Give a critical appreciation and brief summary of Browning's "My Last Duchess."

"My Last Duchess" by Robert Browning has only slight design.  The lines follow each other without formal grouping; the only breaks are dictated by units of meaning, much in the manner of paragraphs in prose.  However, there is some pattern as "My Last Duchess" adds regularity of rhyme, for it is written in rhyming pentameter couplets.  These couplets suggest the next step that the duke takes as he descends the stairs in his opulent home.

As he descends the stairs continuing his monologue, the duke directs an emissary to "glance" at the portrait of his late wife.  This emissary is sent by a wealthy man, whose daughter the duke intends to marry.  With aritocratic arrogance, the duke indicates that others have wondered about the "depth and passion of its earnest glance."  No, he tells, the emissary, the look was not inspired by himself, but by the painter, Fra Pandolf--her lover.  For, his wife had

A heart--how shall I say!--too soon made glad,/Too easily impressed; she liked whate'er/She looked on, and her looks went everywhere.....She thanked men--good! but thanked/Somehow--I know not how--as if she ranked/My gift of a nine-hundred-years-old name/With anybody's gift. 

Here the implication by duke is that he did not mind that she bestowed her looks and favors on other men, so much that he was insulted that she "smiled" and displayed such lack of respect for his name and social position.  For this, she was punished:

I gave commands;/Then all smiles stopped together.  There she stands/As if alive.  Will 'please you rise?  We'll meet/The company below, then.

After implying that the last duchess departed the world because she insulted the family name and did not obey "the commands" given by him, the duke does not miss an iamb as he nonchalantly dismisses the portrait and brings the emissary back to the business at hand.

As they finish their descent of the stairs, the duke points to a statue of Neptune taming a sea-horse, a treasure that suggests both the duke's wealth and his cruelty as he, too, "tames" his duchesses.