全文获取类型
收费全文 | 906篇 |
免费 | 50篇 |
出版年
2021年 | 12篇 |
2020年 | 8篇 |
2019年 | 13篇 |
2018年 | 9篇 |
2017年 | 14篇 |
2016年 | 21篇 |
2015年 | 25篇 |
2014年 | 29篇 |
2013年 | 33篇 |
2012年 | 39篇 |
2011年 | 34篇 |
2010年 | 27篇 |
2009年 | 27篇 |
2008年 | 30篇 |
2007年 | 33篇 |
2006年 | 35篇 |
2005年 | 35篇 |
2004年 | 19篇 |
2003年 | 25篇 |
2002年 | 27篇 |
2001年 | 25篇 |
2000年 | 17篇 |
1999年 | 22篇 |
1998年 | 9篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1994年 | 10篇 |
1993年 | 8篇 |
1992年 | 16篇 |
1991年 | 14篇 |
1990年 | 11篇 |
1989年 | 11篇 |
1988年 | 13篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 24篇 |
1984年 | 9篇 |
1983年 | 20篇 |
1982年 | 12篇 |
1981年 | 11篇 |
1980年 | 19篇 |
1979年 | 19篇 |
1978年 | 12篇 |
1977年 | 8篇 |
1976年 | 18篇 |
1975年 | 11篇 |
1974年 | 13篇 |
1973年 | 11篇 |
1972年 | 10篇 |
1969年 | 7篇 |
排序方式: 共有956条查询结果,搜索用时 15 毫秒
211.
Electrical sizing of particles in suspensions. II. Experiments with rigid spheres 总被引:6,自引:2,他引:4 下载免费PDF全文
Experimental verification is provided for the theoretical expressions (see preceding article, I. Theory) describing the electrical processes that take place during the passage of an aqueous suspension of rigid, nonconducting spheres (ragweed pollen) through an orifice across which there exists an electrical field, for a large range of orifice dimensions; the instrumentation developed is considered in some detail. The effective length of an orifice as deduced from conductivity measurements is shown to be essentially the same as that predicted theoretically. Absolute volume distributions are presented of a suspension of polystyrene latex spheres as determined electrically (mean 11.17 μ3, c. v. 4.2%) and with an electron microscope (mean 11.01 μ3, c. v. 4.1%). Conflicting experimental results reported in the literature are discussed. 相似文献
212.
T4-induced cardiac hypertrophy and the perfused and total microvasculature of the heart 总被引:1,自引:0,他引:1
The purpose of the present investigation was to determine the effects of thyroxine (T4), which induces myocardial hypertrophy, on the number per square millimetre and volume per cubic millimetre of both the total and perfused portions of the arteriolar and capillary beds of the heart. Studies were conducted in the subendocardial and subepicardial regions of the left ventricle of anesthetized open-chest rabbits. Fluorescein isothiocyanate-dextran (i.v.) or radioactive microspheres (intra-atrial) were injected to label the perfused microvessels or to determine coronary flow in three groups of rabbits: controls, and rabbits given 0.5 mg/kg T4 for 3 days and for 16 days. Fluorescent photography was used to identify the perfused microvessels. An alkaline phosphatase stain was employed to locate the total microvascular bed. There were 2369 +/- 638 (SD) capillaries/mm2 and 4 +/- 3 arterioles/mm2 in control hearts. These decreased significantly to 1380 +/- 199/mm2 and 1 +/- 1/mm2, respectively, after 16 days of T4. In controls, 60 +/- 5% of the capillaries and 59 +/- 21% of the arterioles were perfused. This increased significantly to 90 +/- 5 and 86 +/- 18%, respectively, by 16 days of T4 treatment. Similar changes, although smaller, were observed after 3 days of T4. Coronary blood flow increased to 1.7 times control after 3 days and 2.9 times after 16 days of T4. No significant subepicardial versus subendocardial differences were observed in any condition or measurement. Thus, the physiological response to the increased work and increase in anatomic minimum diffusion distance is to increase flow and the proportion of the capillary bed perfused to at least maintain physiological diffusion distances. 相似文献
213.
The question of how an individual bacterial cell grows during its life cycle remains controversial. In 1962 Collins and Richmond derived a very general expression relating the size distributions of newborn, dividing and extant cells in steady-state growth and their growth rate; it represents the most powerful framework currently available for the analysis of bacterial growth kinetics. The Collins-Richmond equation is in effect a statement of the conservation of cell numbers for populations in steady-state exponential growth. It has usually been used to calculate the growth rate from a measured cell size distribution under various assumptions regarding the dividing and newborn cell distributions, but can also be applied in reverse--to compute the theoretical cell size distribution from a specified growth law. This has the advantage that it is not limited to models in which growth rate is a deterministic function of cell size, such as in simple exponential or linear growth, but permits evaluation of far more sophisticated hypotheses. Here we employed this reverse approach to obtain theoretical cell size distributions for two exponential and six linear growth models. The former differ as to whether there exists in each cell a minimal size that does not contribute to growth, the latter as to when the presumptive doubling of the growth rate takes place: in the linear age models, it is taken to occur at a particular cell age, at a fixed time prior to division, or at division itself; in the linear size models, the growth rate is considered to double with a constant probability from cell birth, with a constant probability but only after the cell has reached a minimal size, or after the minimal size has been attained but with a probability that increases linearly with cell size. Each model contains a small number of adjustable parameters but no assumptions other than that all cells obey the same growth law. In the present article, the various growth laws are described and rigorous mathematical expressions developed to predict the size distribution of extant cells in steady-state exponential growth; in the following paper, these predictions are tested against high-quality experimental data. 相似文献
214.
S. K. Grover K. Sridharan K. K. Srivastava 《International journal of biometeorology》1987,31(2):163-168
The sequential changes in serum total protein concentration and in various electrophoretic fractions among lowlanders during two years of stay at high altitude (4,000 m) were determined and compared with that of high altitude natives (Ladakhis). The albumin to globulin ratio decreased during the early period of exposure to altitude among lowlanders and continued to remain at that level with minor fluctuations during the entire period of stay at altitude. Among the high altitude natives, a decreased albumin to globulin ratio with low serum protein concentration was observed. The composition of globulins separated electro-phoretically was of similar pattern in altitude natives and in lowlanders after a stay of two years at altitude. Among these subjects the-globulin was significantly higher as compared to values at sea level. 相似文献
215.
Investigations on the metabolism of 3H-labelled chrysene, benz[a]anthracene, 7-methylbenz[a]anthracene, 7,12-dimethylbenz[a]anthracene, 3-methylcholanthrene, benzo[a]pyrene, dibenz[a,c]anthracene and dibenz[a,h]anthracene by mouse skin maintained in short-term organ culture were carried out. Estimations of the distribution of the metabolites of each hydrocarbon present after 24 h showed that there were wide variations both in the rates at which the hydrocarbons were metabolised and in the amounts of metabolites covalently bound to skin macromolecules. All the hydrocarbons were metabolised to dihydrodiols, which were identified by comparison on high pressure liquid chromatography (HPLC) with the authentic compounds, and these were the same diols as those that were formed in previous experiments with rat-liver microsomal fractions. However, free dihydrodiols represented only relatively small proportions of the total amounts of metabolites formed. All the hydrocarbons yielded dihydrodiols of the type that could give rise to bay-region diol-epoxides, when further metabolised, some of which are thought to be involved in hydrocarbon carcinogenesis. 相似文献
216.
InConvolvulus pluricaulis
Chois. two forms (2n=18, 36) along with one colchicine-autotetraploid have been investigated morphologically and cytologically.
The diploid regularly formed 9II although in the “off-season” plants certain irregularities were observed including formation of unreduced pollen grains.
The natural tetraploid and the colchicine-autotetraploid had mean frequencies of configurations of 1.56IV+14.04II+1.48I and 2.95IV+0.56III+11,43II+0.8I, respectively. The mode of chromosome association in the two types of tetraploids was comparable. A comparison of the morphological
characters of the two tetraploid types, further, suggested close similarity. In addition they were indistinguishable from
the diploid from except being gigas. From these data it is inferred that the natural tetraploid presumably arose as a result
of direct duplication of the diploid form, through the chance fusion of unreduced spores. The lower quadrivalent frequency
of the natural polyploid is ascribed to a gradual shift towards bivalent association accompanying natural selection for fertility.
The taxonomic status of the two forms (2x, 4x) is discussed and the varietal status accorded to the tetraploid form is supported. 相似文献
217.
218.
In an attempt to find a preferential inhibitor of serum FSH, a number of C18, C19 and C21 steroids were evaluated for suppressive effects on LH and FSH and stimulatory effects upon ventral prostate weight in the acutely castrate adult male rat. C21 steroids had no effect. The C19 steroids were listed in order of potency with regard to suppressing LH, FSH and stimulating ventral prostate weight. All C19 steroids that had an inhibitory effect on gonadotropins also stimulated ventral prostate weight and all demonstrated preferential inhibition of LH over FSH. Two C18 steroids were tested, estradiol-17β and estrone; estradiol-17β suppressed LH and FSH in a parallel fashion. Four prostaglandins were evaluated with regard to effect on gonadotropins and ventral prostate weight; none demonstrated effect. 相似文献
219.
The metabolism of γ-2,3,4,5,6-pentachlorocyclohex-1-ene and γ-hexachlorocyclohexane in rats 下载免费PDF全文
1. After intraperitoneal administration, gamma-hexachlorocyclohexane (Gammexane) and gamma-2,3,4,5,6-pentachlorocyclohex-1-ene were converted by rats into 2,3,5- and 2,4,5-trichlorophenol, which were excreted as free phenols and as sulphuric acid and glucuronic acid conjugates. 2. Derivatives of 2,4,5-trichlorophenol and 2,4,5-trichlorophenyl glucosiduronic acid and 2,4-dichlorophenylmercapturic acid were isolated from the urine as metabolites of gamma-2,3,4,5,6-pentachlorocyclohex-1-ene. 3. The phenolic metabolites of gamma-hexachlorocyclohexane and gamma-2,3,4,5,6-pentachlorocyclohex-1-ene isolated from urine were similar to those of 1,2,4-trichlorobenzene, which indicates that the two latter compounds are intermediates in gamma-hexachlorocyclohexane metabolism in rats. 相似文献
220.