首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   599485篇
  免费   59814篇
  国内免费   198篇
  659497篇
  2018年   6155篇
  2017年   5871篇
  2016年   7917篇
  2015年   9501篇
  2014年   11562篇
  2013年   16277篇
  2012年   18741篇
  2011年   19497篇
  2010年   13401篇
  2009年   12231篇
  2008年   17553篇
  2007年   18294篇
  2006年   17299篇
  2005年   16500篇
  2004年   16611篇
  2003年   15709篇
  2002年   15467篇
  2001年   24173篇
  2000年   24036篇
  1999年   19136篇
  1998年   6751篇
  1997年   6827篇
  1996年   6388篇
  1995年   6003篇
  1993年   5805篇
  1992年   15825篇
  1991年   15923篇
  1990年   15587篇
  1989年   15241篇
  1988年   14544篇
  1987年   13890篇
  1986年   12864篇
  1985年   12896篇
  1984年   10695篇
  1983年   9124篇
  1982年   6796篇
  1981年   6054篇
  1980年   5848篇
  1979年   10159篇
  1978年   7978篇
  1977年   7347篇
  1976年   7098篇
  1975年   7905篇
  1974年   8896篇
  1973年   8678篇
  1972年   7990篇
  1971年   7397篇
  1970年   6504篇
  1969年   6350篇
  1968年   6096篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
Further refinements on the triangular flap closure of the cleft lip   总被引:1,自引:0,他引:1  
Refinements in the triangular flap closure of the unilateral cleft lip are presented. Randall's mathematical interpretation of the Tennison repair has been extended by using a series of arcs to determine the crucial points which form the triangular flap. Using this easily taught method, an isosceles triangular flap can be plotted which will interdigitate into the noncleft side of the lip. Two symmetrical vertical distances on either side of the cleft are thus formed. This allows for a standardized repair which may be more readily taught than the rotation-advancement technique. In addition, flaps are created which are turned medically toward the cleft and are used to accentuate the philtral pout, close the nostril floor, and reinforce the lip repair, allowing closure of even extremely wide clefts in one stage. These refinements in using the triangular flap closure for the repair of the unilateral cleft lip are diagrammatically presented. We believe that these refinements enhance the results of this closure.  相似文献   
73.
We studied the effects of ionizing radiation on the morphology of the pulmonary circulation using an in vivo rat model and an in vitro pulmonary artery endothelial cell model. Gamma radiation was given as either an acute (30 Gy) or fractionated (5 X 6 Gy) dose to one hemithorax of rats. An acute 30-Gy dose delivered resulted in a 70% decrease in pulmonary arterial perfusion, using technetium-99m microaggregated albumin (99mTc-MAA), in the irradiated lung by 2-3 weeks after irradiation. Pulmonary microradiographs, using a barium sulfate perfusion method, obtained 2-3 weeks after irradiation demonstrated widespread loss of capillary filling and segmentation of the vessels. Histologic examination demonstrated intact capillaries, suggesting that the alterations in pulmonary perfusion were at the precapillary level. Similar abnormalities in lung perfusion and morphology were found after delivery of fractionated doses of radiation, but the onset of the changes was delayed, occurring 4-6 weeks postirradiation. Using cultured pulmonary endothelial cell monolayers, cell sloughing and retraction from the surface substrate were observed within 24 h after in vitro delivery of 30 Gy. Similar findings occurred in monolayers given fractionated doses (5 X 6 Gy) of radiation 2-3 days after the final dose. The in vivo animal and in vitro endothelial cell models offer a useful means of examining the morphologic alterations involved in radiation lung vascular damage.  相似文献   
74.
The addition of a cytoplasmic fraction, isolated from cells 3h after irradiation of mice, to exposed or intact thymocyte nuclei causes a 2- or 3-fold acceleration of chromatin degradation in the nuclei incubated in conditions optimum for activity of Ca2+,Mg2+-dependent endonuclease to be manifest. In contrast to thymocytes, no chromatin degradation products are found in liver cells of irradiated mice. The cytoplasmic fraction isolated from hepatocytes of irradiated animals fails to activate chromatin degradation in thymocyte nuclei.  相似文献   
75.
The catecholamine content was quantitated and acetylcholinesterase activity determined in rat brain locally exposed to laser radiation. It was shown that the adrenaline/noradrenaline ratio changed in the tissues under study and the cholinergic system was involved in the abscopal effect of laser radiation.  相似文献   
76.
The chemical synthesis and physical data of several new esters of testosterone (17 beta-hydroxyandrost-4-en-3-one), which contain either a halogeno or an alkoxy substituent in the acid chain, are reported.  相似文献   
77.
78.
79.
In experiments on male Albino-Swiss mice weighing 18-22 g insulin given in doses of 2 i.u./kg caused no change in the time of reaction to pain, while the same dose administered daily for 7 days potentiated the analgesic action of morphine (3 mg/kg s.c.). Glucose caused no change in this effect of insulin. After 14 days of insulin treatment the time of reaction to pain in the animals subjected to the action of morphine returned to its initial value. Twenty-four hours after the last administration of morphine the level of gamma-aminobutyric acid (GABA) was found to be decreased in the animals receiving insulin with glucose. These results suggest that the central action of insulin is dependent not only on hypoglycaemia produced by it, but may be due also to its direct action on the central structures and an indirect action mediated by its effect on other neurotransmitter systems.  相似文献   
80.
Preference was studied of one of the forelimbs during performance of different manipulating movements in white rats. High degree of "handedness" was observed in all studied movements. However it was not absolute--no animals performed all movements by one and the same limb. Degree and character of "handedness" were different for different movements, though the number of "righthanded" rats in most tests as a whole exceeded the number of "lefthanded" ones. The "handedness" depends on the individuality of the animal, the character of the motor task, learning and interference of different motor tasks during training.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号