FOOD AND DIGESTION. 353 



second being the next 10 centimetres of the tube, and the third the r& 

 maining portion to the stomach. 



The act of swallowing consists, then, of the contraction in sequence 

 of five muscle-segments: the mylo-hyoids, the constrictors of the phar- 

 ynx, and the three segments of the oesophagus. The computed time of 

 contraction is as follows: 



Seconds. 



Contraction of mylo-hyoids and constrictors of the pharynx . . 0.3 



Contraction of the first part of the oesophagus . . . . ,. . . . o.9 



Contraction of the second part of the oesophagus . . . . . . . . 1.8 



Contraction of the third part of the oesophagus .. . . . .,'.- . 3.0 



If a second attempt at swallowing be made before the first has been 

 completed (that is, before 6 seconds have elapsed), the remaining portion 

 of the first act is inhibited, and the contraction wave reaches the stomach 

 6 seconds after the commencement of the second act. 



In addition to the above, the following facts must be noted: 



During the act of deglutition the posterior nares are closed through 

 the action of the levator palati and tensor palati muscles, which raise 

 the velum; the palato-pharyngei, drawing the posterior pillars of the 

 fauces together; and the azygos uvulaB, which raises the uvula thus 

 forming a complete curtain. Otherwise the food would pass into the 

 nose, as happens in the case of cleft palate. At the same time the lar- 

 ynx is closed by the adductor muscles of the vocal cords and the descent 

 of the epiglottis, the larynx being drawn upward as a whole through the 

 action of the mylo-hyoid, gonio-hyoid, thyro-hyoid, and digastric mus- 

 cles. The presence of the epiglottis is not necessary for the completion 

 of the act of deglutition. 



Nervous Mechanism. The nerves engaged in the reflex act of deglu- 

 tition are -.sensory, branches of the fifth cerebral supplying the soft pal- 

 ate; glosso-pharyngeal, supplying the tongue and pharynx; the superior 

 laryngeal branch of the vagus, supplying the epiglottis and the glottis; 

 while the motor fibres concerned are : branches of the fifth, supplying 

 part of the digastric and mylo-hyoid muscles, and the muscles of masti- 

 cation; the facial, supplying the levator palati; the glosso-pharyngeal, 

 supplying the muscles of the pharynx; the vagus, supplying the muscles 

 of the larynx through the inferior laryngeal branch, and the hypoglos- 

 sal, the muscles of the tongue. The nerve-centre by which the muscles 

 are harmonized in their action, is situate in the medulla oblongata. In 

 the movements of the oesophagus, the ganglia contained in its walls, 

 with the pneumo-gastrics, are the nerve-structures chiefly concerned. 



It is important to note that the swallowing both of food and drink is 

 a muscular act, and can, therefore, take place in opposition to the force 

 of gravity. Thus, horses and many other animals habitually drink up- 

 hill, and the same feat can be performed by jugglers. 



