280 DIGESTION 



the mouth cavity from which the reflex can invariably be started has yet been 

 made out. We can only say that it is induced when the bolus is forced back 

 of the soft palate into the region of the tonsils. Thus far the process is under 

 control of the will and the swallowing reflex therefore is inaugurated volun- 

 tarily, but thereafter it is independent of the will. 



The first muscles to contract are the mylohyoids, and the pressure in the 

 mouth cavity is raised by their contraction alone to about 20 c.c. of water. 

 Almost at the same instant the hyoglossi contract causing the free surface 

 of the base of the tongue, which in rest is directed backward and upward, 

 to execute a movement backward and downward. By this contraction, ac- 

 cording to Kronecker and Meltzer, fluid foods are spirted through the whole 

 length of the oesophagus to its lower end before the contractions of the phar- 

 yngeal and cesophageal muscles can be brought to bear on them. Against this 

 view, however, it may be observed that at the moment of the mylohyoid con- 

 traction the oesophagus is still closed by an elastic pressure exerted by the 

 larynx. This closure is broken only when the larynx is drawn upward and 

 forward, which takes place some 0.2 (Schreiber) to 0.3 of a second (Eykman) 

 after the inception of the act i. e., at a time when the effect of the mylo- 

 hyoid is already past. If this be true, the movements of the above-named 

 muscles of the tongue can only force the food mass into the pharynx (cf. 

 Fig. 112, in which a sagittal section of the floor of the mouth and of the 

 neck in both the normal position (A) and the open position (B) of the pas- 

 sageway is represented after X ray photographs; from B it is plain that the 

 tongue in swallowing lies against the posterior wall of the throat). 



When the food reaches the pharynx there follows in order the contractions 

 of the pharyngeal and the oesophageal muscles. In the oesophagus the move- 

 ment is executed at two different rates of speed. The muscle fibers in the 

 cervical part of the oesophagus are cross-striated and the movement conse- 

 quently is rapid; in the thoracic portion the cross-striated muscle layers are 

 replaced more and more by smooth muscles and the movement of the food 

 is consequently slower. The total time from the beginning of the act of 

 deglutition until immediately before the opening of the cardia is about seven 

 to eight seconds. 



If the oesophagus be cut in the neck or even if a long piece of it be extir- 

 pated, and the superior laryngeal nerve, which very easily discharges the swal- 

 lowing reflex, be stimulated, a perfectly regular act of swallowing results in 

 which the contraction not only extends over the upper part, but appears just as 

 usual in the part lying below the section (Mosso). The peristaltic wave of con- 

 traction therefore runs from above downward, under the direct control of the 

 central nervous system. In deeply anesthetized dogs this experiment does not 

 succeed (Wild) ; but if in such animals the anatomical continuity be preserved, 

 the contraction wave travels over the entire oesophagus. From this it follows 

 that the contraction can be propagated through the oesophagus without the 

 cooperation of the central nervous system, a circumstance which Meltzer sup- 

 poses to be due to the action of peripheral reflex centers. 



We have, therefore, to conceive the act of deglutition as taking place in 

 such a way that once the reflex is induced, the different divisions of the 

 passageway (floor of the mouth and tongue, pharynx, the various divisions 



