One's-self I sing, a simple separate person,
Yet utter the word Democratic, the word En-Masse.
Of physiology from top to toe I sing,
Not physiognomy alone nor brain alone is worthy for the Muse, I say
the Form complete is worthier far,
The Female equally with the Male I sing.
Of Life immense in passion, pulse, and power,
Cheerful, for freest action form'd under the laws divine,
Physiognomy, most popular in the 18th and 19th centuries, is the study of a person’s facial features in order to determine their character.  For instance, a dude with sharp features may be likened to an eagle and thought to be keen, quick-witted, or perhaps a bit severe. 
 
 Physiognomy is generally thought to be a pseudoscience, which Whitman seems to agree with in saying that it “alone” is not “worthy for the Muse” 
 He may also mean more simply that he is singing of the body as well as the soul.
Ones-Self I Sing