The study of the structure of an organism is called [1] anatomy , and it relies heavily on the careful cutting and separation of tissues, called [2] dissection . Microscopic study of the structure of cells and tissues is called [3] microscopic anatomy or histology . In the physical examination of a living person, the body is studied not only by observation but also by [4] palpation , or touch, and by [5] auscultation , listening to the sounds it makes. The study of function is called [6] physiology . Much has been learned about the human body through [7] comparative anatomy and physiology, in which generalizations are drawn from the study of a variety of animal species. A major theme of this book, the [8] unity of form and function, stresses that anatomy and physiology are not isolated sciences but are mutually dependent.

The Greek physician [9] Hippocrates was one of the first to argue that all natural phenomena such as diseases have natural causes that can be understood through rational inquiry. The philosopher [10] Aristotle believed in both natural and supernatural causes of things, but he distinguished between the two, and it is from his term for natural causes that we get the words physiology and physician. The Roman physician [11] Claudius Galen was a careful anatomist, although hampered by the prohibition against dissecting [12] cadavers in his time. He advised readers to trust their own observations more than books, including his own, but his advice was ignored and scholars of the Middle Ages made a dogma of his work. The medieval Muslim scholar [13] Avicenna studied the Greek literature and added many new observations of his own, becoming a leading medical authority of his era.

After the Renaissance, [14] Andreas Vesalius came to be called the father of modern anatomy because he dissected cadavers first-hand, published detailed illustrations of their anatomy, and revolutionized the teaching of medical students. His counterpart in physiology was [15] William Harvey , best known for his innovative experimental studies of blood circulation. A simple microscope developed by Dutch textile merchant [16] Antony van Leeuwenhoek opened the door to a new level of understanding of anatomy. His admirer, [17] Robert Hooke , further improved on the device by adding a second lens, creating a [18] compound microscope more like those in common use today. He also was the first to use the word cell in biology. However, it was not generally appreciated that all living things are made of cells until the nineteenth century when botanist Mathias Schleiden and zoologist [19] Theodor Schwann studied a broader range of species with the microscope.

The habit of making numerous observations and drawing generalizations from them, called the [20] inductive method of science, is the primary method used in human anatomy. The [21] hypothetico-deductive method, employed in studies of physiology, attempts to answer a question by formulating an educated guess called a/an [22] hypothesis , then making a [23] deduction about what should be observed if that guess is correct and a certain observation or experiment is carried out. A valid experiment requires a [24] control group, which is treated the same as the experimental group except for the one variable being studied. In human research, it is especially important to control for effects of the subject’s mental state, which are called [25] psychosomatic effects. One way of doing this is to give a control group a neutral substance called a/an [26] placebo , and preferably to use a [27] double-blind procedure in which neither the person receiving treatment nor the person giving it knows whether the substance administered is a drug or a 26. After data have been collected, [28] statistical tests are needed to determine whether the data suggest a significant effect of the treatment. Another way to help maintain the quality and objectivity of science is [29] peer review , the process of having one’s research evaluated by other experts in the field. Many scientific findings are expressed as concise statements or mathematical formulae called [30] laws of nature , which express predictable relationships between natural causes and effects. When many hypotheses and laws have been confirmed by repeated observations, the findings may be summarized in a [31] theory , which then provides suggestions and direction for further research.

Any genetic change in a population from generation to generation is called [32] evolution . Through the process of [33] natural selection , as Darwin called his theory, some genetic variants leave more offspring than others. Environmental factors called [34] selection pressures lead to the establishment of [35] adaptations in a species--features than enable it to function more effectively in its environment. The prehensile hands of primates, for example, are a/an 35 to their [36] arboreal , or treetop, habitat. Three-dimensional or [37] stereoscopic vision, as well as color vision, also adapted primates to this habitat; thus we can understand the origins of these traits, which were eventually passed down to humans. However, much of human anatomy became remodeled as an adaptation to [38] bipedalism , the upright locomotion favorable to survival in the African grasslands. The helpless state of our newborn infants can be traced to a combination of skeletal remodeling and the dramatic increase in [39] brain volume seen over the course of human evolution.

Among other qualities, living things can be recognized by the fact that they are all composed of [40] cells ; they all grow and maintain themselves through chemical changes collectively called [41] metabolism ; and they are all capable of responding to environmental changes called [42] stimuli . Of the five kingdoms into which living things are often divided, humans belong to the [43] Animal kingdom. The presence of a dorsal supportive rod, the [44] notochord , and pharyngeal [45] gill pouches at some stage of development places humans in the phylum Chordata. We are classified as mammals because of our hair, mammary glands, varied types of teeth, the generation of most of our body heat by our own metabolism, or [46] endothermy , and the presence of only one movable jawbone but three [47] middle-ear bones . Our flat nails, clavicles, and [48] opposable thumbs help define us as primates, and we are placed in the family Hominidae because of our [49] bipedal locomotion, the ability to walk easily on two legs. The human body has a hierarchy of structural complexity. [50] Cells are the smallest units of an organism that carry out the functions of life, mainly by still smaller internal structures called [51] organelles . 50 combine to form [52] tissues , which form a discrete region of an organ and perform a specific function. There are 11 [53] organ systems , each composed of several organs.

French physiologist [54] Claude Bernard pointed out in the nineteenth century that the body maintains a relatively stable internal environment, although Hippocrates had realized essentially the same point. In 1932, Walter Cannon called this tendency [55] homeostasis . This principle states that for a given physiological variable (such as temperature, pH, or blood glucose), the body has a certain average called a [56] set point and maintains a state of [57] dynamic equilibrium , in which there are limited fluctuations around this value. This state is maintained by a self-corrective mechanism called [58] negative feedback . The opposite of this, a self-amplifying process called [59] positive feedback , sometimes contributes to maintenance of health, especially in processes where rapid change is needed, but it often results in dangerous physiological imbalances. Recovery from disease is typically a process of stopping 59 and restoring 55.