Political songs and poems were written about the evil times of King Edward II, the military triumphs of King Edward III, and the complaints of the poor against their oppressors, such as "Song of the Husbandman". John Gower wrote moralizing poems on the villein's revolt, the sins of the clergy and lawyers, and the bad rule of King Richard II. Robin Hood ballads were popular. The minstrel, who was a honorable person, replaced the troubadour of older times. There were many colleges at Oxford and Cambridge due to the prohibition of gifts to the church. Laymen instead of ecclesiastics were appointed as Chancellor. The Masters at Oxford got rid of ecclesiastical supervision by a bishop and archdeacon by 1368. One could be admitted as a student at age thirteen. A Bachelor of Arts degree was granted after four years of study and an oral exam. Required reading in 1340 for the Bachelor's Degree was Aristotlean logic and a selection from these works: "Of Heaven and Earth", "On the Soul", "Of meteors", "Of Birth and Decay", "Of Feeling and What is Felt", "Of Memory and Recollection", "Of Sleep and Waking", "Of the Movement of Animals", "Of Minor Points in Natural History". A Master of Arts degree could be awarded after three more years of study and teaching. A Doctorate degrees in theology required ten more years of study. A Doctorate in civil or canon law required eight more years. A man with a degree in canon law who wanted to practice in a certain bishop's court had to first satisfy this bishop of his competence. The guilds gave rise to the Inns of Court in London. They used the Register of Writs, the case law of the Year Books, and disputation to teach their students. For a doctorate in medicine from Oxford or Cambridge, five more years plus two years of practice were required. Surgery was not taught because it was considered manual labor. Humans were thought to be influenced by four humors: sanguine, phlegmatic, choleric, and melancholic. Urinalysis and pulse beat were used for diagnosis. Epilepsy and apoplexy were understood as spasms inside the head. It was known what substances served as laxatives and diuretics. Teeth were extracted, eye cataracts were removed with a silver needle, and skin from the arm was grafted onto a mutilated face. Englishmen who had collected books on philosophy, medicine, astronomy, and history and literature books from the continent gave their collections to the universities, which started their libraries. Marco Polo's discoveries on his journey to China were known.