• Full text available at: DOAJ Directory of Open Access Journals
• Available from 2005.
  
• Full text available at: Free E- Journals
• Available from 1974 volume: 1.
  
• Full text available at: Oxford Journals 2018 Open Access Titles Collection
• Available from 1996 volume: 24 issue: 1.
  
• Full text available at: Oxford Journals Open Access Collection
• Available from 1996 volume: 24 issue: 1.
  
• Full text available at: PubMed Central
• Available from 1974 volume: 1.
  

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