Medical Subject Headings
MeSH Tree Structures
MeSH descriptors are organized in 16 categories: category A for anatomic terms, category B for organisms, C for diseases, D for drugs and chemicals, etc. Each category is further divided into subcategories. Within each subcategory, descriptors are arrayed hierarchically from most general to most specific in up to eleven hierarchical levels. These trees should not be regarded as representing an authoritative subject classification system but rather as arrangements of descriptors for the guidance and convenience of persons who are assigning subject headings to documents or are searching for literature. The trees are not an exhaustive classification of the subject matter but contain only those terms that have been selected for inclusion in this thesaurus. Their structure frequently represents a compromise among the views and needs of particular disciplines and users, in the absence of any single universally accepted arrangement.
Because of the branching structure of the hierarchies, these lists are sometimes referred to as "trees". Each MeSH descriptor appears in at least one place in the trees, and may appear in as many additional places as may be appropriate. Those who index articles or catalog books are instructed to find and use the most specific MeSH descriptor that is available to represent each indexable concept. For example, articles concerning Streptococcus pneumoniae will be found under the descriptor Streptococcus Pneumoniae rather than the broader term Streptococcus, while an article referring to a new streptococcal bacterium which is not yet in the vocabulary will be listed directly under Streptococcus. Accordingly, the user may consult the trees to find additional subject headings which are more specific than a given heading, and broader headings as well. For example, under Abnormalities, there are specific abnormalities:
Abnormalities, Drug Induced C16.131.42
Abnormalities, Multiple C16.131.77
Alagille Syndrome C220.127.116.11
Angelman Syndrome C18.104.22.168
In the printed version of the trees, each descriptor is followed by the number that indicates its tree location. It may also be followed by one or more additional numbers, in smaller type, and truncated at the third level, indicating other tree locations of the same term. The numbers serve only to locate the descriptors in each tree and to alphabetize those at a given tree level. They have no intrinsic significance; e.g., the fact that D12.776.641 and D12.644.641 both have the three digit group 641 does not imply any common characteristic. The numbers are subject to change when new descriptors are added or the hierarchical arrangement is revised to reflect vocabulary changes.
The trees also contain geographic descriptors (Category Z), which also do not appear in the printed Alphabetic List. These descriptors may be used in searching MEDLINE.