Nipple for nursing bottles

A nipple for nursing bottles, comprising:

• a nipple body having a pair of openings provided at front and bottom ends thereof and including an inverted funnel-shaped intermediate portion;
• an annular flange provided at the bottom end of said nipple body, said annular flange extending outwardly along a direction at a right angle with respect to a longitudinal axis of said nipple body and having an annular rib at a bottom surface thereof;
• at least one air valve provided at said annular flange, said air valve having a recessed portion extending from said annular flange into said annular rib and a slit provided at said annular rib for communicating an inside of said nipple to an outside thereof;
• a partition diaphragm provided inside of said nipple body and disposed in a region remote from said front and bottom ends of the nipple body in a vicinity of a boundary between said front end and said inverted funnel-shaped intermediate portion and at a position where pressure is exerted by an infant during feeding;
• and
• a cut in said partition diaphragm forming a milk sucking passage therein, said cut being normally closed, and wherein said partition diaphragm has a spherical recess at said milk sucking passage,
• wherein said cut is opened and closed in response to said pressure exerted by an infant during feeding, and
• wherein the partition diaphragm is upwardly arcuate, and a central portion of said upwardly arcuate partition diaphragm comprises a thickened portion, the lower side of said thickened portion being formed with said spherical recess defining a thin diaphragm portion above said spherical recess, said cut being formed in a central region of said thin diaphragm portion.