 The final letter may be dropped if the resulting name would be difficult to pronounce otherwise.Examples include "-ci(r)-" for the circulatory system, "-li(m)-" for the immune system ("lim" stands for lymphocyte) and "-ne(r)-" for the nervous system.The final letter is usually omitted if the following source substem begins with a consonant (such as "-zu-" or "-xi-"), but not all target substems are used in their shortened form.