Though many rational theories have been advanced on the subject, it is not known whether nervous, chemical or glandular agents operate to start the flow of milk. Even seemingly convincing experimental evidences are contradictory concerning the causes of the hypertrophy of the mammary glands during pregnancy and their secretory activity immediately after parturition. There is a general opinion that nervous shock, certain psychic states caused by the presence or absence of the suckling reflex and the emotional status and environment of the mother affect the flow of milk. The perturbation accompanying these factors is evidently not a direct nervous mechanism, since it has been shown experimentally that the severance of the spinal cord in woman1 and in other mammals2 and of the nerves supplying the mammary glands3 causes no untoward influence on the secretory activity of the glands. This evidence is further substantiated by the results of
MACY IG, HUNSCHER HA, DONELSON E, NIMS B. HUMAN MILK FLOW. Am J Dis Child. 1930;39(6):1186–1204. doi:10.1001/archpedi.1930.01930180036004
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