Crying (also called sobbing, weeping, wailing, whimpering, bawling, or blubbering) is the shedding of tears in response to an emotional state in humans. The act of crying has been defined as "a complex secretomotor phenomenon characterized by the shedding of tears from the lacrimal apparatus, without any irritation of the ocular structures". A related medical term is lacrimation, which also refers to non-emotional shedding of tears. A neuronal connection between the lacrimal gland (tear duct) and the areas of the human brain involved with emotion has been established. Some scientists believe that only humans produce tears in response to emotional states, while others disagree. Charles Darwin wrote in The Expression of the Emotions in Man and Animals that the keepers of Indian elephants in the London Zoo told him that their charges shed tears in sorrow. Tears produced during emotional crying have a chemical composition which differs from other types of tears. They contain significantly greater quantities of the hormones prolactin, adrenocorticotropic hormone, Leu-enkephalin, and the elements potassium and manganese.