Is there a grammatical category for what might be called the universal determiner "the" - as in "the spleen."  That is, not using the word "the" to refer to a specific example or member of a class (as in "the spleen was transplanted yesterday"), and not to refer to all the members as in the class (as in "the speens are all organs"), but rather to refer to all the members of a class in terms of a single member (as in "the spleen has many cells" or "the spleen is found in the abdomen").  This is found in scientific writing all the time in phrases such as, "the brain has many neurons," "the heart pumps blood," and "the human body is a living organism."