全文获取类型
收费全文 | 10856篇 |
免费 | 549篇 |
国内免费 | 27篇 |
专业分类
耳鼻咽喉 | 186篇 |
儿科学 | 232篇 |
妇产科学 | 255篇 |
基础医学 | 2149篇 |
口腔科学 | 106篇 |
临床医学 | 1030篇 |
内科学 | 2236篇 |
皮肤病学 | 236篇 |
神经病学 | 757篇 |
特种医学 | 659篇 |
外科学 | 1249篇 |
综合类 | 92篇 |
预防医学 | 491篇 |
眼科学 | 264篇 |
药学 | 1025篇 |
中国医学 | 16篇 |
肿瘤学 | 449篇 |
出版年
2021年 | 93篇 |
2020年 | 58篇 |
2019年 | 82篇 |
2018年 | 92篇 |
2017年 | 80篇 |
2016年 | 110篇 |
2015年 | 138篇 |
2014年 | 190篇 |
2013年 | 288篇 |
2012年 | 468篇 |
2011年 | 535篇 |
2010年 | 315篇 |
2009年 | 320篇 |
2008年 | 469篇 |
2007年 | 573篇 |
2006年 | 541篇 |
2005年 | 540篇 |
2004年 | 528篇 |
2003年 | 501篇 |
2002年 | 476篇 |
2001年 | 234篇 |
2000年 | 230篇 |
1999年 | 234篇 |
1998年 | 130篇 |
1997年 | 144篇 |
1996年 | 110篇 |
1995年 | 78篇 |
1994年 | 78篇 |
1993年 | 55篇 |
1992年 | 116篇 |
1991年 | 115篇 |
1990年 | 104篇 |
1989年 | 88篇 |
1988年 | 78篇 |
1987年 | 68篇 |
1986年 | 83篇 |
1985年 | 68篇 |
1984年 | 59篇 |
1933年 | 85篇 |
1932年 | 84篇 |
1931年 | 91篇 |
1930年 | 104篇 |
1929年 | 88篇 |
1928年 | 87篇 |
1927年 | 83篇 |
1926年 | 85篇 |
1925年 | 82篇 |
1924年 | 88篇 |
1923年 | 86篇 |
1922年 | 72篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
A newly developed noninvasive tissue reflectance oximeter utilizes 5 light emitting diodes operating at the wavelengths of 0.635, 0.665, 0.795, 0.910, and 0.955 μ, and photodiodes to sample the tissue reflectance spectra. Since the tissue reflectance is affected by changes in both hemoglobin content (Hb T) and hemoglobin oxygen saturation (OS T),Hb T is first determined using the reflectance at 0.795 μ. The hemoglobinOS T is then estimated using the reflectances at the 5 wavelengths in conjunction with the diffuse reflectance equation which has previously been verified applicable to tissue reflectance oximetry. A quantitative estimation of bothHb T andOS T in intestinal mucosa of dogs obtained using this instrument showed thatHb T values agreed fairly well with those of others and that the standard errors ofOS T were around 5.0% inOS as compared with theOS values of blood samples for minimized arterial-venousOS differences. The continuous on-line measurement ofHb T andOS T should be possible using the reflectance technique and should be valuable for clinical evaluation of the patients. 相似文献
72.
Epithelial membranes are multicompartment structures of microscopic and submicroscopic dimensions. Therefore, interpretations
of kinetic data on solute fluxes, based on the standard three compartment model are open to criticism. We have obtained an
integrated view of the kinetics of Na+ transport in frog skin epidermis by application of the computer simulation method. Epidermis and whole skin models were designed
which resemble photomicrographs of these tissues. Justification is given for the way in which internal and external chamber
compartments are connected (topology). The epidermis model has eight passive, and two active transfer sites. Our primary aims
were 1) simulation of the transepidermal Na+ influx and the concomitant Na+ backflux saturation kinetics, and 2) localization of the so-called “outer” and “inner” Na+ responsive borders in epidermis. The analysis, based on methodical variations of transfer coefficients, suggests involvement
of the “composite desmosomes” and the transepithelial Na+-pump leak pathway. These are located in the outer and the inner region of the epidermis, respectively. Reasonable functional
agreement between epidermis and model was also seen in 1) Na+ saturation kinetics in ouabain, poisoned system, 2) relative independence of the two borders to the “trans” [Na+] in the external solutions, and 3) equal energy requirement, for the transmembrane Na+ pump, Na+/O2∼-20.
This work was supported by NIH Grant GM 03545-23 相似文献
73.
Modification of collagen matrices for enhancing angiogenesis 总被引:3,自引:0,他引:3
Yao C Prével P Koch S Schenck P Noah EM Pallua N Steffens G 《Cells, tissues, organs》2004,178(4):189-196
The vascularization of engineered tissues in many cases does not keep up with the ingrowth of cells. Nutrient and oxygen supply are not sufficient, which ultimately leads to the death of the invading cells. The enhancement of the angiogenic capabilities of engineered tissues therefore represents a major challenge in the field of tissue engineering. The immobilization of angiogenic growth factors may be useful for enhancing angiogenesis. The most potent angiogenic growth factor specific to endothelial cells, vascular endothelial growth factor (VEGF), occurs in several splice variants. The variant with 165 amino acids both has a high angiogenic activity and a high affinity for heparin. We therefore incorporated heparin molecules into collagen matrices by covalently cross-linking them to amino functions on the collagen. Physical binding of VEGF to the heparin may then prevent a rapid clearance from the implant, while the release rate may become coupled to the degradation of the collagen matrix. The modified matrices were characterized by determination of the extent of the heparin immobilization, the in vitro degradation rate by collagenase. For testing the angiogenic properties, non-modified and heparinized collagen specimens were--either loaded with VEGF or non-loaded--subcutaneously implanted on the back of rats. Specimens were explanted after varying periods of implantation, the dry weights and the hemoglobin contents, as well as immunostained histological sections were evaluated: heparinized collagen matrices loaded with VEGF are vascularized to a substantially higher extent as compared to non-modified matrices. 相似文献
74.
75.
An efficient method to generate chromosomal rearrangements by targeted DNA double-strand breaks in Drosophila melanogaster 下载免费PDF全文
Homologous recombination (HR) is an indispensable tool to modify the genome of yeast and mammals. More recently HR is also being used for gene targeting in Drosophila. Here we show that HR can be used efficiently to engineer chromosomal rearrangements such as pericentric and paracentric inversions and translocations in Drosophila. Two chromosomal double-strand breaks (DSBs), introduced by the rare-cutting I-SceI endonuclease on two different mobile elements sharing homologous sequences, are sufficient to promote rearrangements at a frequency of 1% to 4%. Such rearrangements, once generated by HR, can be reverted by Cre recombinase. However, Cre-mediated recombination efficiency drops with increasing distance between recombination sites, unlike HR. We therefore speculate that physical constraints on chromosomal movement are modulated during DSB repair, to facilitate the homology search throughout the genome. 相似文献
76.
Tumor-associated macrophages express lymphatic endothelial growth factors and are related to peritumoral lymphangiogenesis 总被引:53,自引:0,他引:53 下载免费PDF全文
Schoppmann SF Birner P Stöckl J Kalt R Ullrich R Caucig C Kriehuber E Nagy K Alitalo K Kerjaschki D 《The American journal of pathology》2002,161(3):947-956
Formation of lymphatic metastasis is the initial step of generalized spreading of tumor cells and predicts poor clinical prognosis. Lymphatic vessels generally arise within the peritumoral stroma, although the lymphangiopoietic vascular endothelial growth factors (VEGF)-C and -D are produced by tumor cells. In a carefully selected collection of human cervical cancers (stage pT1b1) we demonstrate by quantitative immunohistochemistry and in situ hybridization that density of lymphatic microvessels is significantly increased in peritumoral stroma, and that a subset of stromal cells express large amounts of VEGF-C and VEGF-D. The density of cells producing these vascular growth factors correlates with peritumoral inflammatory stroma reaction, lymphatic microvessel density, and indirectly with peritumoral carcinomatous lymphangiosis and frequency of lymph node metastasis. The VEGF-C- and VEGF-D-producing stroma cells were identified in situ as a subset of activated tumor-associated macrophages (TAMs) by expression of a panel of macrophage-specific markers, including CD68, CD23, and CD14. These TAMs also expressed the VEGF-C- and VEGF-D-specific tyrosine kinase receptor VEGFR-3. As TAMs are derived from monocytes in the circulation, a search in peripheral blood for candidate precursors of VEGFR-3-expressing TAMs revealed a subfraction of CD14-positive, VEGFR-3-expressing monocytes, that, however, failed to express VEGF-C and VEGF-D. Only after in vitro incubation with tumor necrosis factor-alpha, lipopolysaccharide, or VEGF-D did these monocytes start to synthesize VEGF-C de novo. In conclusion VEGF-C-expressing TAMs play a novel role in peritumoral lymphangiogenesis and subsequent dissemination in human cancer. 相似文献
77.
Noise-induced effects within the inner ear have been well investigated for several years. However, this peripheral damage cannot fully explain the audiological symptoms in noise-induced hearing loss (NIHL), e.g. tinnitus, recruitment, reduced speech intelligibility, hyperacusis. There are few reports on central noise effects. Noise can induce an apoptosis of neuronal tissue within the lower auditory pathway. Higher auditory structures (e.g. medial geniculate body, auditory cortex) are characterized by metabolic changes after noise exposure. However, little is known about the microstructural changes of the higher auditory pathway after noise exposure. The present paper was therefore aimed at investigating the cell density in the medial geniculate body (MGB) and the primary auditory cortex (AI) after noise exposure. Normal hearing mice were exposed to noise (10 kHz center frequency at 115 dB SPL for 3 h) at the age of 21 days under anesthesia (Ketamin/Rompun, 10:1). After 1 week, auditory brainstem response recordings (ABR) were performed in noise exposed and normal hearing animals. After fixation, the brain was microdissected and stained (Kluever-Barrera). The cell density in the MGB subdivisions and the AI were determined by counting the cells within a grid. Noise-exposed animals showed a significant ABR threshold shift over the whole frequency range. Cell density was significantly reduced in all subdivisions of the MGB and in layers IV-VI of AI. The present findings demonstrate a significant noise-induced change of the neuronal cytoarchitecture in central key areas of auditory processing. These changes could contribute to the complex psychoacoustic symptoms after NIHL. 相似文献
78.
Lisa Werner Ernst Winkelmann Annelore Koglin J?rg Neser Heiko Rodewohl 《Anatomy and embryology》1989,180(6):583-597
Summary The morhological features of 298 neurons impregnated according to Golgi-Kopsch in areas 17 and 18 of Macaca mulatta were analyzed, and the same neurons were deimpregnated to visualize structural details of the somata in different types of neurons. The following cell types were investigated: Pyramidal and pyramid-like cells, spiny stellate cells, double bouquet cells, bipolar cells, chandelier cells, neurogliaform cells, basket and related cells. This procedure allows the evaluation of the nuclear-cytoplasmic proportion and the position of the nucleus besides shape and size of the cell body. Pyramidal and pyramid-like cells (N=43), spiny stellate cells (N=26), basket and related cells (N=126) are variable in these features. A positive correlation between soma size and width of the cytoplasm is found in pyramidal, pyramid-like cells and spiny stellate cells. With the exception of some large somata in both these types of neurons the nucleus is found in a central position. Double bouquet cells (N=6), bipolar cells (N=13) and chandelier cells (N=11) exhibit small cytoplasmic rims and centrally located nuclei. The small somata of neurogliaform cells (N=37), however, and the small to very large somata of basket and related cells show broad cytoplasmic portions surrounding the eccentrically located nuclei. These findings allow the identification of different neuronal types in Nisslstained sections on the basis of these soma features. This is a prerequisite for further detailed quantitative studies on the laminar distribution of different neuronal types in the visual cortex of the monkey. 相似文献
79.
Defective antigen presentation by Mycobacterium tuberculosis-infected monocytes. 总被引:4,自引:1,他引:4 下载免费PDF全文
In this study we investigated the effect of an in vitro infection with Mycobacterium tuberculosis on the ability of human monocytes to present the soluble antigen tetanus toxoid to T cells. We observed that tetanus toxoid-specific T-cell proliferation was markedly reduced when monocytes were infected with large numbers (bacterium-to-monocyte ratio, 50:1) of both viable and heat-killed mycobacteria. The level of antigen-induced gamma interferon release also was decreased when M. tuberculosis-containing monocytes were used as antigen-presenting cells. However, mycobacterium-infected monocytes did not show or trigger suppressive activity, because the presence of mycobacterium-infected monocytes did not affect the T-cell response induced by tetanus toxoid-pulsed control monocytes. When M. tuberculosis-infected monocytes were fixed with paraformaldehyde, they were not able to serve as antigen-presenting cells even in the presence of untreated accessory monocytes. Moreover, the uptake of both viable and heat-killed M. tuberculosis cells reduced the expression of human leukocyte antigen DR on monocytes. With regard to accessory function, monocytes infected with large numbers of mycobacteria were less efficient as accessory cells than were control monocytes in cultures of T cells activated with pokeweed mitogen. These results indicate that infection with large numbers of M. tuberculosis cells impairs the ability of monocytes to process and/or present soluble antigen and to serve as accessory cells in T-cell activation. 相似文献
80.
Tumor cell growth fractions in human malignant melanomas and the correlation to histopathologic tumor grading. 总被引:1,自引:1,他引:1 下载免费PDF全文
P. Kaudewitz O. Braun-Falco M. Ernst M. Landthaler W. Stolz J. Gerdes 《The American journal of pathology》1989,134(5):1063-1068
The growth fraction (GF) of 72 human malignant melanomas was determined by immunostaining with monoclonal antibody Ki-67. A positive correlation of GF and histopathologically-assessed prognostic variables, such as tumor thickness, mitotic rate, and prognostic index, was found. Individual Ki-67 values were considerably scattered in all histologically defined groups of malignancy. Thus, GF as determined by Ki-67 was used to calculate a modified prognostic index. In contrast to the histologically defined prognostic index, the Ki-67 based prognostic index allows further subdivision of thin malignant melanomas with no or few mitotic figures. This may be of help in identifying tumors with a high recurrence potential. 相似文献