THE MECHANICAL FACTORS OF DIGESTION 



Then, breathing being inhibited, there follows a short, sharp con- 

 traction of the mylohyoid and hyoglossus muscles. The action of the 

 mylohyoid is to press the tongue upwards against the hard palate, 

 that of the hyoglossus to pull it backwards. The result of the com- 

 bined action is a piston-like thrust, which propejs the bolus into the 

 pharynx. Its entrance into the naso-pharynx is prevented by the 

 contraction at the same time of the palato-pharyngeus and levator 

 palati muscles. The levator palati pulls the soft palate down 

 against the posterior pillars of the fauces, which are approximated by 

 the contraction of the palato-pharnygeus muscles. The bolus first 

 strikes the soft palate, then the back wall of the pharynx; it next 

 passes between the pharyngeal wall and the epiglottis, the oesophagus 

 in the meantime being kept closed by the pressure of the larynx; 

 the hyoid bone and the larynx are now raised, the glottis approximated 

 to the epiglottis, the respiratory tract thus shut off, and the gullet 

 opened; so that the bolus, propelled by the mylohyoid, glides into 

 the open oesophagus (Fig. 201). 



This is the end of the first stage of deglutition the stage voluntarily 

 initiated. Then follows the second stage the involuntary stage 

 namely, the passage of food down the oesophagus proper to the cardiac 

 orifice of the stomach. In time past, conflicting opinions were held 

 as to the relative importance of the initial impulse imparted in the 

 first voluntary stage and of the peristaltic action of the oesophagus 

 itself. From recent experiment by means of X rays, it would appear 

 that this depends largely upon the nature of the food, solids and 

 pappy foods being passed down by the peristaltic action of the 

 oesophagus itself liquids, on the other hand, passing quickly down by 

 the impetus given by the piston action of the mylohyoid. The rate 

 of the transmission in the different parts of the oesophagus is variable. 

 It depends upon the nature of the muscle. Thus, in the goose, where 

 the muscle is smooth, a uniform slow peristalsis takes place. It takes 

 twelve seconds for a solid bolus to traverse 15 centimetres of gullet. 

 In the cat, the peristalsis is rapid as far as the heart level (4 seconds), 

 and slow (6 or 7 seconds) for the remainder less than a third of the 

 whole distance. It is at the heart level that the muscle changes 

 from striated to smooth. In the dog, the peristalsis is quick through- 

 out, the time taken for a solid bolus being 4 or 5 seconds from larynx 

 to cardia. In the dog, the whole oesophagus is composed of striated 

 muscle. In both the cat and dog, liquids travel much more quickly 

 than the solid or semi-solid bolus. 



In man, the lower end of the oesophagus is composed of smooth 

 muscle, and a slower rate of peristalsis is observed in this region. 

 X-ray observations upon man show that solids and semi-solids are 

 moved down the oesophagus by peristaltic action, irrespective of the 

 position of the body, and owe practically nothing to the preliminary 

 impetus, the time required for a well-lubricated bolus being from 

 8 to 18 seconds, for a dry bolus several minutes. Liquids, on the 

 other hand e.g., milk containing bismuth are shot rapidly through 

 the greater part of the oesophagus. In the head-down position, they 



