DIGESTION. 239 



posterior orifice of the nose and the upper opening of the larynx without 

 mching them. When it has been brought, by the first act, between the 

 anterior arches of the palate, it is moved onward by the movement of 

 the tongue backward, and by the muscles of the anterior arches contract- 

 ing on it and then behind it. The root of the tongue being retracted, 

 and the larynx being raised with the pharynx and carried forward under 

 the base of the tongue, the epiglottis is pressed over the upper opening 

 >f the larynx, and the morsel glides past it; the closure of the glottis 

 being additionally secured by the simultaneous contraction of its own mus- 

 cles: so that, even when the epiglottis is destroyed, there is little danger 

 of food or drink passing into the larynx so long as its muscles can act 

 freely. At the same time, the raising of the soft palate, so that its pos- 

 terior edge touches the back part of the pharynx, and the approximation 

 of the sides of the posterior palatine arch, which move quickly inward 

 like side curtains, close the passage into the upper part of the pharynx and 

 the posterior nares, and form an inclined plane, along the under surface 

 of which the morsel descends; then the pharynx, raised up to receive it, 

 in its turn contracts, and forces it omvard into the oesophagus. (3.) In 

 the third act, in which the food passes through the oesophagus, every 

 part of that tube, as it receives the morsel and is dilated by it, is stimu- 

 lated to contract: hence an undulatory contraction of the oesophagus, 

 which is easily observable in horses while drinking, proceeds rapidly along 

 the tube. It is only when the morsels swallowed are large, or taken too 

 fjiiickly in succession, that the progressive contraction of the oesophagus 

 is slow, and attended with pain. Division of both pnetimogastric nerves 

 paralyzes the contractile power of the oesophagus, and food accordingly 

 accumulates in the tube. The second and third parts of the act of deglu- 

 tition are involuntary. 



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

 tition are: sensory, branches of the fifth cerebral supplying the soft 

 } Dilate; glosso-pharyngeal, supplying the tongue and pharynx; the supe- 

 rior laryngeal branch of the vagus, supplying the epiglottis and the glot- 

 tis; while the motor fibres concerned are: branches of the fifth, supply- 

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

 mastication; the facial, supplying the levator palati; the glosso-pharyn- 

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

 muscles of the larynx through the inferior laryngeal branch, and the 

 hypoglossal, 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 pneumogastrics, 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 



