convex this series of four reverse curves accomplishes a smoothly decreasing interior diameter that supports laminar flow into orifice 112 teat wall 191 generally increases in thickness from portion 72 and along at least part of wall 206 up to where walls 210 and 214 are located this helps to maintain the stiffness of the nipple in the portion that delivers the fluid in one non limiting embodiment that illustrates the disclosure the radii of curvature and dimensions of a teat of the type shown in fig5 9 are as follows note that the radii and dimensions are adjustable subject to finite element analysis to determine that the flow is relatively laminar on average the radii can be defined as about u00b1 0 5 mm for smaller radii to as much as about u00b1 1 mm for larger radii distance variation can be more liberal likely as much as plus 3 mm more dimension 132 the width of the slit 132 in curved lower wall 163 of valve 110 5 mm quantitative tests were run on teat 100 as compared to two standard teats with a single concave internal nipple wall leading to the orifice for a given mass flow rate out of the teat the required pressure vacuum to be created by the infant was at least 26 less than the other two.