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As suggested above, the active pressure gradient could make it possible for milk to be delivered continuously from the breast to the baby’s mouth, were it not for the “gating” effect of the baby’s jaws. Accordingly, the milk available on each suck is limited to that captured in the milk ducts lying within the baby’s mouth; milk cannot be extracted directly from the breast. A further fact, which should not be overlooked, is that the milk duct openings are very much narrower (by up to 50 times) than the dilated milk-filled ducts leading to them. So, an essential corollary to the question above is: “What force is responsible for opening the duct ends?”

Based on the proposition of Geddes and colleagues [10–14], can it be the case that localised added suction (ETD) at the nipple surface is the force responsible? The answer is likely to be an emphatic “No.” Any level of suction pressure applied outside the nipple surface (if this exceeds the positive milk pressure created by the mother’s MER), is likely to cause collapse of the teat openings. While suction can be transmitted through a fixed aperture, and propagated back along a rigid tube, this cannot occur in the flexible, collapsible milk duct system of the breast. Nipple duct opening, therefore, cannot be achieved from outside the nipple surface.

Instead, this can only be achieved from within, by increasing intra-ductal pressure. This is precisely what the peristaltic tongue movements do. Having captured milk within the milk ducts held in the oral cavity, the peristaltic wave of compression squeezes this milk towards the nipple end; the resulting rise in intra-ductal pressure forces the milk duct ends open. Only when this has happened, might extra-ductal pressure (added intra-oral suction from an ETD) be capable of enhancing either the rate of milk extraction, or the net volume of milk transferred during that suck. The mechanism by which added suction is likely to achieve this is by extending the suck duration, potentially achieving more effective emptying of the ducts.

From this perspective, not only are peristaltic tongue movements (PTMs) the obligate, primary tongue movement, present throughout active sucking, they also appear to be the primary mechanism by which milk is forced towards the duct openings, and out into the baby’s mouth. It may be deduced from this that the efficiency of such a mechanism will depend on the surface area of the nipple-breast “teat” complex lying against the baby’s tongue. In addition, the wider the baby’s mouth is flanged, the better will be its apposition to the breast; resulting in a greater mouthful of breast tissue being taken by the baby. Both these key features will be enhanced by maximising the “positioning” and “attachment” of the baby at the breast.