首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   16883篇
  免费   191篇
  国内免费   1篇
  17075篇
  2023年   130篇
  2013年   757篇
  2012年   162篇
  2011年   179篇
  2010年   184篇
  2009年   335篇
  2008年   238篇
  2007年   234篇
  2006年   207篇
  2005年   199篇
  2004年   162篇
  2003年   164篇
  2002年   173篇
  1999年   116篇
  1998年   118篇
  1996年   159篇
  1995年   157篇
  1994年   123篇
  1993年   238篇
  1992年   221篇
  1991年   210篇
  1990年   238篇
  1989年   407篇
  1988年   358篇
  1987年   465篇
  1986年   403篇
  1985年   396篇
  1984年   327篇
  1983年   265篇
  1982年   323篇
  1981年   341篇
  1980年   325篇
  1979年   316篇
  1978年   368篇
  1977年   411篇
  1976年   427篇
  1975年   455篇
  1974年   310篇
  1973年   177篇
  1972年   175篇
  1971年   172篇
  1970年   173篇
  1969年   141篇
  1968年   142篇
  1967年   132篇
  1964年   165篇
  1960年   115篇
  1959年   113篇
  1958年   131篇
  1957年   163篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
Two sessions of relaxation instructions were administered under high and low expectancy conditions. Fifty-four college students scoring high on a self-report measure of anxiety served as subjects. Live and taped abbreviated progressive muscle relaxation instructions and a self-relaxation condition were equally effective in reducing within-session self-report and physiological indices of anxiety. High expectancy instructions led to greater reductions in heart rate than did low expectancy instructions. Factors controlling anxiety reduction during relaxation therapies are discussed.This experiment was submitted by the first author in partial fulfillment for a master of science degree at VPI and SU. The authors would like to thank Jean Sales and Dan Fones for their help in this investigation. The First author is now at Johns Hopkins Hospital, Baltimore. The second author is currently at the Department of Neurology, University of Wisconsin-Madison.  相似文献   
52.
Summary In captive African catfish, Clarias gariepinus, ovulation was induced with human chorionic gonadotropin (HCG) 4 I.U./g body weight to study the function of postovulatory follicles (POFs). Ultrastructural and enzyme-histochemical data indicate that, apart from special theca cells, the granulosa of relative young POFs (i.e., from 16 h and 28 h after HCG-injection) is capable of producing steroids. Possible functions of the synthesized steroids are discussed. Histological comparison of POFs from stripped and from unstripped fish, as well as histochemical investigation of the contents of ovulated ova and granulosa of POFs at 48 h after HCG-injection, showed that the latter structure is involved in phagocytosis of the disintegrating ovulated eggs. The polysaccharide-lipid-protein material, initially taken up by heterophagolysosomes of the granulosa cells, subsequently undergoes fatty degeneration. The granulosa cells of the POFs showed strong acid phosphatase activity and abundant granular endoplasmic reticulum from 16 h after HCG-injection onward; heterophagolysosomes appeared at 32 h. These results indicate that after ovulation the phagocytotic function of the granulosa develops progressively. Autophagolysosomes, responsible for the final disintegration of POFs, become increasingly evident in the granulosa cells with increasing time after ovulation.  相似文献   
53.
Summary Distribution of glycogen particles in semithin and ultrathin sections of biopsy samples from human muscles subjected to either short- or long-term running were investigated using PAS and Periodic Acid-ThioSemiCarbazide-Silver Proteinate (PA-TSC-SP) staining methods. Glycogen particles were predominantly found immediately under the sarcolemma or aligned along the myofibrillar Iband. After long-term exhaustive exercise type-1 fibers with a few or no glycogen particles in the core of the fibers were frequently observed. The subsarcolemmal glycogen stores of these depleted type-1 fibers were about three times as large as after exhaustive short-time exercise. Another indication of utilization of subsarcolemmal glycogen stores during anaerobic exercise was that many particles displayed a pale, rudimentary shape. This observation suggests fragmental metabolization of glycogen. Thus, depending on type of exercise and type of fiber differential and sequential glycogen utilization patterns can be observed.  相似文献   
54.
Comparisons are made of self-reported medical costs from a sample of headache patients who underwent various combinations of relaxation training and biofeedback training. The average costs for the 2 years prior to self-regulatory treatment were $955±480 (3 SEM) for 45 patients; for the 2 years after completing treatment the average costs were $52±28 (3 SEM) for patients. Within the limitations of the study, medical costs do seem to have been markedly reduced.This research was supported by a grant from NINCDS, NS-15235.  相似文献   
55.
Summary The distribution of specific nuclear binding sites for androgens and estrogens in the male accessory sex organs of the mouse was assessed by autoradiography with3H dihydrotestosterone (3H DHT) and3H estradiol (3H E2). With3H DHT nuclear labeling differed among the epithelia of the organs. It was high in seminal vesicle and ampullary gland, moderate in ventral prostate, urethral gland, prostatic excretory ducts and the ampulla ductus deferentis, low in dorsal prostate and low or absent in coagulation gland. With3H E2, in contrast, epithelial nuclear labeling was high only in coagulation gland, moderate or low in seminal vesicle, low or absent in ventral and dorsal prostate and absent in ampullary gland and ampulla ductus deferentis. In the lamina propria of all organs nuclear labeling with3H DHT was generally moderate and existed only in some cells, with the highest number in the ampulla ductus deferentis. With3H E2, nuclear labeling in the lamina propria showed a high intensity in all organs, except in ventral and dorsal prostate which remained unlabeled. Many labeled cells were found in the deferent duct and its ampulla, while in the other organs only a few cells showed nuclear labeling with3H E2. In the smooth muscle sheath of all organs, some muscle cells were moderately labeled with3H DHT, but not with3H E2. The results indicate the presence of nuclear receptors in male accessory sex organs for both dihydrotestosterone and estradiol. The differential patterns of3H DHT and3H E2 nuclear uptake suggest differential sensitivities of the individual organs and their tissue compartments for androgens and estrogens. Supported by PHS grant NSO9914 to W.E.S. and Deutsche Forschungsgemeinschaft Dr94/4 to U.D. The work of Dr. Schleicher and his stay in Chapel Hill were additionally sponsored by Studienstiftung des Deutschen Volkes and Boehringer-Ingelheim Fonds  相似文献   
56.
57.
We developed a method of hybrid selection between homothallic wild-type and heterothallic strains. The hybrids obtained were used to study the heredity of ethanol tolerance and production. Both characters segregated independently, but no ethanol-sensitive strains were able to produce high levels of ethanol. At least four genes are implicated in ethanol tolerance.  相似文献   
58.
We examined the effect of interacting dipicolinic acid and its calcium chelate on the wet and dry density of DNA. Complexes are produced whose densities are different from those of the individual components. Also, we observed two modes of binding, one strong the other weak, between DPA or CaDPA and DNA. The strength of the binding modes was reflected in the rate of dissolution of the complexes as monitored by changes in wet density with time and temperature. We conclude from these and other data in the literature that the interaction of dipicolinic acid with DNA not only influences the spore wet density and the ratio of core/core+ cortex volume, but may also influence the spore heat resistance.  相似文献   
59.
A Gram-negative, psychrophilic bacterium, designated GB-2, was isolated from fetal calf serum and analyzed for its morphological, physiological, and biochemical characteristics. Gliding motility, sensitivity to actinomycin D, the presence of the pigment flexirubin and cytochrome c, growth on a variety of carbohydrates, the production of acid fermentation products, and a 34.9 mol% guanine+cytosine (G+C) content of bacterial DNA indicate GB-2 to be a member either of the generaCytophaga orFlexibacter. Growth of GB-2 was optimized in a simple defined medium to facilitate isolation and characterization of bacterial products. Liquid growth of GB-2 resulted in the release of significant quantities of a macromolecule free of both endotoxin and protein into the growth supernatant, which activated the proliferation of murine lymphocytes. The relationship between this bacterium and its end-products to other species of theCytophaga/Flexibacter group is discussed.  相似文献   
60.
The frequency of ultraviolet(UV)-induced mutations drops rapidly whenEscherichia coli Hcr+ cells (strains WP-2 Hcr+; B/r) are incubated on phosphate-buffered agar (PBA), but is reduced only slightly if chloroquine or quinacrine are incorporated into the medium. The excision-deficient WP-2 Hcr strain shows little reduction in the number of mutants when incubated on PBA. During postirradiation incubation on PBA, cell viability was relatively unaffected by the presence of the chemicals in the PBA (25 g/ml quinacrine; 50 g/ml chloroquine). When cells were given optimal doses of photoreactivating light, no further decline in mutations was obtained during subsequent incubation on PBA. Approximately 64% of the mutants seen when cells are treated with UV-PBA-chloroquine and 90% seen with UV-PBA-quinacrine can be repaired if cells are incubated on PBA. When these chemicals were added to the PBA, both excision-proficient strains (WP-2 Hcr+; B/r) demonstrated a marked reduction in the repair of UV-induced mutations to streptomycin resistance. Our results indicate that these chemicals interfere with the excision of UV-induced pyrimidine dimers, a process that normally occurs during postirradiation incubation on PBA.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号