316 ALIMENTARY CANAL 



faces are elongated and narrow instead of short and round like 

 those of primitive men. 



II. The act of swallowing. The food, after being chewed, is 

 collected on the surface of the tongue by the action of the bucci- 

 nator and other voluntary muscles. The tip and sides of the tongue 

 are pressed against the hard palate and teeth. A rapid contrac- 

 tion of the mylo-hyoid muscles, which form "a floor for the front 

 portion of the mouth (diaphragma oris), pushes the tongue up 

 against the hard palate. At the same time the hyoglossus pulls 

 the tongue backwards and the bolus is shot towards the gullet. 

 This closes the voluntary stage of deglutition. 



Several other muscles come into play at this point. As will 

 be seen from Fig. 42, just above the larynx is a busy crossing 

 common to two routes. Gaseous food and gaseous excreta pass 

 to and fro right across the track of the descending bolus. At 

 the moment of swallowing, the nose-to-lung and lung-to-nose 

 traffic is reflexly held up. Further, the escape of the food mass 

 by either of these incorrect routes is prevented as follows : 



(a) The naso-pharynx is closed by the action of the pharyngo- 

 palatini muscles, which form the posterior pillars of the fauces. 

 The pharynx is thus drawn to a narrow cleft. Against this narrow 

 opening the soft palate is pressed by the action of the levator and 

 tensor veli palatini muscles, (b) The laryngeal aperture is kept 

 closed by the action of the crico-arytaenoideus lateralis, arytae- 

 noidei, and the thyreoarytaenoidei muscles, which pull the 

 arytaenoid cartilages forwards against the back of the epiglottis. 

 Accompanied by a quick downward motion of the tip of the 

 epiglottis, the bolus is pushed over the back of this structure 

 and is impelled into the gullet. 



III. The stomach. By the impulse imparted to it at its entry 

 into the gullet, aided generally by gravity and to a questionable 

 extent by peristalsis, the bolus is forced down to the gateway to 

 the stomach. This aperture, in common with the exit from the 

 stomach, is guarded by a thick ring of visceral muscle. When 

 contracted, i.e. during the normal state of tonus, these sphincters 

 prevent the too hurried passage of material along the alimentary 

 canal and also prevent its regurgitation. They are not controlled 

 by the will but by local nerve centres. By its weight, the bolus 

 usually exerts sufficient pressure to cause the opening of the 

 cardiac sphincter and gain admission to the stomach, in which 

 it is locked by the operation of a local automatic arrangement. 



The processes of digestion or splitting of the foodstuffs by 



