全文获取类型
收费全文 | 234343篇 |
免费 | 7585篇 |
国内免费 | 4314篇 |
学科分类
医药卫生 | 246242篇 |
出版年
2024年 | 247篇 |
2023年 | 1021篇 |
2022年 | 2314篇 |
2021年 | 3476篇 |
2020年 | 2683篇 |
2019年 | 1943篇 |
2018年 | 23790篇 |
2017年 | 19399篇 |
2016年 | 21240篇 |
2015年 | 3812篇 |
2014年 | 4362篇 |
2013年 | 4400篇 |
2012年 | 12185篇 |
2011年 | 26540篇 |
2010年 | 23025篇 |
2009年 | 15016篇 |
2008年 | 23327篇 |
2007年 | 25271篇 |
2006年 | 3842篇 |
2005年 | 4951篇 |
2004年 | 5386篇 |
2003年 | 6315篇 |
2002年 | 3985篇 |
2001年 | 1523篇 |
2000年 | 1388篇 |
1999年 | 855篇 |
1998年 | 539篇 |
1997年 | 561篇 |
1996年 | 369篇 |
1995年 | 333篇 |
1994年 | 293篇 |
1993年 | 169篇 |
1992年 | 167篇 |
1991年 | 184篇 |
1990年 | 231篇 |
1989年 | 143篇 |
1988年 | 122篇 |
1987年 | 91篇 |
1986年 | 63篇 |
1985年 | 53篇 |
1984年 | 37篇 |
1983年 | 33篇 |
1982年 | 33篇 |
1980年 | 43篇 |
1974年 | 24篇 |
1938年 | 60篇 |
1937年 | 25篇 |
1934年 | 30篇 |
1932年 | 56篇 |
1930年 | 46篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
A newly developed microscope-based imaging system was used to measure the oxygen tension (PO2) inside the retinal and choroidal vessels of mice and to generate in vivo maps of retinal PO2. These maps were generated from the phosphorescence lifetimes of an injected palladium–porphyrin compound using a frequency-domain measurement. The system was fully calibrated and used to produce retinal PO2 maps at different inspiratory oxygen fractions. PO2 rose accordingly and predictably as inspiratory O2 was stepped from hypoxic to hyperoxic conditions. Important experimental and acquisition parameters necessary for applying phosphorescence lifetime imaging to the mouse eye were investigated, including camera exposure and intensifier gain settings. Because of a need to limit light exposure to the retina, PO2 map quality as measured by the coefficient of determination was investigated as a function of signal-to-noise and accumulated excitation energy deposition. With the development of this technology for use in mice, the potential for investigating the oxygen dynamics in genetically engineered mouse models of retinal disease, including diabetic retinopathy, glaucoma, and age-related macular degeneration, is advanced. © 2003 Biomedical Engineering Society.
PAC2003: 4266Ew, 8763Lk, 8719Dd 相似文献
962.
Karch R Neumann F Neumann M Szawlowski P Schreiner W 《Annals of biomedical engineering》2003,31(5):548-563
Topological and metric properties of Voronoi polyhedra (VP) generated by the distal end points of terminal segments in arterial tree models grown by the method of constrained constructive optimization (CCO) are analyzed with the aim to characterize the spatial distribution of their supply sites relative to randomly distributed points as a reference model. The distributions of the number N
f
of Voronoi cell faces, cell volume V, surface area S, area A of individual cell faces, and asphericity parameter of the CCO models are all significantly different from the ones of random points, whereas the distributions of V, S, and are also significantly different among CCO models optimized for minimum intravascular volume and minimum segment length (p < 0.0001). The distributions of N
f
, V, and S of the CCO models are reasonably well approximated by two-parameter gamma distributions. We study scaling of intravascular blood volume and arterial cross-sectional area with the volume of supplied tissue, the latter being represented by the VP of the respective terminal segments. We observe scaling exponents from 1.20 ± 0.007 to 1.08 ± 0.005 for intravascular blood volume and 0.77 ± 0.01 for arterial cross-sectional area. Setting terminal flows proportional to the associated VP volumes during tree construction yields a relative dispersion of terminal flows of 37% and a coefficient of skewness of 1.12. © 2003 Biomedical Engineering Society.
PAC2003: 8719Uv, 8710+e, 4720Ky, 0260Pn, 0230Oz 相似文献
963.
Classification of Breast Masses Using Selected Shape,Edge-sharpness,and Texture Features with Linear and Kernel-based Classifiers 总被引:2,自引:0,他引:2
Breast masses due to benign disease and malignant tumors related to breast cancer differ in terms of shape, edge-sharpness, and texture characteristics. In this study, we evaluate a set of 22 features including 5 shape factors, 3 edge-sharpness measures, and 14 texture features computed from 111 regions in mammograms, with 46 regions related to malignant tumors and 65 to benign masses. Feature selection is performed by a genetic algorithm based on several criteria, such as alignment of the kernel with the target function, class separability, and normalized distance. Fisher's linear discriminant analysis, the support vector machine (SVM), and our strict two-surface proximal (S2SP) classifier, as well as their corresponding kernel-based nonlinear versions, are used in the classification task with the selected features. The nonlinear classification performance of kernel Fisher's discriminant analysis, SVM, and S2SP, with the Gaussian kernel, reached 0.95 in terms of the area under the receiver operating characteristics curve. The results indicate that improvement in classification accuracy may be gained by using selected combinations of shape, edge-sharpness, and texture features. 相似文献
964.
Letizia Penolazzi Ilaria Lampronti Monica Borgatti Mahmud Tareq Hassan Khan Margherita Zennaro Roberta Piva Roberto Gambari 《BMC complementary and alternative medicine》2008,8(1):59
Background
Osteoclasts (OCs) are involved in rheumatoid arthritis and in several pathologies associated with bone loss. Recent results support the concept that some medicinal plants and derived natural products are of great interest for developing therapeutic strategies against bone disorders, including rheumatoid arthritis and osteoporosis. In this study we determined whether extracts of Emblica officinalis fruits display activity of possible interest for the treatment of rheumatoid arthritis and osteoporosis by activating programmed cell death of human primary osteoclasts. 相似文献965.
Transcranial magnetic stimulation: new insights into representational cortical plasticity 总被引:15,自引:5,他引:15
Siebner HR Rothwell J 《Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale》2003,152(1):1-16
In the last decade, transcranial magnetic stimulation (TMS) has been used increasingly as a tool to explore the mechanisms
and consequences of cortical plasticity in the intact human cortex. Because the spatial accuracy of the technique is limited,
we refer to this as plasticity at a regional level. Currently, TMS is used to explore regional reorganization in three different
ways. First, it can map changes in the pattern of connectivity within and between different cortical areas or their spinal
projections. Important examples of this approach can be found in the work on motor cortex representations following a variety
of interventions such as immobilization, skill acquisition, or stroke. Second, TMS can be used to investigate the behavioural
relevance of these changes. By applying TMS in its "virtual lesion" mode, it is possible to interfere with cortical function
and ask whether plastic reorganization within a distinct cortical area improves function. Third, TMS can be used to promote
changes in cortical function. This is achieved by using repetitive TMS (rTMS) to induce short-term functional reorganization
in the human cortex. The magnitude and the direction of rTMS-induced plasticity depend on extrinsic factors (i.e. the variables
of stimulation such as intensity, frequency, and total number of stimuli) and intrinsic factors (i.e. the functional state
of the cortex targeted by rTMS). Since conditioning effects of rTMS are not limited to the stimulated cortex but give rise
to functional changes in interconnected cortical areas, rTMS is a suitable tool to investigate plasticity within a distributed
functional network. Indeed, the lasting effects of rTMS offer new possibilities to study dynamic aspects of the pathophysiology
of a variety of diseases and may have therapeutic potential in some neuropsychiatric disorders.
Electronic Publication 相似文献
966.
Satoh Y Ishikawa Y Miyoshi T Mukai H Okumura S Nakagawa K 《Virchows Archiv : an international journal of pathology》2003,442(2):173-178
Pulmonary metastasis from low-grade endometrial stromal sarcomas (ESSs) occasionally are found after long, disease-free periods, mostly as incidental histological or radiological discoveries. We describe a case of low-grade ESS presenting as nodular pulmonary metastases finally diagnosed by estrogen-receptor staining, cytogenetic and fluorescence in situ hybridization (FISH) analyses, and perusal of the histology of hysterectomy material. An abnormal nodule in the lung field was discovered by means of chest X-ray of a 47-year-old woman. She had been disease free for 13 years after hysterectomy for an alleged leiomyoma. A computed tomographic scan revealed nodules, with fluctuation in size over the 2-year period, in both lungs. Finally the lesion in the left lung was resected, and pulmonary endometriosis was suspected because of the lack of stromal cell nuclear atypia and positive immunohistochemical reactions for estrogen and progesterone receptors. However, a characteristic karyotype was identified cytogenetically: 46, XX, t(7;17)(p15;q11), the translocation of which, specific to ESS, was confirmed by FISH analysis. A final diagnosis of pulmonary metastases from an ESS could be made by reviewing the histology of the previous uterine tumor. In this case, metastatic lesions from an ESS showed a decrease as well as an increase in size, despite the malignant potential. Immunostaining for estrogen and progesterone receptors and cytogenetic and FISH analyses, together with clinical information on the past gynecological history, are valuable diagnostic keys. 相似文献
967.
Kallakuri S Cavanaugh JM Ozaktay AC Takebayashi T 《Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale》2003,148(4):419-424
Diffuse axonal injury (DAI) is seen as widespread damage in the white matter of brain characterized by morphological changes
to axons throughout the brain and brain stem. The current study attempted to investigate the effect of increasing impact energy
on the presence and severity of DAI in corpus callosum (CC). DAI was induced in adult male Sprague-Dawley rats using an injury
model adapted from Marmarou et al. in 1994. A 450-g cylindrical brass weight was dropped from three different heights (2.0 m,
1.5 m and 1.0 m) on to a metal helmet affixed to the skull of the rats. In the sham group, rats underwent a surgical procedure
with no impact. After a 24-h survival period the animals were transcardially perfused. The brain was removed and the cerebral
hemispheres were sectioned with a vibrotome and stained by silver impregnation technique. The CC of all the impacted rats
showed DAI in the form of beaded axons, retraction balls and vacuole-like enlargements. The axonal injury was most severe
in the 2-m group, while mildest in the 1-m group. In the sham group, axons appeared to be normal. This study demonstrates
evidence of graded DAI depending on the impact energy. Such data is useful for mathematical modeling of axonal injury in rat
brain using the same impact parameters and potential determination of injury thresholds for neural trauma.
Electronic Publication 相似文献
968.
LJM, a 41-year-old schizophrenic Chinese man with bilateral anterior cingulate gyrus (Brodmann's area 24) lesions and also a small lesion in right amygdala after an operation, was compared with normal as well as brain-damaged and schizophrenic controls in identification of morphed facial expressions of six basic emotions. In repeated administrations of the test for recognition of facial emotions, over a 1- year period, LJM performed significantly worse for expressions of fear compared with the three groups of controls. Recognition of other emotions was not significantly different from that of the controls, except that his recognition of disgust during the first session (but not in two subsequent sessions) was worse than normal and brain-damaged controls but not worse than schizophrenic controls. The dissociation between recognition of fear and other emotions supported the view that the brain has separable networks for processing different emotions, and that the right amygdala as well as the anterior part of bilateral cingulate gyrus are possible substrates involved in the special network for perception of fear. The results from the various groups of Chinese subjects indicate that they perceive emotions in a categorical manner, and that the six basic emotions are likely to be cross-cultural universals. 相似文献
969.
Reda M. El-S. Hassanine Mohammed O. Al-Jahdali 《Acta parasitologica / Witold Stefański Institute of Parasitology, Warszawa, Poland》2008,53(3):289-295
During June and July of 2007, a total of 130 specimens of the fish Rastrelliger kanagurta Cuvier (Teleostei, Scombridae), ranging between 19–31 cm in total length, were caught in the Red Sea off the coast of Sharm
El-Sheikh, South Sinai, Egypt, and examined for infections by acanthocephalans (65 fish/month). Of this number, 29 (22.30%)
were slightly or heavily parasitized by the acanthocephalan Diplosentis nudus (Harada, 1938) Pichelin et Cribb, 2001 (Cavisomidae); no other helminth parasites were found in the intestine of R. kanagurta. Twenty-nine infrapopulations of D. nudus, ranging from 23–218 individuals were collected from the infected fishes. These infrapopulations were distributed in a well-defined
fundamental niche along the intestine of R. kanagurta, where the distribution of male worms was not random with respect to female worms size and position and suggests that the
male-male competition for access to female may be intense and may select for large males. No correlation between fish size
and infrapopulation size was observed. Correlations between female-to-male sex ratio and infrapopulation size, numbers of
females and their mean lengths, numbers of males and their mean lengths, mean female length and mean male length within infrapopulation
were very strong, and clearly suggest that as the infrapopulation size increased, the number of females and their mean lengths
decreased and the number of males and their mean lengths increased. Combination of these results strongly suggests density-dependent
effects and competition between male worms. The relationship between the mean female length or size and the number of eggs
within its pseudocoel was strongly positive; egg production by female worm significantly decreases as the infrapopulation
size increases, suggesting density-dependent reduction in female worm fecundity. Tendency for the variability in male testes
size was not significant in infrapopulations of D. nudus. All of these results are discussed. 相似文献
970.
系统性红斑狼疮临床特征与HLA—DR,DQ基因的相关研究 总被引:2,自引:0,他引:2
采用聚合酶链反应结合顺序特异物寡核苷酸(PCR/SSO)探针杂交方法对HLA-DR,DQ亚区作DNA分型,分析了系统性红斑狼疮(SLE)的各种临床特征与HLA-DR、DQ基因的关联,结果发现汉族SLE中肾脏损害与DR2相关,而与DR4呈负相关,口、鼻腔粘膜溃疡与DR12相关,未发现SLE的其他临床特征与DR、DQ基因有关,发病年龄小于35a者,DR2,DQ6阳性率高,而DR3则多见于发病年龄大于3 相似文献