全文获取类型
收费全文 | 399544篇 |
免费 | 45019篇 |
国内免费 | 598篇 |
专业分类
445161篇 |
出版年
2018年 | 3346篇 |
2016年 | 4574篇 |
2015年 | 6311篇 |
2014年 | 7278篇 |
2013年 | 10492篇 |
2012年 | 11592篇 |
2011年 | 11684篇 |
2010年 | 7771篇 |
2009年 | 7234篇 |
2008年 | 10433篇 |
2007年 | 10515篇 |
2006年 | 10084篇 |
2005年 | 9645篇 |
2004年 | 9463篇 |
2003年 | 9300篇 |
2002年 | 9040篇 |
2001年 | 20895篇 |
2000年 | 20891篇 |
1999年 | 16288篇 |
1998年 | 5171篇 |
1997年 | 5517篇 |
1996年 | 5226篇 |
1995年 | 4651篇 |
1994年 | 4529篇 |
1993年 | 4511篇 |
1992年 | 12662篇 |
1991年 | 12396篇 |
1990年 | 11576篇 |
1989年 | 11554篇 |
1988年 | 10566篇 |
1987年 | 9851篇 |
1986年 | 8912篇 |
1985年 | 9106篇 |
1984年 | 7377篇 |
1983年 | 6271篇 |
1982年 | 4854篇 |
1981年 | 4349篇 |
1980年 | 4079篇 |
1979年 | 6947篇 |
1978年 | 5418篇 |
1977年 | 5132篇 |
1976年 | 4670篇 |
1975年 | 5139篇 |
1974年 | 5560篇 |
1973年 | 5489篇 |
1972年 | 4876篇 |
1971年 | 4571篇 |
1970年 | 4048篇 |
1969年 | 3890篇 |
1968年 | 3579篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
102.
103.
104.
105.
106.
107.
The ultrastructure of the interstitial cells of Cajal (ICC) in the oesophagus of the monkey resembled that described in the oesophagus of other mammalian species but differed in their paucity and almost lack of smooth endoplasmic reticulum, caveolae and filaments. The plasmalemma of the ICC was in close contact (20- to 30-nm gaps) with that of smooth muscle cells. This may occasionally take the form of a desmosome, but gap junctions have not been observed. Vesiculated axon profiles, containing large granular or agranular vesicles were in close contact (20- to 30-nm gaps) with the plasmalemma of ICC. In a few vesiculated profiles a presynaptic density could be recognized. The intercalation of the ICC between the vesiculated axon profiles and the smooth muscle cells suggest a role in oesophageal motility. Between 3 and 21 days following bilateral vagotomy some ICC showed regressive changes such as increased electron density and shrinkage of the cytoplasm, crowding of the organelles and dissolution of the nuclear chromatin material. Axon profiles in the vicinity of the affected ICC contained glycogen granules suggesting injury. In late stages, the number of ICC and smooth muscle contacts was reduced. The results suggest that the vagus nerves exert a trophic influence on the ICC and that the intercellular relationships between ICC and smooth muscle cells possess a degree of plasticity. It is tentatively suggested that these vagal effects may be mediated via the oesophageal myenteric ganglia. 相似文献
108.
Stewart A. Hill Stephen E. Scheckler James F. Basinger 《American journal of botany》1997,84(1):85-103
A new fern-like fossil plant is described from the lower Upper Devonian of southern Ellesmere Island, Canadian Arctic Archipelago. The plant occurs in an Archaeopteris-dominated flora preserved in the Nordstrand Point Formation (Mid-Late Frasnian) near Bird Fiord. The plant has a pinnate vegetative system with three branch orders and laminate sphenopteroid pinnules. Primary pinnae usually diverge from the main axis in distichous pairs (quadriseriate), but can depart singly (biseriate). Each primary pinna bears a basal catadromic aphlebia. Anatomically, the plant exhibits a mesarch, bipolar protostele that is ribbon- to clepsydropsoid-shaped in the main axis. Primary pinna traces are also initially bipolar and crescent-shaped, but may become four-ribbed before dividing into a pair of bipolar traces. The morphology and anatomy of this plant are nongymnospermous and are most similar to Zygopteridales (particularly Rhacophytaceae and Zygopteridaceae). The Frasnian age of Ellesmeris shows that laminated foliage had evolved in some zygopterid ferns much earlier than previously recognized. The Sphenopteris-like pinnules of Ellesmeris indicate the need for caution when attributing such a convergent foliar design to other plant groups, such as the Devonian gymnosperms. 相似文献
109.
Ultrastructure of the epithelial cells of the ventral prostate from the hopping mouse Notomys alexis
The large ventral prostate of the hopping mouse has abundant secretory units whose epithelial cells vary in height and which often have nuclei in the apical region of the cell. TEM observations indicated two epithelial cell types in which some unusual features occurred. Type A cells had granular endoplasmetic reticulum (GER) whose membranes often formed intracytoplasmic confronting cisternae. Type B cells had more fragmented and vesiculated GER with sparse ribosomes and less frequently also intracytoplasmic confronting confronting cisternae. In the latter cells, two types of granules were found, one of which was derived from the Golgi and the other possibly directly from the GER. Type A cells only had one type of granule present. A highly convoluted membrane was also found at the basal region in many of the cells. The significance of these unusual ultrastructural features has yet to be ascertained. 相似文献
110.
A method is described which allows a clear demonstration of capillaries and muscle fibers in the heart and skeletal muscle of experimental animals. The fluorescent dyes fluorescein isothiocyanate (FITC) and lissamine rhodamine B 200 (RB 200) were conjugated with a protein of high (gamma-globulin) and low (myoglobin) molecular weight, respectively, and were intravitally injected into the vascular system of rats. FITC globulin distributes itself in the intravasal space and RB 200 myoglobin in the extracellular. In histological sections the capillary lumina and the borderlines of the muscular fibers can be clearly identified and quantitatively evaluated because of the selective fluorescence in the respective structures. 相似文献