174 THE ENGINES OF THE HUMAN BODY 



The air or breath entering by the nasal aperture above 

 leaves by the laryngeal doorway below. The nasal 

 aperture is guarded by a valve-like movable curtain, the 

 soft palate ; the laryngeal doorway, as we have already 

 seen, can be automatically opened and closed. Food and 

 drink, when they enter the pharynx, cut across the air- 

 way. They enter from the mouth through the faucial 

 threshold, which is guarded by the movable pillars of 

 the palate ; they leave the lower end or bottom of the 

 pharyngeal bag by an aperture leading into the oesophagus, 

 which is guarded and kept shut by a muscular stopcock 

 mechanism known as a sphincter. The regulation of 

 traffic in the pharynx has been compared to a police- 

 controlled crossing in London. When a Royal procession 

 is to pass through the city, the police, at a given signal, 

 stop ordinary traffic and give the procession a free passage. 

 At meal-times the pharyngeal police hold up the ordinary 

 breath traffic and give the food and drink an open way. 

 With the arrival of each bolus the nasal and laryngeal 

 gateways are shut, and only reopened when the bolus has 

 entered the oesophagus and the doorway through which 

 it has passed has again become closed. Such a picture of 

 the pharynx, while expressing certain truths, gives a false 

 conception of the mechanism involved. The pharynx is 

 a muscular force-pump for pressing its contents of food 

 or of drink into the oesophagus as quickly as possible. 

 The muscular sheets which are placed within its walls are 

 made up of cylinders exactly like those of the voluntary 

 muscles of our limbs ; they are richly provided with 

 nerves, some for conveying messages from the brain to 

 the muscles, others for carrying messages in a reverse 

 direction. Yet we cannot set the pharyngeal pump going 

 by a direct effort of will ; we must first perform a piston- 

 like movement with the tongue as if we really intended 

 to swallow something, and only in that way can we set 

 the pharynx going as a force-pump. Tides of air may 

 cross and recross the pharynx as we breathe without 

 setting the pharyngeal pump into motion, but the instant 

 that a bolus of food or a mouthful of water enters it, 



