A TRANSPORT SYSTEM 175 



then we find that the nasal and laryngeal apertures are 

 automatically closed and the pharynx at once contracts 

 and acts as a force-pump. As it passes into action not 

 only are the breath openings closed, but at the same time 

 the pillars of the fauces shut the doorway to the mouth, 

 while the sphincter at the orifice of the oesophagus relaxes, 

 thus leaving a single passage of escape for the contents 

 of the pharyngeal force-pump (see fig. 37). 



In the contrivance of the pharynx we have one of the 

 most remarkable examples of Nature's reflex or " touch- 

 the-button " mechanisms. Modern life has made us 

 familiar with inventions of this kind, but they were dis- 

 covered by Nature long ago. We press a button at the 

 door of a high building, thereby setting an electric current 

 flowing and a bell ringing in a chamber in its upper storey, 

 with the result that a bolt is drawn and the door opens to 

 let us enter. In the hall we find a lift or elevator provided 

 with a row of buttons — one for each storey. We press one, 

 an electric motor sets the lift in motion, arid we stop at the 

 desired landing. The lining membrane of the pharynx 

 is studded with " nerve-buttons " or transmitters. The 

 breath passes and repasses without influencing them, but 

 the instant a bolus of food, a mouthful of water, or 

 the root of the tongue, which forms a piston for the 

 pharynx as well as for the mouth, comes in contact with 

 or touches them, then a stream of messages is thereby 

 dispatched which flows upwards in the nerves until certain 

 busy nerve exchanges are reached in the medullary part 

 of the brain stem. From these exchanges relays of orders 

 are issued which reach the muscular engines controlling 

 the doorways of the pharynx ; those leading to the larynx, 

 to the nose, and to the mouth are promptly shut, while 

 the sphincter engine, which guards the opening to the 

 oesophagus, is ordered to relax. At the same time the 

 muscular walls of the pharynx are thrown into action ; 

 the pharynx then becomes a force-pump ; all its openings 

 are closed save one, and it is through that one that the 

 bolus is forced within the upper end of the oesophagus. 

 How essential the machinery of the pharynx is to life we 



