52O THE MECHANICAL PROCESSES OF DIGESTION. [CH. xxxv. 



the posterior palatine arch, which move quickly inwards 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 ; than the 

 pharynx, raised up to receive it, in its turn contracts, and forces 

 it onwards into the oesophagus. The passage of the bolus of food 

 through the three constrictors of the pharynx is the last step in 

 this stage. (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 stimulated to contract: hence an 

 undulatory or peristaltic contraction of the oesophagus occurs, 

 which is easily observable through the skin in long-necked animals 

 like the swan. If we suppose the bolus to be at one particular 

 place in the tube, it acts stimulatingly on the circular muscular 

 fibres behind it, and inhibitingly on those in front ; the contraction 

 therefore squeezes it into the dilated portion of the tube in front, 

 where the same process is repeated, and this travels along the 

 whole length of the tube. The second and third parts of the act 

 of deglutition are involuntary. The action of these parts is more 

 rapid than peristalsis usually is. This seems to be due to the 

 large amount of striated muscular tissue present. It serves the 

 useful purpose of getting the bolus as quickly as possible past 

 the opening of the respiratory tract. 



Nervous Mechanism. The nerves engaged in the reflex act of 

 deglutition are : sensory, branches of the fifth cranial nerve sup- 

 plying the soft palate and tongue; 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 mastication ; 

 the bulbar part of the spinal accessory through the pharyngeal 

 plexus, supplying the levator palati, probably by rootlets which are 

 glosso-pharyngeal in origin ; the glosso-pharyngeal and vagus, and 

 possibly the bulbar part of the spinal accessory, supplying the 

 muscles of the pharynx through the pharyngeal plexus ; the vagus, 

 in virtue of its spinal accessory roots, supplying the muscles of 

 the larynx through the inferior laryngeal branch ; and the hypo- 

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

 muscles are harmonised in their action, is situated in the medulla 

 oblongata. Stimulation of the vagi gives rise to peristalsis of the 

 oesophagus. The cell stations of these fibres are in the ganglion 

 trunci vagi. Division of both pneumo-gastric nerves gives rise to 

 paralysis of the oesophagus and stomach, and firm contraction of 

 the cardiac orifice. These nerves therefore normally supply the 



