 duties. rather, each person is expected to perform a great variety of tasks, though duties may differ between the sexes. most goods are handmade and subsistence economy prevails. individualism is weakly developed in folk cultures as are social classes. unaltered folk cultures no longer exist in industrialized countries such as the united states and canada. perhaps the nearest modern equivalent in anglo america is the amish, a german american farming 19 that largely 20 the products and labor saving devices of
the industrial age. in amish areas, horse 21 buggies still serve as a local transportation device and the faithful are not permitted to own 22. the amish’s central religious concept of demut “humility”, clearly reflects the weakness of individualism and social class so typical of folk cultures and there is a corresponding strength of amish group identity. rarely do the amish marry outside their sect. the religion, a variety of the mennonite faith, provides the principal 23 for maintaining order.
by contrast a popular culture is a large 24 group often highly individualistic and a pronounced many specialized professions. 25 institutions of control such as the police and army take the place of religion and family in maintaining order, and a money-based economy prevails. because of these contrasts, “popular” may be viewed as clearly different from “folk”. the popular is replacing the folk in industrialized countries and in many developing nations. folk-made objects give way to their popular equivalent, usually because the popular item is more quickly or cheaply produced, is easier or time saving to use or leads more prestige to the owner.
48 bacteria
bacteria are extremely small living things. while we measure our own sizes in inches or centimeters, 26 size is measured in microns. one micron is a thousandth of a millimeter: a pinhead is about a millimeter across. rod-shaped bacteria are usually from two to four microns long, while rounded ones are generally one micron in diameter. thus if you enlarged a rounded 27 a thousand times, it would be just about the size of a pinhead. an adult human magnified by the same amount would be over a mile(1.6 kilometer) tall.
even with an ordinary microscope, you must look closely to see bacteria. using a magnification of 100 times, one finds that bacteria are barely visible as tiny rods or dots. one cannot make out anything of their structure. using special stains, one can see that some bacteria have attached to them wavy-looking “hairs” called 28. others have only one flagellum. the flagella rotate, pushing the bacteria through the water. many bacteria lack flagella and cannot move about by their own power, while others can 29 along over surfaces by some little-understood mechanism.
from the bacteria point of view, the world is a very different place from what it is to humans. to a bacterium water is as thick as molasses is to us. bacteria are so small that they are influenced by the movements of the chemical 30 around them. bacteria under the microscope, even those with no flagella, often bounce about in the water. this is because they collide with the 31 molecules and are pushed this way and that. molecules move so rapidly that within a tenth of a second the molecules around a bacteria have all been replaced by new ones; even bacteria without flagella are thus constantly exposed to a changing environment.
49 sleep
32 is part of a person’s daily activity cycle. there are several different stages of sleep, and they too occur in cycles. if you are an average 33, your sleep cycle is as follows. when you fist drift off into 34, your eyes will roll about a bit, you temperature will drop slightly, your muscles will relax, and your breathing well slow and become quite regular. your brain waves slow and become quite regular. your brain waves slow down a bit too, with the alpha rhythm of rather fast waves 1 sleep. for the next half hour or so, as you relax more and more, you will drift down through stage 2 and stage 3 sleep. the lower your stage of sleep. slower your brain waves will be. then about 40to 69 minutes after you lose consciousness you will have reached the deepest sleep of all. your brain will show the large slow waves that are known as the 35 rhythm. this is stage 4 sleep.
you do not remain at this deep fourth stage all night long, but instead about 80 minutes after you fall into slumber, your brain activity level will increase again slightly. the delta rhythm will disappear, to be replaced by the activity pattern of brain waves. your eyes will begin to 36 around under your closed 37 as if you were looking at something occurring in front of you. this period of rapid eye movement lasts for some 8 to 15 minutes and is called rem sleep. it is during rem sleep period, your body will soon relax again, your breathing will slip gently back from stage 1 to stage 4 sleep----only to rise once again to the surface of near consciousness some 80 minutes later.
50. cells and temperature
cells cannot remain alive outside certain limits of temperature and much narrower limits mark the boundaries of effective functioning. 38 systems of mammals and birds are most efficient only within a narrow range around 37c;a departure of a few degrees from this value seriously 39 their functioning. even though cells can survive wider 40 the integrated actions of bodily systems are 41. other animals have a wider 42 for changes of bodily temperature.
for centuries it has been recognized that mammals and birds differ from other animals in the way they regulate body temperature. ways of characterizing the difference have become more accurate and meaningful over time, but popular 43 still reflects the old division into “warm-blooded” and “cold-blooded” species; warm-blooded included mammals and birds whereas all other creatures were considered cold-blooded. as more species were studied, it became evident that this classification was 44. a fence 45 or a desert iguana—each cold-blooded----usually has a body temperature only a degree or two below that of humans and so is not cold. therefore the next distinction was made between animals that maintain a constant body temperature, called home0therms, and those whose body temperature varies with their environments, called poikilotherms. but this classification also proved inadequate, because among mammals there are many that vary their body temperatures during 46. furthermore, many 47 that live in the depths of the ocean never experience change in the depths of the ocean never experience change in the chill of the deep water, and their body temperatures remain constant.
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