294 THE TISSUES AND MECHANISMS OF DIGESTION. 



the steady onward movement, such as is seen when more solid contents pass 

 along the straight and somewhat firmly attached oesophagus, is complicated 

 by movements due to a loop being projected forward by the entrance of fluid 

 from above, or being dragged down by the weight of its new contents, or, 

 on the other hand, due to a loop being retracted by the driving onward of 

 its contents and the emptying of itself, and the like. In this way a peculiar 

 writhing movement of the bowel is brought about, and the phrase " peristal- 

 tic movements " is generally used to denote this total effect of the contrac- 

 tion of the muscular coats ; it will, however, be best to restrict the meaning 

 to the progressive contraction of the circular coat assisted, in most cases, by 

 a similar progressive contraction of the longitudinal coat. 



229. Mastication. This in man consists chiefly of an up-and-down 

 movement of the lower jaw, combined in the grinding action of the molar 

 teeth, with a certain amount of lateral fore-and-aft movement. The lower 

 jaw is raised by means of the temporal, rnasseter, and internal pterygoid 

 muscles. The slighter effort at depression brings into action chiefly the 

 digastric muscle, though the mylo-hyoid and genio-hyoid probably share in 

 the matter. Contraction of the external pterygoids pulls forward the con- 

 dyles and thrusts the lower teeth in front of the upper. Contraction of the 

 pterygoids on one side will also throw the teeth on to the opposite side. The 

 lower horizontally placed fibres of the temporal serve to retract the jaw. 



During mastication the food is moved to and fro and rolled about by the 

 movements of the tongue. These are affected by the muscles of that organ 

 governed by the hypoglossal nerve. 



The act of mastication is a voluntary one, guided as are so many volun- 

 tary acts, not only by muscular sense, but also by contact sensations. The 

 motor fibres of the fifth cranial nerve convey motor impulses from the brain 

 to the above-mentioned muscles ; but paralysis of the sensory fibres of the 

 same nerve renders mastication difficult by depriving the will of the aid of 

 the usual sensations. 



230. Deglutition. The food when sufficiently masticated is, by the 

 movements of the tongue, gathered up into a bolus on the middle of the 

 upper surface of that organ. The front of the tongue being raised partly 

 by its intrinsic muscles and partly by the stylo-glossus the bolus is thrust 

 back between the tongue and the palate through the anterior pillars of the 

 fauces or isthmus faucium. Immediately before it arrives there the soft 

 palate is raised by the levator palati, and so brought to touch the posterior 

 wall of the pharynx, which, by the contraction of the upper margin of the 

 superior constrictor of the pharynx, bulges somewhat forward. The elevation 

 of the soft palate causes a distinct rise of pressure in the nasal chambers? 

 this can be shown by introducing a water manometer into one nostril and 

 closing the other just previous to swallowing. By the contraction of the 

 palato-pharyngeal muscles which lie in the posterior pillars of the fauces the 

 curved edges of those pillars are made straight, and thus tend to meet in 

 the middle line, the small gap between them being filled up by the uvula. 

 Through these manoeuvres the entrance into the posterior nares is blocked, 

 while the soft palate is formed into a sloping roof, guiding the bolus down 

 the pharynx. By the contraction of the stylo-pharyngeus and palato- 

 pharyngeus, the funnel-shaped bag of the pharynx is brought up to meet 

 the descending morsel, very much as a glove may be drawn up over the 

 finger. 



Meanwhile in the larynx, as shown by the laryngoscope, the arytenoid 

 cartilages and .vocal cords are approximated, the latter being also raised so 

 that they come very near to the false vocal cords ; and the cushion at the 

 base of the epiglottis covers the rima glottidis, while the epiglottis itself is 



