Crying is a common way for humans to vent their emotions. People not only cry when they are sad, but also when they are happy and excited. However, this is only half the story. In fact, crying can be divided into two types: emotional crying and reflexive crying.
Psychologists believe that emotional crying is an effective "panacea" for relieving stress and soothing emotions. In fact, crying bitterly when feeling upset and releasing the pent-up frustration and worries can greatly improve one's mood. If one does not cry when needed, it may lead to mental depression and many physical diseases, such as peptic ulcer.
Reflexive crying is caused by some kind of stimulation to the eyes. For example, when cutting onions, the irritating odor will stimulate the eyes, and a large amount of tears will flow out of the eyes in a short time, which can wash away the irritating odor of onions or foreign objects. When the eyes are stimulated by strong light, reflexive crying will also occur. These are normal physiological phenomena.
Tears are a colorless and transparent liquid, most of which is water with a small amount of salt, so they are slightly salty.
So where do tears come from? It turns out that everyone's eyes have a "small factory" that "produces" tears, called the lacrimal gland. In fact, the lacrimal gland secretes tears non-stop every day. The use of tears is significant - when blinking, tears are evenly applied to the surface of the eyeball, making the surface of the eyeball moist and not feeling dry, and also washing away the dust on the surface of the eyeball, keeping its surface clean; tears can also kill bacteria, because tears contain lysozyme and some immune active substances.
The lacrimal gland secretes tears, some of which flow into the nasal cavity through the nasolacrimal duct, and some are drained to the surface of the eyeball through the lacrimal duct. Under normal circumstances, the lacrimal gland can secrete 0.5 to 0.6 milliliters of tears during the day, some of which are used to moisten the surface of the eyeball and then evaporated, and the rest enter the lacrimal sac through the puncta and finally flow into the nasal cavity through the nasolacrimal duct. Because the amount of tear production and the amount of tear excretion are in balance, there will be no tears in normal circumstances. If the tear secretion is excessive or the tear excretion system is blocked, the tears cannot be evaporated or enter the lacrimal sac in time, and tears will occur. When people encounter sad things or are stimulated by foreign objects, the lacrimal gland will secrete more than dozens or even hundreds of times the normal amount of tears in a short time, so tears will flow down their cheeks.