Except For Fornication: Is Adultery of the Heart Scriptural Grounds For Divorce?

In Matthew 5:31-32 the Lord stated, "It hath been said, Whosoever shall put away his wife, let him give her a writing of divorcement: But I say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery."  A few verses earlier Jesus also stated, "Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart." (Mathew 5:27-28) When these two passages are combined, does the Bible teach that adultery of the heart is Scriptural grounds for divorce?  We will try to answer this question by considering both passages within context and in light of Scripture.