Sentence:  To a much less full extent , the hebephrenic person 's belching or flatus has a comparable communicative function ; in working with these patients the therapist eventually gets to do some at least private mulling over of the possible meaning of a belch , or the passage of flatus , not only because he is reduced to this for <WSD>lack</WSD> of anything else to analyze , but also because he learns that even these animal like sounds constitute forms of communication in which , from time to time , quite different things are being said , long before the patient can become sufficiently aware of these , as distinct feelings and concepts , to say them in words . 
Meaning of the word marked with <WSD> tags:  the state of needing something that is absent or unavailable