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   926篇
  免费   0篇
综合类   1篇
化学工业   2篇
建筑科学   1篇
轻工业   1篇
冶金工业   921篇
  2022年   1篇
  2016年   1篇
  2013年   1篇
  1999年   26篇
  1998年   266篇
  1997年   144篇
  1996年   105篇
  1995年   65篇
  1994年   50篇
  1993年   53篇
  1992年   6篇
  1991年   8篇
  1990年   9篇
  1989年   12篇
  1988年   9篇
  1987年   6篇
  1986年   10篇
  1985年   3篇
  1983年   2篇
  1982年   3篇
  1981年   8篇
  1980年   8篇
  1979年   1篇
  1978年   2篇
  1977年   44篇
  1976年   78篇
  1975年   1篇
  1966年   1篇
  1965年   1篇
  1962年   1篇
  1955年   1篇
排序方式: 共有926条查询结果,搜索用时 31 毫秒
31.
In the course of the past few years, sonography has become firmly established in clinical practice. The accuracy of the investigations does not only depend on the quality of the equipment used but also to a considerable degree on the standards of investigator training. To reduce the discrepancy between the number and quality of the investigations in the German Federal Republic, the "Kassen?rztliche Vereinigung" has released guidelines requiring the users of sonographic equipment to undergo theoretical and practical training. These guidelines are reviewed and commented.  相似文献   
32.
Anterograde and retrograde tracing experiments have been used to demonstrate the origin and terminal distribution of commissural fibers in the first somatosensory cortex (SI) of the rat. The commissural fibers originate from pyramidal cells of all layers, but predominantly from layers III and V. The fibers terminate in a series of approximately vertical bands. In each of these there are concentrations of terminals extending from the inner portion of the molecular layer to the deep portion of layer III as well as in the superficial part of layer V, and in layer VI. Discrete vertical bands of cortex are reciprocally connected across the midline to give both the origin and terminal regions of the projection a patchy or "columnar" appearance. The commissural fibers arise from and terminate in areas of the cortex that lie between and alongside the aggregations of granule cells that distinguish SI of the rat. No commissural fibers terminate within the aggregations of layer IV cells themselves but the more superficial terminal ramifications may come to overlie these aggregations. A heterotopic projection to the contralateral second somatosensory cortex has been observed and is similar in form to the homotopic projection to SI. Many commissural fibers have crossed the midline in the corpus callosum by the day of birth but lie in the underlying white matter and do not enter the cortical plate until at least the third postnatal day. During the first postnatal week these fibers grow somewhat diffusely into the maturing cortex and their topographic and laminar pattern of distribution attains its adult characteristics by the end of the first week. Commissural axons, thus, arise from immature cells but the maturation of cell form seems to precede the ingrowth of these axons and the acquisition of commissural synapses.  相似文献   
33.
MANZANO  C  FERNANDEZ  PC  HILL  JG  LUFT ALBARRACIN  E  VIRLA  EG  COLL ARÁOZ  MV 《Journal of chemical ecology》2022,48(7-8):650-659

Parasitoids are known to exploit volatile cues emitted by plants after herbivore attack to locate their hosts. Feeding and oviposition of a polyphagous herbivore can induce the emission of odor blends that differ among distant plant species, and parasitoids have evolved an incredible ability to discriminate them and locate their hosts relying on olfactive cues. We evaluated the host searching behavior of the egg parasitoid Cosmocomoidea annulicornis (Ogloblin) (Hymenoptera: Mymaridae) in response to odors emitted by two taxonomically distant host plants, citrus and Johnson grass, after infestation by the sharpshooter Tapajosa rubromarginata (Signoret) (Hemiptera: Cicadellidae), vector of Citrus Variegated Chlorosis. Olfactory response of female parasitoids toward plants with no herbivore damage and plants with feeding damage, oviposition damage, and parasitized eggs was tested in a Y-tube olfactometer. In addition, volatiles released by the two host plant species constitutively and under herbivore attack were characterized. Females of C. annulicornis were able to detect and significantly preferred plants with host eggs, irrespectively of plant species. However, wasps were unable to discriminate between plants with healthy eggs and those with eggs previously parasitized by conspecifics. Analysis of plant volatiles induced after sharpshooter attack showed only two common volatiles between the two plant species, indole and β-caryophyllene. Our results suggest that this parasitoid wasp uses common chemical cues released by many different plants after herbivory at long range and, once on the plant, other more specific chemical cues could trigger the final decision to oviposit.

  相似文献   
34.
35.
Men develop perioperative ulnar neuropathies more frequently than women. To determine the role of anatomical gender differences in the development of these neuropathies, we performed several studies of the anatomy of the ulnar nerve, cubital tunnel, and elbow region. These studies included detailed dissection of male and female embalmed and unembalmed cadavers, ultrasound measurements of the tissue layers at the elbow, and measurement of various dimensions of the coronoid process of the ulna in multiple skeletal sets. No gross anatomical differences were found between genders regarding the course of the ulnar nerve through the upper limb. However, there was a strikingly larger (2-19 times greater) fat content on the medial aspect of the elbow in women compared to men, and the tubercle of the coronoid process was approximately 1.5 times larger in men (P < or = .002, rank sum test). Our finding suggest that the tubercle of the coronoid process is a likely area for external compression-induced ischemia of the ulnar nerve because the nerve and its arterial supply (the posterior ulnar recurrent artery) are covered at the tubercle only by skin, subcutaneous fat, and a very thin aponeurosis of the flexor carpi ulnaris. Importantly, this tubercle is larger and the nerve and blood vessels passing by it are less protected by subcutaneous fat in men than in women. These two anatomical differences between men and women may contribute to the increased frequency of perioperative ulnar neuropathy induced by external pressure at the medial aspect of the elbow in men.  相似文献   
36.
37.
OBJECTIVE: To assess the outcomes of changes in mental health policy introduced in Italy in 1978. METHODS: Data on psychiatric services, before and after the policy change, are presented. Effects of change are evaluated through indicators related to four issues: transfer of care, criminalisation of the mentally ill, suicides, and homelessness. RESULTS: Admissions of new patients to mental hospitals have been stopped and the size of the mental hospital population is now very low (26 per 100,000 population). Psychiatric care has been shifted to community services including general hospital psychiatric units. There has been an overall reduction of psychiatric hospitalisation. However, the provision of residential facilities is inadequate and community services are unevenly distributed across the country. Few negative effects of changing patterns of care have been reported, although the low quality of data limits the validity of such a conclusion. Outcome of care in areas where the full range of community services is available has been rated as satisfactory. CONCLUSIONS: Although care of the mentally ill has been shifted to community services, we lack hard data on the social and clinical outcome of community care at the nation-wide level. Long-term monitoring and evaluation of community services is a high priority in Italy.  相似文献   
38.
Glycosyltransferases in the Golgi apparatus synthesize cell wall polysaccharides and elaborate the complex glycans of glycoproteins. To investigate the targeting of this type of enzyme to plant Golgi compartments, we generated transgenic Arabidopsis plants expressing alpha-2,6-sialyltransferase, a glycosyltransferase of the mammalian trans-Golgi cisternae and the trans-Golgi network. Biochemical analysis as well as immunolight and immunoelectron microscopy of these plants indicate that the protein is targeted specifically to the Golgi apparatus. Moreover, the protein is predominantly localized to the cisternae and membranes of the trans side of the organelle. When supplied with the appropriate substrates, the enzyme has significant alpha-2,6-sialyltransferase activity. These results indicate a conservation of glycosyltransferase targeting mechanisms between plant and mammalian cells and also demonstrate that glycosyltransferases can be subcompartmentalized to specific cisternae of the plant Golgi apparatus.  相似文献   
39.
40.
1. The mean incidence of deaths from ascites in the UK in 1993 was 1.4% (0.7% in 1991 and 0.9% in 1992) and 0.8% from sudden death syndrome (SDS). In total, the economic loss to the UK Broiler Industry in 1993 as a result of these 2 conditions was 24 Pounds M. 2. Clear geographical differences emerged in the occurrence of ascites, with, not only the lowest incidences being observed in Northern Ireland, but also the peak of the mortality from ascites occurring much later in the rearing cycle than in other regions on the mainland. 3. In all regions the incidence of SDS was lower than that of ascites but the reason for this disparity remains to be established. 4. Some of the variables associated with the road transportation of day-old chicks from the hatchery to the farm appeared to influence the incidence of ascites. These included distance or time travelled, stocking density, internal lorry temperature and the length of time the lorry was heated before transport as well as the time the shed was heated before chick arrival. Temperature was also an important factor during growth (brooding and finishing). 5. Negative pressure-powered ventilation was preferred in most organisations but more ascites was seen with positive pressure ventilation. However, the lowest incidence of ascites occurred with natural ventilation. There was more ascites relative to shed orientation when the wind direction was from the west compared to the east. 6. This survey identifies the extent of the problem of broiler ascites in the UK and also highlights the importance of good management control of day-old chicks, not only following placement, but even before their arrival on the farm.  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