120 MANUAL OF PHYSIOLOGY. 



By the second stage is meant the period occupied by the passage 

 of the food bolus through the pharynx and past the top of the 

 larynx. Although we are not able to influence it in any way by 

 our will, we are conscious of the food passing in this region. It is 

 a rapid, involuntary spasm in which a great number of muscles 

 take part, all of which are made up of striated muscle tissue. 



The third stage includes all the rest of the time during which 

 the bolus is passing from the grasp of the lower pharyngeal con- 

 strictor and along the oesophagus. Not only has our will no in- 

 fluence over this stage of deglutition, but we are hardly conscious 

 of its taking place, since no sensations accompany the greater 

 part of it. Thus the more essential movements of the act of swal- 

 lowing are purely reflex and involuntary, though we can call 

 forth this series of reflections by voluntary stimulation of a cer- 

 tain part of the fauces by means of a morsel of food or a drop of 

 liquid. And without such a stimulus as food or liquid we cannot 

 by our will excite swallowing. We think we can perform the 

 muscular movements of swallowing when we please, without any 

 food of fluid, but in this we are mistaken, as careful observation 

 of our own performance of the act will show. 



The pharyngeal spasm is always preceded by the deposition in 

 the region of the isthmus faucium of some drop of saliva collected 

 from the mouth or fauces themselves. In fact, without a slight 

 preliminary movement of the posterior part of the tongue which 

 might be called the last act of mastication the more essential 

 stages of deglutition cannot be excited. 



Nervous Mechanism. The voluntary influences which 

 regulate the motions of the muscles of mastication pass along 

 the efferent branches of the fifth nerves (trigemini) which accom- 

 pany its inferior division. The muscles which depress the jaw to 

 open the teeth and the intrinsic muscles of the tongue are supplied 

 by the ninth pair of nerves (except the posterior belly of the 

 digastric, which has a branch from the facial, and the mylohyoid 

 and anterior belly of the digastric, which are supplied from the 

 third division of the fifth). The coordination of the movements 

 of mastication and suction seem to reside in the medulla oblon- 

 gata, but are obviously under the control of the will. 



