The Index to the National Library of Medicine Classification consists of Medical Subject Headings (MeSH). Some non-MeSH terms are used as cross references when no appropriate MeSH term is available. All MeSH entries in the Index are updated annually to be consistent with the latest edition of MeSH. The index terms are arranged in alphabetical order with Roman numerals filed as letters in this arrangement. Arabic numerals are found at the beginning of the Index.
The classification numbers assigned to the index terms are usually general numbers for the concept represented or numbers reflecting a medical view when that is more appropriate.
Indented terms represent more specific aspects of the subject or aspects of the subject to which a number different from the general number has been assigned. The indented terms are often elliptical and should be interpreted broadly. For example, when “Organic chemistry” is used as a term indented under the name of a chemical, the number following it is to be selected if the principal focus of the work being classified is the organic chemistry of the chemical. Some subheadings refer the user to another heading. General references or see also references are listed at the end of the alphabetical sequence of the indented terms under the index term.
How to use the Index
The Index is not a substitute for the main schedules. A user should always refer to the schedules for confirmation of the proper application of the number and its relationship to other numbers.
The number assigned to a heading in this Index should not be used unless it represents the principal subject of the material being cataloged.
The Index does not provide annotations nor does it show relationships between headings; these are found in the Annotated Alphabetic List and the Tree Structures, respectively, or the Legal Index Browser. To view descriptor data, when available, click on the “tree” icon on the left of the index term, which links to the term in the current Browser.
The number of indented terms under an index term varies greatly. The choice was dependent upon the needs that arose in the past. Therefore, the list is in no way exhaustive of the possibilities that can occur.
The Index contains over ten thousand index terms to which classification notations are assigned. Many terms are found only in the index and will not appear in the schedules. They refer to a number in the schedule where only a broader term or a related term appears.
There are several types of cross references used in the index.