the air-pipe; and, to keep food and drink from taking the wrong road, there is an effective system of valves. The parts act spasmodically whenever they are irritated by the pressure of solids and liquids.
Saliva is found in nearly all animals. Its universal office is to lubricate the food and so help it to glide easily along the pharynx and
gullet. This kind of saliva is a glairy mucus, and is the only kind in animals which do not chew the food—for example, birds, reptiles, and fishes. As an aid to digestion, the saliva of mammals will be considered later.
The most astonishing feats in swallowing are performed by the snakes. The boa can certainly swallow a goat or deer. Our common little snakes, the size of a finger, can swallow a large frog, a performance sufficiently remarkable. The process is very slow and tedious, and one would suppose painful. The boa first kills its prey by crushing it in its tightening coils, which break down the ribs and limbs and reduce the victim to a shapeless mass. By this horrible proceeding the carcass is gotten into condition to be more easily swallowed. After coating it with mucus, the boa begins the difficult operation of forc-
