Lacrimal Gland Structure

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Human Uniqueness Compared to "Great Apes": 
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Humans are unique among primates in producing emotional, or psychic, tears. All mammals produce basal tears that moisten the conjunctiva and cleanse the eye, and reflex tears that flush the eye in response to pressure, foreign objects, or irritation. Emotional tears differ from basal and reflex tears in being richer in proteins (especially hormones), potassium, and manganese. Emotional tears also differ from other tear types in volume, being so copious that they overwhelm the normal drainage pathway (the nasolacrimal ducts) and stream visibly onto the face. While multiple glands contribute to the production of basal and reflex tears, it is thought that the bulk of psychic tears derive from the lacrimal glands, and that human lacrimal glands are more richly innervated than those of other primates (having neuronal connections with more areas of the brain, especially those concerned with the sensation and expression of deep emotion). The human capacity for emotional crying is thought to represent an honest signal of emotional condition that evolved in the context of heightened cooperation, social bonding, and social communication that characterizes human interactions.

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