762 



PHYSIOLOGY 



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 associated together, 

 while the lower third, in the neighbourhood of the stomach, consists 

 almost entirely of unstriated muscle. Corresponding to these differ- 

 ences 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 propulsive 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 reflexly with the beginning of the act of 

 deglutition. When a second act of deglutition succeeds the first 

 within a sufficiently short interval, the reflex inhibition due to the 

 second act may prevent the development of any wave of contraction 

 in the oesophagus. This tube thus remains in a lax condition and 

 allows the free rapid passage "of the food downwards until the move- 

 ments of deglutition have come to an end, when a peristaltic con- 

 traction occurs and sweeps all remaining adherent particles of food 

 into the stomach. The circular fibres of the lower end of the 

 oesophagus which form the cardiac sphincter of the stomach are 

 normally in a state of tonic contraction. When one mouthful of 

 food is swallowed it may be either squirted directly into the stomach, 

 or may remain at the lower end of the oesophagus until the following 

 peristaltic wave forces it through the orifice. When several acts of 



