There are two kind of tear. One is the tear generated physical stimulation, for example the tears comes out when crushing onion. The other one is emotional tears. When people are happy, upset, or mad, they will well up with tears. There are several theories about why people crying with tears. The one I believe in is that tear is the medium to transport some extra substance out of body. It is said that foreign stimulation is transmitted between nerves by central neurotransmitter. The amount of neurotransmitter shall be comparatively stable. When there are more central neurotransmitters than needed, special lytic enzyme will resolve the extra neurotransmitters. When people get mad, upset or excited, the extreme emotion will promote the generation of too much neurotransmitters which could be totally resolved by lytic enzyme. Too much neurotransmitter is harmful to people’s health. It will improve the morbidity of ulcer and enteritis. So, the tear gland secretes tears to rinse the extra neurotransmitters out on purpose of protection.
