Why does anger has something to do with spleen in both Chinese and English? The English word spleen has two meanings in Cambridge Advanced Learner's Dictionary, 


an organ near the stomach which produces and cleans the body's
blood.
a feeling of anger and disagreement.


Interestingly, Chinese character 脾, apart from the meaning of an organ, can also mean a feeling of anger in the word 脾气. Is this a coincidence?

I checked 《漢語大詞典》 for 脾气, and found that when interpreted as anger, all the examples are from modern Chinese literatures:


  曹禺 《雷雨》第二幕： “哦， 太太怪可憐的， 為什麼老爺回來， 頭一次見太太就發這麼大的脾氣？”
  趙樹理 《張來興》：“以前聽說這位老師傅有脾氣， 只是聽說。”
  沙汀 《航線》： “那個買辦， 早被頑固的訴苦弄出脾氣來了。”


but when interpreted as disposition, there is examples from earlier literatures, like 《红楼梦》 from Qing dynasty.


  人的習性。 亦借指事物的特性。 ▶ 《紅樓夢》第八四回： “都像 寶丫頭 那樣心胸兒、脾氣兒， 真是百裏挑一的。”


No corresponding meaning for spleen in English. Does this count as evidence that the meaning anger of 脾气 may be borrowed English?

In American Heritage Dictionary, there is a meaning labeled as Obsolete that reads: This organ conceived as the seat of emotions or passions. And in 《漢語大詞典》, there is a meaning of 脾气 with examples from literatures dating back to before Christ:


  脾臟之氣. 《素問‧生氣通天論》： “是故味過於酸， 肝氣以津， 脾氣乃絕。”


This seems to has something to do with traditional Chinese medicine. Why does spleen is deemed to have something to do with emotion in both English and Chinese?
It is my speculation


  https://baike.baidu.com/item/%E8%84%BE/20358750
  
  中医解读:
  
  脾气的运动特点是主升举。
  
  脾为太阴湿土，又主运化水液，故喜燥恶湿。


Translation:


  Interpretation of Chinese medicine:
  
  The typical movement of the air in the spleen is upward rising 
  
  Spleen is a cool, wet, water filtering organ. it likes dry and hot and dislikes wet and cool


Anger is also associate with 燥(dry) and hot . We say a person is easy to anger when his being 燥.

My assumption is: A person being angry because his 脾 is overly active causing 燥 (dryness) inside one's body and dryness cause anger

Related phrase:


發脾气 (being angry)
大脾气 (hot temper)
脾气好 (mild temper)
脾气差 (bad temper)