It is that of the mouth of the young kangaroo.

In all mammals, as in ourselves, the air-passage from the lungs opens in the floor of the mouth behind the tongue, and in front of the opening of the gullet, so that each particle of food as it is swallowed passes over the opening, but is prevented from falling into it (and thus causing death from choking) by the action of a small cartilaginous shield (the epiglottis), which at the right moment bends back and protects the orifice.

Now the kangaroo is born in such an exceedingly imperfect and undeveloped condition, that it is quite unable to suck.

The mother therefore places the minute blind and naked young upon the nipple, and then injects milk into it by means of a special muscular envelope of the mammary gland.

Did no special provision exist, the young one must infallibly be choked by the intrusion of the milk into the windpipe.