Say, I want to know what a "dog" is, so I read the definition for dog in the OED. But, of course, it contains a list of words too so I set out to read the definitions for those as well. This forms a directed graph. Was there any research done on this? I presume, for example, you won't need to understand what a "ventriloquist" is just to understand "dog" -- but , for example, is there a list of words that eventually the definition chains of all words reach?