724 



PHYSIOLOGY 



of the lower air passages are the external thyro-arytenoid, arytenoid, 

 aryepiglottidean, and the lateral crico -arytenoid muscles. Since the 

 approximation of the posterior to the anterior boundary of the laiyngeal 

 opening is only rendered possible by the elevation of the whole larynx under 

 the hyoid bone, the act of deglutition cannot be carried out unless the larynx 

 is free to move. 



The two openings from the back of the pharynx into the air passages 

 being thus closed, the bolus is shot rapidly past them into the region of the 

 middle and inferior constrictors of the pharynx. If the bolus be liquid or 

 semi-fluid, the movement of the back part of the tongue may be sufficient to 

 propel the substance past the constrictors through the lax oesophagus to its 

 lower end. It is on this account that, when corrosive fluids are swallowed 

 by accident, we very often find the damage to the oesophagus limited 

 to the three points where it is narrowed and there is a slight hindrance 

 to the onward flow of fluid. If the bolus be large and solid or 

 semi-solid, it is seized in the grasp of the middle constrictors on passing 

 through the upper part of the pharynx, and is thrust by successive con- 

 tractions of this muscle and of the inferior constrictor gradually down the 

 oesophagus. The walls of the cervical part of the oesophagus are composed 

 of striated muscle. In the thorax striated and unstriated muscles are asso- 

 ciated together, while the lower third, in the neighbourhood of the stomach, 

 consists almost entirely of unstriated muscle. Corresponding to these 

 differences in structure, Kronecker and Meltzer have found differences in the 

 duration and rapidity of propulsion of the contractional waves in each part. 

 The following Table shows the time-relations of the chief muscles engaged 

 in deglutition as determined by Kronecker and Meltzer and by Marckwald : 



The free passage of food down the oesophagus under the influence of the. 

 prppulsive force exercised by the mylohyoid muscles shows that the walls 

 of this tube must be lax, and in fact one must assume that the first act of 

 deglutition, so far as concerns the oesophagus, is an inhibition initiated 



