全文获取类型
收费全文 | 5879篇 |
免费 | 589篇 |
国内免费 | 1篇 |
专业分类
6469篇 |
出版年
2021年 | 64篇 |
2020年 | 37篇 |
2018年 | 71篇 |
2017年 | 54篇 |
2016年 | 104篇 |
2015年 | 168篇 |
2014年 | 199篇 |
2013年 | 231篇 |
2012年 | 303篇 |
2011年 | 337篇 |
2010年 | 207篇 |
2009年 | 172篇 |
2008年 | 280篇 |
2007年 | 290篇 |
2006年 | 296篇 |
2005年 | 306篇 |
2004年 | 286篇 |
2003年 | 257篇 |
2002年 | 265篇 |
2001年 | 108篇 |
2000年 | 88篇 |
1999年 | 101篇 |
1998年 | 99篇 |
1997年 | 90篇 |
1996年 | 72篇 |
1995年 | 62篇 |
1994年 | 60篇 |
1993年 | 55篇 |
1992年 | 73篇 |
1991年 | 58篇 |
1990年 | 46篇 |
1989年 | 68篇 |
1988年 | 58篇 |
1987年 | 67篇 |
1986年 | 53篇 |
1985年 | 66篇 |
1984年 | 68篇 |
1983年 | 50篇 |
1982年 | 74篇 |
1981年 | 70篇 |
1980年 | 66篇 |
1979年 | 82篇 |
1978年 | 51篇 |
1977年 | 52篇 |
1976年 | 51篇 |
1975年 | 44篇 |
1974年 | 58篇 |
1973年 | 51篇 |
1972年 | 36篇 |
1970年 | 37篇 |
排序方式: 共有6469条查询结果,搜索用时 0 毫秒
101.
Physical illness or disability inevitably has a damaging effect on sexual relationships. Physicians are usually unaware of the sexual consequences of illness on their patients, and lack experience in treating sexual dysfunctions.The report of treatment of a couple with serious cardiovascular disease illustrates the potential efficacy of brief sex therapy for improving the quality of a patient''s life. If a primary physician lacks the skills to conduct sex therapy, he may collaborate with nonphysician therapists. The physician''s knowledge of the physiological and psychological effects of a specific illness on his patient is essential to successful therapy. Often, simple education, encouragement or reassurance by the physician is enough to overcome the damaging effects of illness on a patient''s sex life. 相似文献
102.
Fred C. June 《Environmental Biology of Fishes》1977,2(3):285-296
Synopsis The timing of ovarian maturation and spawning of 17 warmwater fish species in Lake Oahe (South and North Dakota) was estimated from changes in the mean ovary indices (ratios of ovary weight to fish length). The onset of vitellogenesis varied within species (up to 2 months). Maturation of the ova took from 7.5 to 10 months, depending on species. Annual variations in the mean date of peak spawning of individual species during 1964–1971 were usually less than a week. There was little overlap of the annual mean peak spawning dates of the 17 species, and an established sequence of spawning among species was shown. A relatively high incidence of atresia in the shovelnose sturgeon, northern pike, and carp indicated that these species had apparently not yet adapted to the altered and variable spawning conditions in this reservoir. Regularity of spawning would seem to provide the best chance for spawning success in variable environments such as Lake Oahe. 相似文献
103.
104.
Measurements of stress relaxation in uniaxial extension and associated time-dependent birefringence have been made on bovine fibrin film, prepared by gentle compaction of coarse fibrin clots, containing 13–22% fibrin plasticized with either aqueous buffer or glycerol. Both unligated and ligated (i.e., with α-α and γ-γ ligation by fibrinoligase, factor XIIIa) films were studied. Both types showed two stages of stress relaxation, with time scales of approximately 10 and 103–104 s, respectively, with a plateau region between. In the plateau, the nominal (engineering) stress for ligated glycerol-plasticized film is proportional to In λ, where λ is the stretch ratio, up to λ ? 2, and it decreases with increasing temperature. For unligated glycerol-plasticized film, the stresses are smaller by a factor of one-half to one-third. For ligated film, the second stage of relaxation is relatively slight, and recovery after release of stress is often nearly complete. For unligated film, the second stage involves a substantial drop in stress, and after recovery there is a significant permanent set. A second relaxation for ligated film reproduces the first, but for unligated film it reproduces the first only if the initial relaxation is terminated before the second stage; otherwise, the second relaxation shows a weaker structure. The behavior of water-plasticized film is similar to that of glycerol-plasticized except that the second stage of relaxation occurs at shorter times. During the first stage of stress relaxation, up to about 100 s, the birefringence and the stress-optical coefficient increase; during the plateau zone of stress relaxation, the birefringence of ligated films is approximately constant and is proportional to 2λ2/(λ2 + 1) ? 1, where λ is the stretch ratio. This dependence is predicted by a two-dimensional model in which rodlike elements in the plane of the film are oriented with independent alignment. During the final stage of stress relaxation, the birefringence of ligated films decreases slightly; that of unligated films decreases substantially, but less rapidly than the stress, corresponding to a further increase in the stress-optical coefficient. With additional information from small-angle x-ray scattering reported in an accompanying paper, the first stage of relaxation is attributed to partial release of bending forces in the fibers by orientation, accompanied by increased birefringence. The second stage is attributed, for ligated films, to an internal transition in the fibrin units accompanied by elongation of some of the fibers; and in the unligated films, to a combination of the latter transition with slippage of protofibrils lengthwise within the fiber bundles that causes some loss of orientation, which diminishes the birefringence. 相似文献
105.
106.
G. Hartmann 《Helgoland Marine Research》1991,45(1-2):165-198
The present paper deals with ostracods collected by H. Kunz during October and November 1979 on the Hawaiian Islands (Big
Island, Maui and Kauai). 26 species were found, nine species are described, two were already known to science, 15 remain in
open nomenclature. Although some of the new species are represented only by a single specimen, the morphologic characters
permit to distinguish them from other species of the genus. The ostracods of the Hawaiian Islands are very imperfectly known.
相似文献
107.
J P Kleman D J Hartmann F Ramirez M van der Rest 《European journal of biochemistry》1992,210(1):329-335
The biosynthesis of collagen by the A204 cell line was examined using polyclonal antibodies raised against collagen type V and type XI. The study of the pepsin-digested collagen showed that it is composed mainly of alpha 1(XI) and alpha 2(V) collagen chains in an apparent 2:1 ratio, suggesting the formation of heterotypic molecules [alpha 1(XI)]2 alpha 2(V). The existence of this chain stoichiometry was further demonstrated by immunoprecipitation of the molecule with an antibody recognizing alpha 2(V) but not alpha 1(XI) collagen chains. Electron microscopy analyses of 24-h cultures showed that this matrix is composed of thin fibrils, that can be decorated with immunogold-labelled anti-(type-V collagen) IgG, but not with anti-(type-XI collagen) IgG. The collagen matrix laid down by A204 cells is highly insoluble. In the presence of beta-aminopropionitrile, an inhibitor of lysyl oxidase, only a small proportion of intact collagen could be extracted without proteolytic treatment. Immunoblotting of intact medium collagen from cultures performed in the presence of beta-aminopropionitrile showed four distinct bands with each antibody. The migration of the bands, stained with anti-(type-V collagen) IgG, had apparent molecular masses of 127, 149, 161 and 198 kDa (compared to globular standards) while the bands stained with anti-(type-XI collagen) IgG had apparent masses of 145, 182, 207 and 225 kDa. These data indicate that type-V and type-XI collagen chains can assemble in heterotypic isoforms. In this system, the synthesized isoforms are able to aggregate into a highly cohesive matrix and they undergo a proteolytic processing closely similar to that of other fibrillar collagens. 相似文献
108.
Amyloidogenicity of beta A4 and beta A4-bearing amyloid protein precursor fragments by metal-catalyzed oxidation. 总被引:10,自引:0,他引:10
T Dyrks E Dyrks T Hartmann C Masters K Beyreuther 《The Journal of biological chemistry》1992,267(25):18210-18217
Previously we have shown that the COOH-terminal 100 residues (A4CT) of the amyloid protein precursor (APP), which carry the sequence of the amyloid beta A4 protein of Alzheimer's disease at N-terminal position, form highly insoluble aggregates if expressed in the rabbit reticulocyte lysate and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (Dyrks, T., Weidemann, A., Multhaup, G., Salbaum, J.M., Lemaire, H.-G., Kang, J., Müller-Hill, B., Masters, C. L., and Beyreuther, K. (1988) EMBO J. 7, 949-957). Here we report that aggregation of this COOH-terminal APP fragment A4CT and also of beta A4 itself depends on additional factors. In contrast to the reticulocyte expression system, expression of A4CT and beta A4 in the wheat germ expression system resulted in only monomeric forms. We have identified the factors which are capable of transforming both soluble A4CT and beta A4 into insoluble and aggregating molecules. Monomeric A4CT or beta A4 expressed in the wheat germ lysate could be transformed into aggregating molecules by the addition of metal-catalyzed oxidation systems. The addition of radical scavengers such as ascorbic acid, trolox, and amino acids prevented the aggregation process induced by the radical initiators. Thus, the aggregation of amyloidogenic APP fragments if analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis requires amino acid oxidation and protein cross-linking induced by radical generation systems. 相似文献
109.
V Papa K K Hartmann S M Rosenthal B A Maddux P K Siiteri I D Goldfine 《Molecular endocrinology (Baltimore, Md.)》1991,5(5):709-717
Insulin-like growth factor-I (IGF-I) receptors are present in breast cancer cells and may play a role in breast cancer cell growth. We have studied the effect of progestins on IGF-I receptors in T47D human breast cancer cells. T47D cells constitutively express high levels of progesterone receptors and are a model for studying the regulation of cellular functions by progestins. Treatment of T47D cells with either progesterone or the synthetic progestin promegestone (R5020) decreased IGF-I receptor content by approximately 50%, as measured by Scatchard analysis and receptor biosynthesis studies. In contrast to progestins, estradiol, dexamethasone, and dihydrotestosterone did not influence IGF-I receptor content. No effect of R5020 was seen after 12 h of incubation, a near-maximal effect was seen after 24 h, and greatest effects were seen after 72 h. R5020 decreased IGF-I receptor mRNA abundance, indicating that progestins acted at the level of gene expression. However, progestins also increased the secretion of IGF-II, a ligand for the IGF-I receptor. In contrast to IGF-II, T47D cells did not express IGF-I. The addition of exogenous IGF-II to T47D cells down-regulated both IGF-I receptor binding and IGF-I receptor mRNA abundance. This study indicates, therefore, that progestins regulate IGF-I receptors in breast cancer cells and suggests that this regulation occurs via an autocrine pathway involving enhanced IGF-II secretion. 相似文献
110.