全文获取类型
收费全文 | 749648篇 |
免费 | 76142篇 |
国内免费 | 637篇 |
出版年
2018年 | 11309篇 |
2017年 | 11060篇 |
2016年 | 12146篇 |
2015年 | 11611篇 |
2014年 | 13808篇 |
2013年 | 20009篇 |
2012年 | 23710篇 |
2011年 | 28105篇 |
2010年 | 19657篇 |
2009年 | 18206篇 |
2008年 | 24298篇 |
2007年 | 26091篇 |
2006年 | 19546篇 |
2005年 | 19339篇 |
2004年 | 18898篇 |
2003年 | 18391篇 |
2002年 | 17748篇 |
2001年 | 36113篇 |
2000年 | 36004篇 |
1999年 | 27845篇 |
1998年 | 8684篇 |
1997年 | 9273篇 |
1996年 | 8705篇 |
1995年 | 8050篇 |
1994年 | 7720篇 |
1993年 | 7671篇 |
1992年 | 21973篇 |
1991年 | 21112篇 |
1990年 | 20589篇 |
1989年 | 20054篇 |
1988年 | 18417篇 |
1987年 | 17257篇 |
1986年 | 15927篇 |
1985年 | 15980篇 |
1984年 | 13003篇 |
1983年 | 11022篇 |
1982年 | 8051篇 |
1981年 | 7316篇 |
1980年 | 6865篇 |
1979年 | 12041篇 |
1978年 | 9244篇 |
1977年 | 8520篇 |
1976年 | 7882篇 |
1975年 | 8798篇 |
1974年 | 9468篇 |
1973年 | 9343篇 |
1972年 | 8891篇 |
1971年 | 8114篇 |
1970年 | 6740篇 |
1969年 | 6265篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
891.
The perfusion procedure described in this paper produces high quality impregnation of pig visual and somatosensory cortical neurons with a Golgi-Cox solution. Starting within 30 min after death, pig heads were perfused with a fixative solution composed of a mixture (v/v) of liquid phenol, 5%; formalin, 14%; ethylene glycol, 25%; methanol, 28%; and water, 28% for two periods of 4 hr each. After perfusion, the heads were chilled for at least 18 hr. The entire brain was removed from the skull and then placed in 10% buffered formalin, where it remained for at least 10 days before taking the blocks that were to be immersed in the Golgi-Cox solution. Three weeks spent in the Golgi-Cox solution typically produced uniform neuron impregnation. The tissue blocks were then embedded in celloidin and sectioned at 120 micron. This procedure avoids the following difficulties: Golgi-Cox methods that produced excellent results with rodent or primate tissue were unsuccessful with pig tissue, placing fresh tissue in Golgi-Cox solution resulted in incomplete neuron impregnation, and immersion fixation in 10% buffered formalin without perfusion resulted in excessive staining of glia. 相似文献
892.
The paper submits the results of studies on the kinetics of spermatogenous epithelium cell number after exposure to fast neutrons (60-300 cGy) and gamma-radiation (200-600 cGy). It was shown that a relative decrease in the quantity of spermatocytes is determined by an exponential dose-response curve with D0 of 35 and 120 cGy for neutrons and gamma-radiation respectively. For spermatides and spermatozoa a single D0 value of 20 and 55 cGy was obtained for neutrons and gamma-radiation respectively. As the radiation dose increases the recovery process in the epithelium is substantially decelerated. The equation T1/2 = T1/2(0)e0.0009D well describes the dependence of the half-recovery period T1/2 upon the equivalent dose. 相似文献
893.
894.
Frank M. Sulzman Leland N. Edmunds Jr. 《Biochimica et Biophysica Acta (BBA)/General Subjects》1973,320(3)
Possible factors that could generate the circadian oscillations in alanine dehydrogenase (EC 1.4.1.1.) activity observed in cultures of non-dividing Euglena gracilis (Z) have been examined in an effort to learn more about the basic timekeeping mechanism of biological clocks. No differences in Km, pH optimum or electrophoretic mobility could be demonstrated between enzyme extracted from the minimum part of the 24-h oscillation in activity and that extracted from the maximum part. Also, no evidence for the presence of activators or inhibitors was found in mixing experiments. The effect of cycloheximide on the rhythm was examined; it was shown that the oscillation ceases in the presence of the inhibitor, but that if the inhibitor is removed after 12 h, the rhythm resumes with no apparent change in phase. Analyses of gel scans of enzyme preparations partially purified by (NH4)2SO4 fractionation and polyacrylamide gel electrophoresis indicated that there was more alanine dehydrogenase protein present at the maximum part of the cycle than there was at the minimum part. In view of these and other data, an operational model of a circadian biological clock is discussed. 相似文献
895.
896.
897.
898.
THERE are two principal groups of theories of ageing—those which hold that random cell damage is chiefly responsible for the events characteristic of ageing, which culminate in death and those which hold that ageing and death are genetically controlled. It is too soon to decide between these points of view and in any case Bullough1 has shown that they are not mutually exclusive. So far experiments to test the random error theories of ageing, involving exposure of organisms to unnaturally large or even small amounts of agents such as X-rays and mutagenic agents (for reviews, see refs. 2 and 3), have been controversial and inconclusive. 相似文献
899.
Jürgen Sühnel 《Bulletin of mathematical biology》1998,60(2):197-213
A possible experimental design for combination experiments is to compare the doseresponse curve of a single agent with the
corresponding curve of the same agent using either a fixed amount of a second one or a fixed dose ratio. No interaction is
then often defined by a parallel shift of these curves. We have performed a systematic study for various types of doseresponse
relations both for the dose-additivity (Loewe additivity) and for the independence (Bliss independence) criteria for defining
zero interaction. Parallelism between doseresponse curves of a single agent and those of the same agent in the presence of
a fixed amount of another one is found for the Loewe-additivity criterion for linear doseresponse relations. For nonlinear
relations, one has to differentiate between effect parallelism (parallel shift on the effect scale) and dose parallelism (parallel
shift on the dose scale). In the case of Loewe additivity, zero-interaction dose parallelism is found for power, Weibull,
median-effect and logistic doseresponse relations, given that special parameter relationships are fulfilled. The mechanistic
model of competitive interaction exhibits dose parallelism but not effect parallelism for Loewe additivity. Bliss independence
and Loewe additivity lead to identical results for exponential doseresponse curves. This is the only case for which dose parallelism
was found for Bliss independence. Parallelism between single-agent doseresponse relations and Loewe additivity mixture relations
is found for examples with a fixed doseratio design. However, this is again not a general property of the design adopted but
holds only if special conditions are fulfilled. The comparison of combination doseresponse curves with single-agent relations
has to be performed taking into account both potency and shape parameters. The results of this analysis lead to the conclusion
that parallelism between zero interaction combination and single-agent doseresponse relations is found only for special cases
and cannot be used as a general criterion for defining zero-interaction in combined-action assessment even if the correct
potency shift is taken into account. 相似文献
900.
V T Kondakov A B Okulov M A Filippkin R Z Tandilava I U Artamonov O A Voronenko A N Sazonov 《Vestnik rentgenologii i radiologii》1991,(4):31-35
The paper is concerned with analysis of the results of intraoperative phlebotesticulography, performed in 50 patients with varicocele of degree I-II during Ivanissevich's operation. The effect of surgical intervention was shown to depend upon the quality of ligation of the testicular vein, some parts of which are anastomosed between themselves. The localization of this anastomosis is revealed by means of intraoperative phlebotesticulography, which permits increasing the results of surgical treatment and predicting a course of a postoperative period. 相似文献