Introduction
The mammary glands of mammals from the platypus to man are identical in fine structure, and consist of alveolar tissue within which milk is continuously secreted during lactation. An alveolar structure increases enormously (perhaps by 10 000-fold) the surface area for secretion relative to the external size of the gland, but at the same time complicates the problem of milk removal. Small ducts generate substantial surface tension forces that oppose the movement of fluids: suction is therefore a relatively ineffective method for removal of alveolar milk. The problem has been overcome by investing the alveoli in a basket-like reticulum of myoepithelium which contracts in response to oxytocin released from the posterior pituitary. When stimulated by oxytocin the alveoli are compressed and milk is expelled into the larger collecting ducts for removal by the sucking of the young. Further, the establishment of a nervous link between the nipple and the
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