Translated by Ralph Anzarouth and an anonymous friend
According to research, anger is disorder of the thinking process coming as a results of increasing disturbance in the flow of blood around the heart resulting from an intense desire for revenge.
The master of the physicians said that this is visible in the expression of somebody who is angry and that his body heats up and his heart-rate becomes stronger and faster.
You should know that anger generates resentment since as a result of blood pollution the heart remains remains full of resentment, which subsequently takes the form of hatred and continues to intensify.
Numerous transgressions and grave sins stem out of these three: anger, resentment and hatred; in particular dispute, war and rivalry, which are the opposite of peace, source of all the good as we have mentioned.
Dispute occurs when one party rejects or opposes what the other says or wishes and this can happen even between brothers, relations or friends. War is when one party raises his hand against the other. Rivalry involves intense hostility between antagonizing parties. And innumerable evils will result from anger, apart from displaying an obvious indication of a person lacking soundness of mind.
King Solomon said (Kohelet 7, 9): "Don't rush into anger, since anger dwells within the mindless". He also said (Proverbs 29, 22): "A bad-tempered person has an inclination to misdeeds".
And our Sages of blessed memory said that losing one's temper is comparable to idol worship etc. And Talmud says (treatise Pesachim 66b): "Resh Lakish says: 'One who loses his temper, if he is a wise man his wisdom leaves him, if he is a prophet his gift of prophecy leaves him; and we have proof of this in what happened to Moses and to Elisha etc.'. And Rabbi Mani says that whoever loses his temper has his greatness removed from him, even when it has been allotted to him from the Divine Providence, as we learn from the case of Eliav etc. Our Sages have taught us from this that every person who is a leader or is greater than his fellow man should especially distance himself from this trait. And as King Solomon said (Proverbs 19,11): "The wise man is patient and his honor stems from his not taking offense. In other words, if he is tolerant this will be an honor for him, since he will overlook all offenses, thus he will not fall into the same error and he will cause no harm.
We can find the trait of anger in the bear, which by its nature especially likes to eat honey; and in the course of removing the honey from the beehive, the bees sting its eyes and it chases them in attempt to kill them. If at this stage another bee comes and stings its faces, it will abandon the first bee and chase the second. In this way he will become so angry and furious that even if there will be a thousand bees, he will try to take revenge on every one of them. This being an impossible task, he will always abandon one and chase after another, and all in vain. And King Solomon said (Proverbs 27, 3): "The stone is heavy, sand is burdensome but the anger of the wicked man heavier than both". It is brought down in the book of Ben Sira that anger is like raging fire. Seneca said that an angry man has no eyes and whatever is said or done out of anger cannot be correct. Socrates said that one's mind perceives anger while anger is mindless. And a wise man said that one should treat the angry man in three ways: with conciliatory language, with silence and by distancing oneself. Our Sages of blessed memory have already said in the Ethics of the Father (Avot 4, 18): "Do not appease your fellow man at the height of his anger". And how good it is to behave in accordance with the recommendation of our Sages in the Midrash Hagadol (the great Midrash) as it is brought down in Talmud Bavli (treatise Yoma 23a): "About those who prefer to receive offense rather than give offense, and who hear themselves abused without responding, prophets say (Judges 5, 31): 'And His beloved ones will radiate light like the the sun in its glory'."
And one should examine whether the insult and abuse which he receives is true or false. If it is indeed true, it's better and obligatory to be silent, because one should have refrained from committing acts which could result in him being abused. And if the claim is false, one should despise and treat with scorn those claims, since (Psalms 85, 12) "the truth will sprout forth from the earth" and whenever one's Yetzer1 drives him to anger, he should make self-criticism and remove the anger from his heart, and in this way eradicate evil from his body and soul.
It is written that once there were bought to a prominent minister a number of earthenware and glass vessels which were exceptionally beautiful and a delight to behold. The minister appreciated them and considered them a fitting gift for him to receive; he applauded the donor who brought him the gift. Subsequently, he took all these vessels and smashed them one by one. When he was asked why he did it, he replied: "I know myself how great is the power of anger and fury within me. I said to myself that one day my servants and attendants might break them and I would be furious: I would trample them in the dust and tread upon them in my rage. It is therefore better for me to break those vessels now, rather than be guilty of such misconduct”.
Note of the translators:
 Man has two instincts, the good one and the bad one. Our text refers to the Yetzer Hara, the bad instinct.
The whole book Tzemach Tzadik in Hebrew (printed in Rashi characters) can be downloaded in PDF format and read online at Hebrewbooks.org. Chapter 11 about anger is found at pages 23-25.