全文获取类型
收费全文 | 22253篇 |
免费 | 613篇 |
国内免费 | 5篇 |
专业分类
22871篇 |
出版年
2016年 | 160篇 |
2015年 | 287篇 |
2014年 | 351篇 |
2013年 | 947篇 |
2012年 | 525篇 |
2011年 | 523篇 |
2010年 | 410篇 |
2009年 | 559篇 |
2008年 | 482篇 |
2007年 | 526篇 |
2006年 | 412篇 |
2005年 | 403篇 |
2004年 | 316篇 |
2003年 | 333篇 |
2002年 | 353篇 |
1999年 | 168篇 |
1998年 | 177篇 |
1996年 | 190篇 |
1995年 | 206篇 |
1994年 | 159篇 |
1993年 | 286篇 |
1992年 | 260篇 |
1991年 | 264篇 |
1990年 | 273篇 |
1989年 | 436篇 |
1988年 | 412篇 |
1987年 | 500篇 |
1986年 | 427篇 |
1985年 | 445篇 |
1984年 | 366篇 |
1983年 | 307篇 |
1982年 | 379篇 |
1981年 | 386篇 |
1980年 | 377篇 |
1979年 | 358篇 |
1978年 | 420篇 |
1977年 | 448篇 |
1976年 | 481篇 |
1975年 | 487篇 |
1974年 | 343篇 |
1973年 | 190篇 |
1972年 | 195篇 |
1971年 | 187篇 |
1970年 | 192篇 |
1969年 | 167篇 |
1968年 | 170篇 |
1967年 | 150篇 |
1964年 | 180篇 |
1958年 | 142篇 |
1957年 | 176篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
21.
22.
In a field study, heart rate and motor activity were assessed continuously in 12 male smokers during 2 smoking and 2 abstinence days and in 12 male nonsmokers during 4 days. A circadian analysis revealed earlier activity acrophases in smokers than nonsmokers and earlier heart rate acrophases in abstinent than smoking smokers. Furthermore, heart rate acrophases of smoking smokers significantly anticipated activity acrophases;, whereas in abstinent smokers and nonsmokers the two parameters oscillated in phase. With the use, in smoking smokers, of the individual average smoking interval as a hypothetical ultradian period length, significant periodicities were found for heart rates in 16 and for activity in 15 of 24 observation days. These rhythms were nicotine independent and based on heart rate and activity increases prior to lighting up the cigarettes. Individual frequency spectra for the 16 h after getting up and the 7 h after going to bed did not reveal single dominant frequencies but rather complex frequency distributions. Power spectra of the daytime data revealed no group differences for activity and no heart rate differences between smoking smokers and nonsmokers. In abstinent smokers, however, a significant reduction of heart rate frequencies slower than 1 cycle/135 min and a significant increase of heart rate frequencies faster than 1 cycle/20 min were observed as compared with all other groups. This effect persisted over the 2 abstinence days, suggesting an activity-independent change in the frequency distribution of heart rates after quitting smoking. 相似文献
23.
Three experiments were performed to study the influence of challenging incentives on feedback-assisted heart rate reduction for coronary-prone (Type A) and non-coronary-prone (Type B) males. In the first experiment, when subjects were given a competitive instructional set, Type As were significantly more successful relative to Type Bs in reducing their heart rate; with a noncompetitive set, Type Bs were significantly more successful than were Type As. In the second experiment, when told that heart rate reduction was a scarce ability, Type As reduced heart rate significantly better than did Type Bs; when told that heart rate reduction was a common ability, Type Bs achieved significantly greater heart rate reduction than did Type As. In the third experiment, when heart rate reduction was described as being instrumental to time-urgency (i.e., getting more done in less time), Type As reduced heart rate significantly bettern than did Type Bs; when heart rate reduction was described as being instrumental to relaxation, Type Bs were significantly better able to reduce heart rate. In all three studies, the incentives had no effect on heart rate when feedback was not provided. The results are discussed as support for the notion that Type A behavioral pattern characteristics can be exploited to reduce Type A symptoms. Implications for how coronary-prone individuals may be challenged to modify symptoms within the clinical setting are discussed. 相似文献
24.
Dr. H. Eidmann 《Zoomorphology》1924,2(3-4):567-610
25.
Satoshi Shizukuishi Satoshi Nishii John Ellis Karl Folkers 《Biochemical and biophysical research communications》1980,95(3):1126-1130
The mean basal specific activities and the mean % deficiencies of the activity of the glutamic oxaloacetic transaminase of the erythrocytes were identical (n.s.) for a group of eight patients with a severe carpal tunnel syndrome and for a group of eight university students. There was no significant difference in the increases in the specific activities for the patients and the students at 4 concentrations of pyridoxal 5′-phosphate. The apparent Km for the patients and the students was 95 μM and 61 μM (n.s.) respectively. It is concluded from these data in conjunction with previous findings that the carpal tunnel syndrome is a deficiency disease of vitamin B6, which is probably primary rather than one of a dependency state. 相似文献
26.
27.
28.
29.
Dr. R. Miikkulainen 《Biological cybernetics》1992,66(3):273-282
An approach to episodic associative memory is presented, which has several desirable properties as a human memory model. The design is based on topological feature map representation of data. An ordinary feature map is a classifier, mapping an input vector onto a topologically meaningful location on the map. A trace feature map, in addition, creates a memory trace on that location. The traces can be stored episodically in a single presentation, and retrieved with a partial cue. Nearby traces overlap, which results in plausible memory interference behavior. Performance degrades gracefully as the memory is overloaded. More recent traces are easier to recall as are traces that are unique in the memory.This research was supported in part by an ITA Foundation grant and by fellowships from the Academy of Finland, the Emil Aaltonen Foundation and the Foundation for the Advancement of Technology (Finland) when the author was at UCLA 相似文献
30.
Dipl. agr. S. Borissenko Prof. Dr. sc. G. Schuster Doc. Dr. W. Schmygla 《Journal of Phytopathology》1985,114(2):185-188
A high percentage of plantlets, in which PVX, PVM, and PVS were no longer detectable by precipitin tests was obtained from 100% virus-infected stocks of the potato varieties Rosa and Priobskij rannija by combining meristem (tip) culture with antiphytoviral chemotherapy. When 2,4 - dioxohexahydro 1,3,5-triazinc (DHT) was added to the nutrient media, no virus was detectable serologically in 72% of the corresponding explants of the variety Rosa and in 66% of that of the variety Priobskij rannija, respectively. Contrary to this, all untreated control explants proved virusinfected. Treatment with 2-thiouracil resulted in 46.6%, with cyanoguanidine in 28.6% of plantlets, in which no virus was detectable serologically. 相似文献